ContentsAcknowledgementsForwardExecutive SummaryChapter 1 Introduction History, heritage and operationChapter 2 Cosmic-ray and Heliospheric ScienceChapter 3 Space Weather and operational needs for NMs

Chapter 4 Other science applications of NM dataChapter 5 Current status and outlookChapter 6 Recommendations and prioritiesChapter 7 Citation Listing Chapter 8 References

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AcknowledgementsThe National Science Foundation, the University of New Hampshire and the Bartol Research Institute sponsored a workshop entitled Neutron Monitor Community Workshop on October 24-25, 2015 in Honolulu, Hawaii. The workshop was organized with generous help from the University of Hawaii at Manoa. Its objective was to review and evaluate the US-supported neutron monitor network, and articulate a community consensus on the network utility and value with recommended priorities and actions. This paper is the principal product of that workshop, integrating contributions of the many participants.

Participants Institution

John Bieber University of Delaware

Doug Biesecker NOAA

Veronica Bindi UH of Hawaii Manoa

Mirko Boezio Istituto Nazionale di Fisica Nucleare

Alessandro Bruno University of Bari

Mark Christi NASA Marshall Space Flight Center

John Clem University of Delaware

Cristina Consolandi University of Hawaii

Kyle Copeland Federal Aviation Administration

Claudio Corti University of Hawaii at Manoa

Georgia deNolfo Goddard NASA

Andrew Druetzler University of Hawaii Manoa

Paul Evenson University of Delaware

Paul Goldhagen Department of Homeland Security

Mike Gordon IBM

Paul Goldhagen Department of Homeland Security

Julia Hoffman University of Hawaii at Manoa

Randy Jokipii University of Arizona

Jongil Jung Chungnam National University

Marty Lee University of New Hampshire

Jim Madsen UW River Falls

Kazuoki Munakata Shinshu University

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Roger Pyle Pyle Consulting Group

James Ryan University of New Hampshire

Mohammad Sabra NASA Marshall Space Flight Center

Peggy Shea Unaffiliated

Don Smart Unaffiliated

Christian Steigies University of Kiel

Nicola Tomassetti CNRS/IN2P3, Grenoble

Allan Tylka Unaffiliated

Katie Whitman University of Hawaii Manoa

Ilya Usoskin University of Oulu

Stephen White Air Force Research Laboratory

Yu Yi Chungnam National University

Yihua Zheng Goddard NASA

ForwardThe purpose of the neutron monitor is to detect, deep within the atmosphere, variations of intensity in the interplanetary cosmic ray spectrum. Interactions of the primary cosmic rays with the atmosphere produce, among other things, a lower energy secondary nucleonic component consisting of nucleons, in particular neutrons that are not slowed by ionization loss, These secondaries fall in the energy range of a few hundred MeV up to about one GeV. Because of the falling energy spectrum of the primary cosmic rays, the neutron monitors are most sensitive to the low energy (1-20 GeV) portion of the spectrum.

These nucleons in turn produce further nuclear interactions, either in the atmosphere, or in lead target material surrounding the monitor. The interaction rate may be measured most conveniently and reliably by detecting the reaction product neutrons rather than by detecting the charged fragments directly.

Excerpt from Simpson, J.A., "Cosmic-Radiation Neutron Intensity Monitor", in Annals of the IGY, 1955

Neutron monitors were identified decades ago as useful instruments for studying the near-Earth environment. Since their wide deployment in the International Geophysical Year 1957-1958, they have been used to (1) measure the energetic particle emission from the Sun during periods of intense solar activity, (2) study the dynamics of the near and far heliosphere as sensed through solar modulation over the course of several solar cycles and (3) study the dynamics of what are now known as Coronal Mass Ejections through the transient modulation of galactic cosmic rays.

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Since then other disciplines have found uses for neutron monitor data that go beyond near-Earth science.

The initial chapter addresses the history and workings of the instrument and its modes of deployment. The remainder of the paper is organized into discussions of the utility and importance of neutron monitors for science, service and space weather. The paper finishes with a discussion of recommended actions and priorities to support these diverse needs.

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Executive SummaryA neutron monitor workshop held in October 2015 reviewed the status, utility, science and future of the monitor network. The participants formulated recommended actions to preserve and enhance these aspects of the network. Also in October 2015, two documents from the Executive Branch were published in response to the country’s needs from the risks posed by Space Weather events, the National Space Weather Strategy and the National Space Weather Action Plan. The documents were prepared by the Space Weather Operations, Research, and Mitigation (SWORM) Task Force and published by the National Science and Technology Council. Co-chairs of the task force included the National Oceanic and Atmospheric Administration, Department of Homeland Security and the Office of Science and Technology Policy. The role of neutron monitors in the multi-agency plan is part of the multipronged approach to identify and characterize space weather events and mitigate their negative effects on society and governmental functions.

A neutron monitor is a ground-based instrument that continuously records the rate of high-energy particles (E>500MeV) impacting the Earth’s atmosphere. For historical reasons these particles, mostly protons and helium nuclei, are called "cosmic rays." Because the intensity of cosmic rays hitting Earth is not uniform, it is important to place neutron monitors at multiple locations to form a complete picture of cosmic rays in space. The advent of the neutron monitor came after the discovery by Simpson [1948] that the intensity of the nucleonic component in cosmic ray air showers is several times more sensitive to changes in low energy regime (0.5-3.0GeV) of primary cosmic rays than that of the electromagnetic and muon components, as measured by the ionization chambers. In 1952 the first neutron monitor stations were established at Chicago, Climax, Huancayo, Peru and Sacramento Peak, and by the 1957 IGY, sixty stations were established world-wide. Some these early stations are still in operation today. The reliability and basic simplicity of neutron monitors provide a means for studying the longer-term variations, while the sensitivity to low energy primary cosmic rays and high count rates make possible the measurement of short term intensity changes. As the result of the long term reliability and on-line availability of neutron monitor data, users outside the traditional circles of Space Sciences have discovered innovative ways to utilize these data for new and unforeseen investigations, radiation monitoring or real-time background. Over the past decades neutron monitor data have become an operational resource in many different areas.

Unfortunately, from the point of view of basic science, service to the community and as part of the national space weather strategy, the current state of the US neutron monitor network is not good. The network now consists of aging neutron monitors, many of which have been neglected due to a lack of funding. The network is not functioning as it once did or envisioned during the International Geophysical Year in 1957-1958. Several key measurement sites have been de-commissioned or switched off. A perceived lack of interest by the US makes foreign sites consequently vulnerable to cuts and closings. There is minimal coordination between supporting groups with each neutron monitor station fighting for existence. New and inexpensive technology cannot make its way into the field to modernize and greatly improve the performance of those stations now operating. Recommendations emerged from the workshop that if implemented would bring the network into a condition to conduct the best science and support operational needs. In order of priority, they are:

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Fully restore the scientific functionality and update the existing US network. This will constitute a major step in restoring the global network by restoring coverage provided by the US stations and by making a statement to international funding agencies that the global network is important,

Establish a desired network concept that would fulfill the needs of the science and operational communities. This would involve identifying new key strategic sites to complete the global coverage,

Improve station and data uniformity and accessibility,

Train a new generation of scientists and expand educational outreach,

Given the unavailability of standard detectors, design and deploy a new generation of neutron monitors in the form of inexpensive kits to be widely deployed as part of global strategy for the network, and

Modernize or install new timing electronics to study rapid phenomena, not anticipated in 1957.

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Chapter 1 Introduction History and Heritage

The history of the neutron monitor starts with the discovery of "high altitude radiation" identified with a balloon experiment conducted by Victor Hess in 1912 (Hess, 1912). Soon afterwards temporal and spatial variations in the radiation intensity were observed with ionization chambers and charged particle telescopes. The analysis of these variations led to a better understanding of the nature of primary cosmic-rays. Travelling by sea from Indonesia to Europe in 1927, Jacob Clay observed variation of cosmic ray intensity with latitude indicating primary cosmic rays are deflected by the Earth’s magnetic field and consequently must be electrically charged particles. In 1933 Bruno Rossi observed that the number of particles coming from the west is greater than from the east, indicating most primaries are positive charged (the East-West effect). During the same experiment, while testing his detectors, Rossi also discovered the existence of particle showers produced by interactions of cosmic rays in the atmosphere, a phenomenon studied by Pierre Auger in 1937, whose name became associated with the discovery.

Routine monitoring of the cosmic radiation was initiated in January 1932 with the operation of an ionization chamber at Hafelekar, Austria (Shea and Smart, 2000). The first network of cosmic ray detectors was sponsored by the Carnegie Institution and consisted of four shielded ionization chambers of the same design at Christchurch, New Zealand (April, 1936), Huancayo, Peru (June 1936), Cheltenham, USA (March 1937) and Godhavn, Greenland (October, 1938). The Carnegie Network of ionization chambers was developed primarily to detect the change in cosmic radiation as a function of geomagnetic activity as evidenced by some of Forbush's early papers (Forbush, 1937, 1938). By the early 1940s there were a number of ionization chambers in operation both at stationary locations and on shipboard.

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During the decades from 1930s to 1940s, a wide variety of experiments confirmed that primary cosmic rays entering the top of the atmosphere produced nuclear interactions deep in the atmosphere. An incoming primary cosmic ray with sufficient energy will interact with an air nuclei, initiating a cascade of secondary interactions that ultimately produce a particle shower. A significant fraction of these interactions occur at the nuclear level, which can be thought of as a two stage process (inelastic collision and nuclear de-excitation). In the first stage (inelastic collision), the primary particle reacts with nucleons inside the nucleus creating an intranuclear cascade of high-energy (>20 MeV) protons, neutrons, and pions within the nucleus. Some of these energetic hadrons escape as secondary particles while others deposit their energy in the nucleus leaving it in an excited state. The escaping secondary particles may collide with another air nuclei and produce successively spallation products and excited nuclei. In the second stage (nuclear de-excitation), evaporation takes place when the excited nucleus relaxes by emitting low-energy (< 20 MeV) neutrons, protons, alpha particles, etc., with the majority of the particles being neutrons. Evaporation neutrons interacting with nuclei are typically elastic that transfer

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Figure 1. Anatomy of a cosmic ray air shower.

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energy without changing the structure of the nucleus. After multiple interactions, the evaporation neutron will lose energy and eventually thermalize, and/or be absorbed or undergo decay.

In 1946, Simpson initiated the first investigation on the dependence of nucleonic intensity with incident primary cosmic-ray energy using the geomagnetic cut-off effect. These observations took place at 30,000ft on a B29 aircraft. The nucleonic flux in the atmosphere was inferred by measurements of the evaporation energy neutron intensity (Simpson, 1949). This required a detector that was insensitive to neutron energies lower than the evaporation energy, as thermal neutrons are highly dependently on local conditions. An additional requirement was the exclusion of the muon and electron components at all energies. To achieved these requirements, Simpson designed a detector utilizing a 10BF3 gas proportional counters encapsulated in paraffin. 10B has a neutron capture cross-section inversely proportional to the neutron and responds to neutrons by the exothermic reaction 10B(n,) 7Li with a Q value of 2.3MeV. With properly chosen gas density, the pulse-height threshold could eliminate the muon and electromagnetic component from neutron absorptions. Paraffin serves to reduce the energy of neutrons, thus increasing the probability of an absorption inside the proportional counter while also providing a shielding barrier against unwanted thermal and epithermal energy neutrons.

Utilizing this new neutron detector along with Geiger-Muller counters on an air-borne latitude measurements in 1946–1947, Simpson [1948] discovered the latitude variation of the intensity of evaporation neutrons in the atmosphere is several times larger than that of the ionizing and hard components. It immediately became apparent from this discovery that variations in the low

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Figure 2. The lower curve represents the latitude dependence for ionization chamber. The upper curve represents the evaporation neutron intensity. Both curves are normalized to 1 at zero magnetic latitude. The data for the curves were obtained in June, 1948. (Need Permission?)

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energy cosmic-ray proton intensity down to 1–5 GV could be investigated continuously for the first time. This new detector opened a new window to the primary cosmic-ray energy spectrum.

The observed variation in the nucleonic component intensity at high latitudes from survey to survey motivated the need for a ground base system to continuously monitor the evaporation

neutrons. It was during this time Simpson (2000) considered the effect of atmospheric production and local production of evaporation of neutrons, estimating that the multiplicity per interaction of neutron production in materials with high atomic mass, A, increased as A0.7. The anticipation that the count rate could be greatly enhanced led to the concept of a cosmic-ray neutron monitoring system based on measuring the local production of evaporation neutrons in a high atomic mass target such as lead. The fragmentation of a lead nucleus by an incident high-energy secondary nucleon would yield a multiplicity of evaporation neutrons, which would then become thermalized in the surrounding paraffin wax and be detected with 10BF3 proportional counters embedded in the ‘pile’. The development of a leaded neutron monitor was undertaken in 1949, leading to a basic ‘standard’ pile design with an account of its dependence only on barometric pressure (Simpson, 1953; Simpson et al., 1953b). Drawing of IGY is shown in Figure 3. Furthermore, this design could be extended in size to multiply the counting rate from the pile. The 12 counter configuration became the standard neutron monitor design for Chicago and Climax, Colorado in 1949; and with the influence of Scott Forbush, later it was the design adopted for the International Geophysical Year (1957–1958) at more than sixty (60) sites world-wide (Simpson, 1958). The detector was renamed the IGY neutron monitor.

The IGY neutron monitor opened a new window to cosmic ray intensities and produced an understanding of six basic phenomena:

(1) Variation of cosmic ray intensity with geomagnetic latitude (Simpson, 1948). (2) Variation of cosmic radiation intensity in response to the 11-year solar activity cycle

and the 22-year solar magnetic cycle (Meyer and Simpson, 1955) that was originally indicated in ion chamber data variations from the time period 1937–1952 by Forbush (1954).

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Figure 3. IGY Neutron Monitor

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(3) Correlation of the 27-day modulation of cosmic ray intensity with the Carrington rotation of the Sun (Simpson, Babcock and Babcock, 1955).

(4) Anisotropy of the cosmic rays exhibited as diurnal variations (Simpson 1953). (5) The origin of the rapid decrease in cosmic ray intensity (Forbush Decrease) lies

outside of the Earth’s geomagnetic field (Simpson 1953).(6) Based on the ground level event observation that the sudden and short burst of

relativistic nucleons of a solar flare event (23 February 1956) could only slowly escape to the interstellar medium through a continuous barrier region beyond the orbit of Earth, implying a dynamical heliosphere (Bieber et al., 2000, Meyer et al., 1956; Simpson, 1985; Parker, 1956).

Following these discoveries, Parker (1963) developed a quantitative theory for coronal expansion of the solar wind into interplanetary space. This was the start of our current understanding of the heliosphere and the cosmic ray modulation.

In 1964 a new neutron monitor design was developed (H. Carmichael, 1964, Fowler 1962, Hatton 1971) to greatly increase the neutron counting rates. The new design was then called the super-monitor and now simply the NM64. Such an increase (to over an order of magnitude larger than the IGY monitor) involved designing a neutron monitor to cover a greater area than that of the IGY monitor. This was achieved with a much larger (BP28) BF3 proportional counters developed at Chalk River by Fowler (1962). Another design development was the use of polyethylene as a moderator and reflector which provided a more stable mechanical structure. Although the lead producer and moderator were chosen to be similar to that of the IGY, the reflector thickness was reduced from 11 inches to 3 inches based on an optimization study by

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Figure 4.Cross-sectional view of an NM64 Neutron Monitor

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Hatton and Carmichael (1964) with the objective to maximize neutrons counts above evaporation energies while minimizing the contribution from externally produced evaporation neutrons. Most importantly the NM64 design has achieved a globally accepted standardization (Hatton 1971). Today this same design is still recognized as the “Neutron Monitor” standard.

Neutron Monitors in the IGY(Peggy)

The International Geophysical Year (IGY) from July 1957 to December 1958 provided a unique opportunity for scientists to conduct multi-discipline geophysical studies using data acquired from a large variety of sensors located around the world. One of the objectives of the IGY was the worldwide exchange of scientific data. To meet this objective the World Data Centers were established. These were established at places where there was scientific research in specific disciplines. Four World Data Centers for Cosmic Rays were established with Center A at the University of Minnesota.

The initial plans for the exchange of cosmic radiation data during the IGY called for bi-hourly atmospheric pressure values and bi-hourly neutron monitor data corrected for atmospheric pressure. Special data forms were prepared for uniform reporting; these forms were to be sent to each cosmic ray data center every three months with a letter of data transmittal sent to the IGY

Headquarters in Belgium. An example of a month of data is shown in Figure 1.

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STANDARD COSMIC RADIATION DATA FORMAT SUBMISSIONS TO WORLD DATA CENTERS DURING THE IGY

Standard Cosmic Radiation Data Format Submissions to World Data Centers during the IGY

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The initial publication of the Carnegie ionization chamber data (Lange and Forbush, 1948) contained only bi-hourly data, and this may have influenced the IGY organizers in their initial planning. At the time of the IGY planning meetings, only four high energy solar proton events had been identified: two in 1942, 1946 and 1949, and the publication of those results contained graphs of bi-hourly ionization chamber data (Forbush 1946; Forbush et al., 1950). Relativistic solar proton events were considered so rare that no provision was made for the exchange of data in smaller time increments than the bi-hourly values.

There were other factors to be considered. The scientific data prior to and during the IGY were primarily recorded on paper either handwritten or typed. Digital counters were installed at most sites with a camera recording the counters at set time intervals. After the film was developed, the digital values had to be manually read, and since the counters did not reset themselves at specified intervals, each reading had to be subtracted from the following value to obtain the counting rate within the specified period. The barometric pressure was usually recorded on analog charts with hourly values recorded by hand. Next the cosmic ray counting rate for the specified interval had to be corrected for the atmospheric pressure for that time period using a pre-determined barometric pressure coefficient appropriate for the station. Once the values were obtained, they had to be typed or hand written on the official IGY data forms. The entire process of recording the data was time consuming and laborious.

As preparations were underway for reporting and archiving cosmic radiation data, a major solar proton event occurred on 23 February 1956. The neutron monitor at Leeds, UK recorded an increase of 4581% over a 15-minute period. The event was world-wide in scope with significant increases in the equatorial region indicating the presence of particles in excess of 15 GV at the top of the atmosphere. Sarabhai et al. (1956) estimated the maximum energy of solar protons to be >50 GeV. Plans were quickly revised to recommend recording intervals of 15 minutes (or less) so that unusual "events" would permit detailed post event analyses (Nicolet, 1959). The routine bi-hourly reporting intervals to the World Data Centers remained unchanged.

The ground-level enhancement (GLE) of 23 February 1956 remains as the highest increase in 15 minute data recorded by neutron monitors. While there are higher increases for the 20 January 2005 GLE, these increases are for much shorter time intervals and were recorded at polar stations (Bieber et al., 2005). When averaged over a 15-minute interval, the percentage increase for the 20 January 2005 event is less than the increase at the Leeds neutron monitor on 23 February 1956. The GLE of 23 February 1956 is still actively studied today (Rishbeth et al., 2009).

Many groups planning their neutron monitor data recordings for the IGY had already implemented plans for smaller time interval data with some groups installing "flare alarms" which increased the recording interval if there was a rapid increase in the counting rate. While the IGY was during one of the most active solar cycles, there were no ground-level solar cosmic ray events during this time interval. The shorter time interval data was, however, extremely useful in the study of Forbush decreases of which there were many.

At the start of the IGY in July 1957 there were 43 IGY neutron monitors operating world wide. Nine of these monitors were in the USA; two others at Mexico City and Huancayo were under the auspices of the University of Chicago. (The Thule neutron monitor did not become

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operational until August 1957.) In December 1959, at the end of the International Geophysical Cooperation (the one year extension of the IGY) there were 58 IGY neutron monitors in operation around the world including the same nine monitors in the USA and those in Mexico City, Huancayo and Thule.

During the IGY, in addition to submitting bi-hourly data to the World Data Centers, the scientists initiated an exchange of data amongst themselves. It was relatively common for one group involved in a specific study, to request detailed data from another group, and this cooperation was readily acknowledged in subsequent publications. In addition to the exchange of data within the cosmic ray community, other scientists conducting research on various solar and geomagnetic phenomena were utilizing cosmic radiation data as a baseline describing the cosmic ray intensity during periods of interest. The use of neutron monitor data, particularly from the Climax neutron monitor, greatly increased with the advent of the space era.

Neutron Monitor Operations

In most stations each proportional counter has a dedicated electronics interface that records the total number of pulses that occur above a pre-set threshold over different integration periods. A new generation of electronics recently developed by the Bartol Research Institute, also records the time between pulses. For further details of this instrumentation and its use for determining cosmic ray spectra, see Bieber et al. 2002. These systems are currently in operation at the Newark and Thailand stations. The average number of evaporated neutrons produced in an inelastic interaction is energy dependent and can be roughly described as a power law, consequently spectral information can be in principle extracted this timing data. The capabilities of this new system is describe in Chapter 2 of this paper.

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Most of the US operating stations are fully automated with realtime data over internet or satellite connections. The data is transferred to the home database and the international Neutron Monitor Data Base (NMSB). The property for the station is leased and a local contact/observer is under contract for general building maintenance, technical support and snow removal. Instrumentation maintenance is currently operating on a “parts swap” basis such as electronic components, digital barometers, and GPS systems, however regular visits to remote stations have been discontinued due to lack of funding. Personnel directly associated with station operation are nominally a full-time, PhD-level data manager and for approximately halftime support for an Electronics Technician. Duties of the Electronics Technician include communications with remote observers, identifying and solving technical problems with station operation, and travel to the remote stations for maintenance and repair as needed. The nominal duties of the data manager include: station management, station data streaming, data processing, data quality control, data dissemination, data archiving, and maintenance of real-time data resources. This includes data transfer to the local database and to international Neutron Monitor database http://www.nmdb.eu/

Neutron monitors have been in operation for over sixty five years, and they provide a vital long-term perspective on solar variations with time scales such as the Schwabe cycle (eleven years) or Hale cycle (twenty-two years). At the same time, neutron monitors continue to make valuable measurements at much shorter time scales, providing information on relativistic solar particles and on transient, solar-induced variations of Galactic cosmic rays such as Forbush decreases.

The scientific return from neutron monitors is enhanced when they are linked together in coordinated multi-national arrays. Indeed in most modern applications the “instrument” is the

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Figure 7. International Neutron Monitor station network linked to NMDB data servers

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array itself, and not any single detector in it. Analysis of intensities from different arrival directions permits determination of the cosmic ray anisotropy, while analysis of detectors at different geomagnetic cutoffs provides information on the energy spectrum. Coordinated arrays now in operation include the 12-station Spaceship Earth network, which is optimized for measuring the angular distribution of relativistic solar energetic particles (Bieber et al. 2004), and the Neutron Monitor Database (NMDB) recently organized under auspices of the European Union (Mavromichalaki 2010).

Great strides have been made in providing access to data, both archival and near-real-time, for researchers throughout the field of Space Science. Neutron monitor data, in particular, has for many decades enjoyed a unique history of world-wide collaborative efforts and the unrestricted sharing of datasets among researchers. This is in large part due to the nature of the measurements made by neutron monitors; an understanding of the time-varying, anisotropic galactic or solar cosmic ray spectrum in most cases requires that data from a large array of stations needs to be considered, and often that array must be global in scope.

The main goals of the Neutron Monitor database (http://NMDB.eu) are to provide high-resolution data from all Neutron Monitor stations in a standard format,to provide real-time data, and to make it easily accessible for everyone. The standard data format has been implemented by storing the measurements in a SQL database. The stations are sending the data immediately after the measurement to NMDB, so that the data is available in real-time (ie less than 5 minute delay after the measurement where possible). The data is made available to everyone to an easy to use webinterface at http://nest.nmdb.eu where data can be plotted and downloaded in ASCII format. For real-time applications a direct read-access to the database is available. The database already contains data from over 50 stations, not only real-time measurements but also historical data. To allow all stations to provide real-time data affordable registration systems have been designed during the NMDB project, the designs are freely available to all interested users.

Since the International Geophysical Year 1957, when a world-wide network of standard IGY neutron monitors was put in place, data from these stations and their successors has been, for the most part, freely shared among experimenters. Since each neutron monitor views the cosmic ray flux arriving from a relatively small range of directions, it was early recognized that, especially in the analysis of solar flare particle events, a network of stations, distributed in magnetic latitude and longitude, were necessary to characterize the flux of charged particles in nearby interplanetary space. Between 1956 and 1958 the number of international operating monitors grew from 8 to 52. A good number of these stations were active only during the IGY, and were then closed in the late 1950s. However, in 1964 the NM64 monitor was developed which provided a greatly-increased counting rate, and many of the operating stations were upgraded to this new design. The total number of operating stations (IGY and NM-64) reached a peak of 65in 1964, then declined gradually to its current level of about 45 by the late 1970s. As for the US operated stations, the time profile and map is displayed in Figure XX

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Chapter 2Cosmic-ray and Heliospheric ScienceNeutron monitors, as the name suggests, monitors or counts neutrons in its environment over a wide range of energy (Note at sea-level above 20MeV neutrons contribute to about 80-85% of the counts). However, these neutrons are direct descendants of primary cosmic rays at the top of the atmosphere. Counting these neutrons constitutes a direct measure of the cosmic ray intensity at the top of atmosphere overhead. Faithful, accurate and precise calculations can relate the local intensity of neutrons to the primary cosmic rays in this wide swath of energy. The design of the instrument, the depth of the instrument in Earth’s atmosphere and the Earth’s geomagnetic field conspire to be sensitive to cosmic rays from 500 MeV to almost 20 GeV. As discussed below, this turns out to be an important and interesting energy range that reflects solar activity and its influence on the interplanetary magnetic field.The energy distribution or spectrum of cosmic rays cannot be teased from the count rate of a single instrument. One must use several monitors for this purpose. Many modern applications use the network itself, and not any single monitor. The latitude and longitude distributions of stations allows studies of the cosmic-ray pitch angle distribution and momentum spectrum. The global array constitutes an enormous magnetic spectrometer, where the geomagnetic field sorts cosmic rays based on their momentum or energy with the rates from several monitors being used to piecewise assemble a spectrum responsible for the signal in all the instruments. The mathematical description of this process is described as follows:

In reality, the process and physics is more complicated. One major complicating factor is that the cosmic rays in the Earth environment may not be isotropic, i.e., they come preferentially from a general direction in space, typically linked to the local direction of the interplanetary field. Consequently, cosmic-ray trajectories from this non-uniform distribution are also non-uniform as they map through the geomagnetic field to the ground and the various instruments. This anisotropy can be confused with the non-uniformity intrinsic to the energy response of the neutron monitor network. Disentangling these effects requires care and is especially important for energetic particles coming from the Sun, as we describe below.The original intent of neutron monitors was to measure these Sun-associated effects—some poorly recognized from pre-neutron monitor measurements. The zoo of solar effects can be stated concisely. They are (1) direct solar particles (SEPs) superimposed on the intensity of primary cosmic rays from the Galaxy, (2) solar-cycle modulation of the local intensity of galactic primary cosmic rays and (3) transients in the GCR intensity tied to singular events on the Sun. Below we discuss the progress made over the years in these areas enabled by neutron monitor measurements.Neutron Monitor as a Primary Cosmic Ray DetectorIn order to relate the ground-based neutron monitor (NM) as a primary cosmic ray detector a quantifiable relationship between the count rate and primary flux must be established. Primary particles, not rejected by the geomagnetic field, enter the atmosphere and undergo multiple interactions resulting in showers of secondary particles which may reach ground level and be detected by a NM. Therefore a yield function must incorporate the propagation of particles through the Earth’s atmosphere and the detection response of a NM to secondary particles such

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as neutrons, protons and muons. The response functions can then be determined by convolving the cosmic ray spectra with the yield function. The expected count rate from latitude surveys can be calculated by integrating the resulting response functions.

A neutron monitor latitude survey is conducted with a mobile detector that records counting rates during passage across a range of geomagnetic rigidity cutoffs [Moraal et al., 1989, and references therein]. The scientific motivation behind such a survey is three fold: to improve knowledge of geomagnetic cutoffs; to study the primary cosmic ray spectrum; and to understand the neutron monitor energy response function. The below equation provides a mathematical description of the relationship between parameters relevant to a latitude survey in the usual approximation:

N(Rc) is the neutron monitor counting rate, Rc is the geomagnetic rigidity cutoff, Y(R) is the neutron monitor yield function, and j(R)is the primary differential cosmic ray spectrum. The different response function is define as the product of yield function and primary spectrum

The

integral response function (count rates) is directly measured during a NM latitude survey.

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Figure XX Neutron Monitor Count rate recorded on the Italian Antarctic Program 3-NM-64 survey during 1996–97 (Villoresi, 1997).

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The detection response for NM64 of incident secondary particles at ground level was determined by Clem (2000) and the results of this work are displayed in Figure 5 left. The data in this figure was determined for 6 particle species in the vertical incident direction. As shown the detector response is optimized to respond to the hadronic component. The response to muons above 1 GeV is roughly 3.5 orders of magnitude below the hadrons. In this energy region, the primary mechanisms for muon induced counts are neutron production in photo-nuclear interactions and electromagnetic showers resulting in multiple ionization tracks in a counter. Below 1 GeV, stopping negative charge muons are captured by a lead nucleus into a mesic orbit and absorbed by the nucleus. The de-excitation of the nucleus occurs with the emission of neutrons which is reflected in the rise in detection efficiency with decreasing energy. Through the Giant-dipole resonance interaction for lead, incident gamma rays and electron-induced bremsstrahlung may result in a number of de-excitation events. Muon-decay electrons and positrons also contribute to neutrons through this channel

Utilizing the detection response to ground level particles and particle transport through the atmosphere, the NM64 differential yield function Y(R) to primary cosmic rays as function of primary rigidity can be determined (Clem, 2000). The results of this calculation is shown in Figure X right for protons and alphas primaries for different arriving angles. It is important to note the yield function is independent of the primary cosmic ray spectrum. The product of the yield function and the desired primary spectrum gives the response function.

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Figure XXI. Left Calculated Neutron Monitor NM64 detection response to different ground level incident particles. Symbols represent measurements of NM64 response to a neutron beam measured at an accelerator facility (Shibata, et al., 1997). Right The yield function Y(R) of a NM64 located at Sea-level depth from primary protons (solid lines) and alphas (dashed lines) arriving at 00, 450 and 600 incidence. This result was derived from the detection response to ground level particles (Left) and particle transport through the atmosphere using FLUKA particle physics package.

Detection Efficiency of a NM64

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Figure XXII right shows the response function for NM64 located at sea-level atmospheric depths for two different primary spectra representing different solar cycle epochs. This was determined by multiplying the Yield function shown in Figure XXI with either primary spectra during solar

maximum and solar minimum. ……….. more later. …………..The Yield Function provides the direct link between the variations observed by a ground based Neutron Monitor count rates and the associated activity above the atmosphere.

NEUTRON MONITOR NETWORKS

The Heliosphere is a vast spheroidal cavity in the interstellar plasma, extending out to approximately 140 AU from the Sun, created by the supersonic outward flow of the solar atmosphere (the solar wind). Galactic cosmic rays (GCR) and solar energetic particles (SEP) propagate within this cavity. Because of the low ambient plasma density, the GCRs and SEPs do not collide with each other or the plasma particles. However, they are greatly affected by the ambient plasma electric and magnetic fields.

In addition to their importance in understanding the physics of both the interplanetary and interstellar media, the GCRs and SEPs are an important component of space weather. They

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Figure XXII Right NM64 Sea-level Response Function to primary cosmic rays for different solar modulation spectra determined using the Yield function in the Figure (above) multiplied by primary cosmic ray spectra expected for Solar Max and Solar Min. For comparison the Compton counter, the Forbush ionization monitor, response function is also displayed. It measures the rate of charged particles that pass through the active volume (primarily muons and electrons). As shown the NM is sensitive to lower energy primary particles than that of ionization chamber. Right Neutron Monitor Count rate as shown above in Figure XX with the integrate response function shown on the left figure for solar minimum,

Sea-Level Response of a NM64

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constitute a major threat to astronauts in space when they are outside of the protective geomagnetic field. This danger can be only mitigated slightly by current technology.

SEPs are emitted sporadically by events on the Sun in discrete events, which last only hours to days, and which occur much more frequently during maximum solar activity than during minimum activity. Although their intensity at energies below some tens of MeV is quite high, the average intensity above approximately 100 MeV is dominated by GCRs. The lower energy of SEP makes it possible to shield astronauts effectively against them. For this reason, I will concentrate on GCR for the rest of this document.

The heliosphere and the outflowing solar wind act to decrease (modulate) the intensity of GCRs, preventing the full interstellar intensity from striking Earth. This modulation is most effective during periods of heightened solar activity. Figure 1 illustrates the intensity as reported for the neutron monitor at McMurdo, over the past 5 sunspot cycles. The GCR intensity maxima and minima occurring during sunspot minima and maxima, respectively, are clearly visible. The alternating shapes of the GCR maxima, with a sharply peaked maximum at one solar minimum followed by a more rounded maximum at the following minimum can be understood to be a consequence of the fact that the direction of the interplanetary magnetic field changes at each sunspot minimum. The sharply peaked maxima occur when the northern interplanetary magnetic field is pointed inward toward the Sun.

Because of the eleven-year sunspot cycle, the reliability, stability and robustness of very long term measurements is critical for understanding changes on the Sun that go beyond particular outbursts or series of them. Of particular interest is the fact that the GCR intensity during the last (2010) solar minimum is the greatest over the period covered by the observations—by a significant factor. This would have been unknown without the long duration measurements of neutron monitors. Other measurements, both at other neutron monitors and from spacecraft exhibit the same effect. The high intensity is probably a consequence of the fact that the last solar minimum was anomalously deep and long-lasting, with an unusually weak interplanetary magnetic field and low solar-wind velocity. It important to determine whether this striking behavior is a harbinger of more change in the future or whether it is an anomaly. Analogies quickly appeared in the literature comparing this recent GCR maximum to that inferred from the Maunder Minimum historical record, suggesting that things may be learned about the Maunder Minimum from this unusual (over the last half century) episode. Fortunately, we established a baseline of solar or space climate with which we can compare this activity in the future, i.e., the next several solar cycles—long after the current community of solar and heliospheric physicists has retired.

The observed phenomena during the last solar minimum, particularly in the intensity of GCRs, demonstrate the importance of neutron monitor data in understanding the heliosphere and space weather, or space climate when speaking about secular changes or trends occurring over several solar cycles.

As described above, it is important to note that the basic global structure of the heliosphere, including its 11-year solar activity cycle and 22-year magnetic cycle, was established by neutron monitor measurements of the galactic cosmic ray intensity. Only these high-energy particles are able to sample the entire heliosphere as they propagate from interstellar space to their collision with Earth’s atmosphere.

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Neutron monitors (and earlier, electroscopes) also detected cosmic ray temporal variations on much shorter timescales. “Forbush Decreases” (Forbush, 1946?) in the cosmic ray intensity of a few to several percent were frequently observed to occur with a timescale of hours following large solar flares. The decrease recovers with a timescale of days, or longer in the case of extended recurring solar activity. Recognized to originate as the result of flare-associated transient increases in solar wind speed and magnetic turbulence “sweeping out” the cosmic rays, these variations provided a means of probing the behavior of transient disturbances in the solar wind and their eventual decay. These observations first highlighted the dynamic state of the heliosphere and the nature of the cosmic-ray response. In the case of cumulative decreases occurring with the onset of a new solar activity cycle, it appeared that the decreases do not recover. Rather, the disturbed solar wind may be viewed as characteristic of the solar wind during solar maximum activity, which reduces the cosmic-ray intensity in the heliosphere more effectively. These ideas led to the recognition that periods of multiple flares and coronal mass ejections lead to effective barriers to cosmic-ray penetration; these barriers are now known as “global merged interaction regions.” It should be noted that Forbush Decreases may exhibit a small precursor increase as the cosmic rays are swept ahead of the (shock) disturbance. Such a precursor is indeed an expected signature of diffusive shock acceleration, which is initiated as particles reflect from the approaching shock surface or from the turbulent flow downstream of the shock. Neutron monitors still provide an effective means of studying the time-dependent modulation of the bulk of galactic cosmic rays by the variable structure of the solar wind, in addition to providing a nearly 65-year record of the variable heliosphere. The recent predictions and direct measurement of the extent of the heliosphere by the Voyager spacecraft, IBEX and accompanying theoretical work provide new challenges for the theory of cosmic-ray modulation and opportunities for neutron monitor measurements to play a crucial role in advancing our understanding of the heliosphere.

Often, in association with a Forbush Decrease, neutron monitors observe an impulsive cosmic ray increase with a timescale of an hour, usually commencing about a day before the Forbush Decrease. Unlike the previous variations, these impulsive increases depend sensitively on the location of the neutron monitor. The sensitivity stems from the anisotropy of the energetic particles in the event. In contrast with the galactic cosmic rays, which have a nearly isotropic distribution, these particles accelerated at solar flares or coronal shocks are initially highly anisotropic; their detection depends on the “asymptotic direction” of the neutron monitor at the time of detection. However, combining the measurements of many neutron monitors facilitates the reconstruction of the particle distribution function as a function of time to investigate the propagation of the solar energetic particles (SEPs) to Earth and determine the time and energy dependence of the released particles. These “Ground Level Events” (GLEs) occur only a few times during each solar activity cycle. They constitute a particularly interesting class of SEP events because their energies (beyond about a GeV) are not accessible to spacecraft measurement and represent the highest energies attainable by the solar acceleration process. Tylka and Dietrich (2009) combined many neutron monitor measurements of the GLE event of 15 April 2001 to obtain the form of the energy spectral rollover beyond a rigidity of ~ 0.1 GV. These measurements by neutron monitors are crucial

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in establishing the origin of SEPs since the highest energies subject the proposed acceleration mechanisms (shock acceleration at a coronal shock and as a byproduct of magnetic reconnection) to the most severe requirements.

The mechanism by which energetic particles scatter and diffuse in collisionless plasma remains a key problem in astrophysics. Detailed information on transport conditions in the interplanetary medium, such as the scattering mean free path, can be obtained from modeling the time-intensity and time-anisotropy profiles of solar energetic particles (Palmer 1982; Bieber et al. 1994). Anisotropy information from neutron monitor networks is crucial for this modeling, because it permits diffusive delays in the solar wind to be distinguished from extended acceleration or release at the solar source. In turn, the analysis provides information on the particle injection profile at the Sun for comparison to solar radio and optical signatures. Neutron monitors of the four-nation Spaceship Earth network (Bieber et al. 2004) were deployed to provide optimal coverage of solar particle anisotropies, using existing monitors where possible and constructing new ones as needed. The energy spectrum of cosmic ray variations can be deduced from a network of neutron monitors deployed over a range of geomagnetic cutoffs. For instance, Tylka and Dietrich (2009) used cutoff arrays to extend the spectrum of solar energetic particles from spacecraft energies to the neutron monitor energy range, while Oh et al. (2013) derived information on the spectrum of Galactic cosmic rays during the recent very weak solar minimum and accompanying record maximum in Galactic cosmic rays.

Because cosmic rays of neutron monitor energies are “tuned” to the various spatial dimensions of the heliosphere, they can be used to understand large disturbances in the solar wind. Pioneering observations of the anisotropy of Galactic cosmic rays (Pomerantz and Duggal 1971 and references therein) typically employed a single ground-based detector and relied on Earth’s rotation to provide a range of viewing directions. Often results were expressed as annual means, because it was necessary to average over long periods in order to extract the minuscule anisotropy signal from the many other variations present.

Now, with a network observing multiple directions simultaneously it is possible to extract the anisotropy with much better time resolution, often to one hour or better. Thus it becomes possible to study transient anisotropies in the pre-existing Galactic cosmic ray population produced by solar wind disturbances such as ICMEs. Reported effects include: bidirectional streaming, indicative of a closed magnetic field topology (Dvornikov et al. 1983; Richardson et al. 2000); gradient anisotropies, which permit determination of the cosmic-ray spatial gradient and thereby provide clues to ICME geometry (Bieber and Evenson 1998); and precursor “loss-cone” anisotropies that potentially can provide up to ~twelve hours advance warning of major geomagnetic storms (Leerungnavarat et al. 2003 and references therein).

In the last twenty years, it became clear that neutron monitors are sensitive to purely solar flare particles, in particular the secondary neutrons produced by high-energy ions with the solar atmosphere. The Sun on occasion emits relativistic neutrons with sufficient intensity to be detectable by neutron monitors (Chupp et al. 1987; Shea et al. 1991; Bieber et al. 2005). Such events provide exceptionally clear insights into acceleration/injection processes at the Sun, because the neutrons travel straight from the source to the observer unimpeded by magnetic fields. (Non-relativistic neutrons are often not observed at 1 AU because their decay time, ~15

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min, is shorter than the transit time.) An optimal network for observing these events would employ low-latitude, high-altitude monitors sited to minimize atmospheric attenuation and thus increase both the probability of detection and the size of the neutron signal.Considerable resources have been invested in space-based measurements of energetic particles. Implicit in the success of these missions is the standard baseline measurements performed by neutron monitors. Only using the entire Earth as a magnetic spectrometer, we can achieve quality data above a few GeV. It is difficult to get reasonable signals from instruments at these energies when limited by the limited size of these instruments. NASA missions have from the first have deduced the maximum science when coupled with the neutron monitor measurements. The joint data sets provide a wide vision of the spectrum and energy dependent physical processes when spanning energy ranges from several MeV to several GeV.

REFERENCES

Bieber, J. W., and P. Evenson, CME geometry in relation to cosmic ray anisotropy, Geophys. Res. Lett., 25, 2955-2958, 1998.

Bieber, J. W., W. H. Matthaeus, C. W. Smith, W. Wanner, M.-B. Kallenrode, and G. Wibberenz, Proton and electron mean free paths: The Palmer consensus revisited, Astrophys. J., 420, 294-306, 1994.

Bieber, J. W., P. Evenson, W. Dröge, R. Pyle, D. Ruffolo, M. Rujiwarodom, P. Tooprakai, and T. Khumlumlert, Spaceship Earth observations of the Easter 2001 solar particle event, Astrophys. J. (Lett.), 601, L103-L106, 2004.

Bieber, J. W., J. Clem, P. Evenson, R. Pyle, D. Ruffolo, and A. Sáiz, Relativistic solar neutrons and protons on 28 October 2003, Geophys. Res. Lett., 32, L03S02, doi:10.1029/2004GL021492, 2005.

Bieber, J. W., P. A. Evenson, T. Kuwabara, and C. Pei, IMF prediction with cosmic rays, Am. Geophys. Union Fall Meeting, abstract SH53A-2146, 2013.

Chupp, E. L., H. Debrunner, E. Flückiger, D. J. Forrest, F. Golliez, G. Kanbach, W. T. Vestrand, J. Cooper, and G. Share, Solar neutron emissivity during the large flare on 1982 June 3, Astrophys. J., 318, 913-925, 1987.

Dvornikov, V. M., V. E. Stobnov, and A. V. Sergeev, Analysis of cosmic ray pitch-angle anisotropy during the Forbush-effect in June 1972 by the method of spectrographic global survey, Proc. 18th Internat. Cosmic Ray Conf. (Bangalore), 3, 249-252, 1983.

Kuwabara, T., J. W. Bieber, J. Clem, P. Evenson, R. Pyle, K. Munakata, S. Yasue, C. Kato, S. Akahane, M. Koyama, Z. Fujii, M. L. Duldig, J. E. Humble, M. R. Silva, N. B. Trivedi, W. D. Gonzalez, and N. J. Schuch, Real-Time cosmic ray monitoring system for space weather, Space Weather, 4, S08001, 2006a.

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Kuwabara, T., J. W. Bieber, J. Clem, P. Evenson, and R. Pyle, Development of a GLE alarm system based upon neutron monitors, Space Weather, 4, S10001, 2006b.

Leerungnavarat, K., D. Ruffolo, and J. W. Bieber, Loss cone precursors to Forbush decreases and advance warning of space weather effects, Astrophys. J., 593, 587-596, 2003.

H. Mavromichalaki and 33 co-authors, Applications and usage of the real-time neutron monitor database, Adv. Space Res., 47, 2210-2222, 2011.

Oh, S. Y., J. W. Bieber, J. Clem, P. Evenson, R. Pyle, Y. Yi, and Y.-K. Kim, South Pole neutron monitor forecasting of solar proton radiation intensity, Space Weather, 10, S05004, 2012.

Oh, S., J. W. Bieber, P. Evenson, J. Clem, Y. Yi, and Y. Kim, Record neutron monitor counting rates from Galactic cosmic rays, J. Geophys. Res., 118, 5431-5436, 2013.

Palmer, I. D., Transport coefficients of low-energy cosmic rays in interplanetary space, Rev. Geophys. Space Phys., 20, 335-351, 1982.

Pomerantz, M. A., and S. P. Duggal, The cosmic ray solar diurnal anisotropy, Space Sci. Rev., 12, 75-130, 1971.

Richardson, I. G., V. M. Dvornikov, V. E. Sdobnov, and H. V. Cane, Bidirectional particle flows at cosmic ray and lower (~1 MeV) energies and their association with interplanetary coronal mass ejections/ejecta, J. Geophys. Res., 105, 12579-12592, 2000.

Shea, M. A., D. F. Smart, and K. R. Pyle, Direct solar neutrons detected by neutron monitors on 24 May 1990, Geophys. Res. Lett., 18, 1655-1658, 1991.

Souvatzoglou, G., H. Mavromichalaki, C. Sarlanis, G. Mariatos, A. Belov, E. Eroshenko, and V. Yanke, Real-time GLE alert in the ANMODAP center for December 13, 2006, Adv. Space Res., 43, 728-734, 2009.

Tylka, A. J., and W. F. Dietrich, A new and comprehensive analysis of proton spectra in Ground-Level Enhanced (GLE) solar particle events, Proc. 31st Internat. Cosmic Ray Conf. (Lodz), Paper 0273, 4 pp., 2009.

2) Ilya Usoskin

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Importance of Neutron Monitors for Space-based instrumentation

The Sun is the only player in controlling our heliosphere and in modulating galactic cosmic rays (GCRs). As the tilt angle of the heliospheric current sheet evolves, the velocity and density of

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the solar wind change, and the strength and turbulence characterizing the interplanetary magnetic field (IMF) reorganize. Neutron monitors cover nearly six cycles of activity and the consequent impact on galactic cosmic ray radiation at 1 AU. Various spacecraft have been operating throughout the space age to record the Sun’s activity and its effect on the local radiation environment. These instruments often have improved resolution, albeit at lower energies, but neutron monitors provide the only source of continuous long-term monitoring while offering the possibility to inter-relate spacecraft and high-altitude balloon instruments that operate over much shorter periods of time (see Figure 1).

Due to the long-term coverage by neutron monitors, it has been possible to establish the 22-year galactic cosmic ray modulation, dominated by the 11-year solar activity, but influenced by gradient and curvature drifts in the IMF. The world wide network, which has continually grown since the first neutron monitors in 1950s, has ensured continuous coverage of the Sun’s 11 and 22 year cycles, enabling scientists to identify periods during which solar activity is unusual, e.g., the recent solar minimum of cycle 24. This unusual solar minimum has resulted in the highest intensities of galactic cosmic rays in the space age (see Figure 2) presenting increased radiation hazards to spacecraft instrumentation and astronauts (Mewaldt et al. 2010).

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Correlation of proton fluences from Tylka et al (2009) NM analyses versus corresponding values from GOES/HEPAD

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Of particular importance to aircraft technology and air crew, given the frequency of flights over polar routes, are the transient but intense increases in solar radiation resulting from high-energy solar energetic particle events. The highest energy solar energetic particle events, though relatively rare, occur throughout the solar cycle. The world wide network of neutron monitors offers the only real-time warning for the arrival of such events. While not enough is known of these elusive high-energy events, neutron monitors offer an important measure of their arrival times, spectral shapes, and anisotropies that help to constrain the acceleration processes and transport at play at the Sun (maybe you just need a Figure of a solar flare here).

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Event-integrated integral proton spectra vs. rigidity for two GLEs. Noted are the parameters of the power law fits to the neutron monitors and of the Band-function fits to measurements above 0.137 GV (10 MeV).

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Bruno

THE USE OF THE EARTH AS A MAGNETIC SPECTROMETERDr. Alessandro Bruno - INFN and University of Bari, Italy

The PAMELA space experiment is providing first direct observations of SEPs with energies from about 80 MeV to several GeV in near-Earth orbit, bridging the low energy measurements by in-situ spacecrafts and the GLE data by the worldwide network of NMs. Its unique observational capabilities include not only the possibility of measuring the flux energetic spectrum and composition, but also its angular distribution, thus investigating possible anisotropies associated to SEP events [1]. Cosmic Ray cutoff rigidities and asymptotic arrival directions are commonly evaluated by simulations accounting for the effect of the geomagnetic field on the particle transport. Using spacecraft ephemeris data (position, orientation, time), and the particle rigidity and direction provided by the PAMELA tracking

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Fig.1: asymptotic directions determined during the first polar pass that registered the 2012 May 17 event. Also shown are the asymptotic directions of the NM that registered the primary GLE beam [3].

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system, trajectories of all detected protons are reconstructed by means of a tracing program based on numerical integration methods, and implementing the IGRF-11 and the TS07D [2] models for the description of internal and external geomagnetic sources, respectively. Solar wind and IMF parameters are obtained from the high-resolution Omniweb database. Each trajectory is back propagated from the measurement location with no constraint limiting the total path-length or tracing time, and the corresponding asymptotic arrival direction is evaluated with respect to the IMF direction. Since the PAMELA aperture is 20 deg, the observable pitch-angle range is quite small (a few deg) except in regions close to the geomagnetic cutoff (discarded from the analysis). However, because it is a moving platform, it sweeps through pitch angle space allowing one to construct a pitch angle distribution of the SEPs. Consequently, a quite large pitch-angle range is covered during the whole polar pass. Fig.1 reports PAMELA's vertical asymptotic directions of view (0.39-2.5 GV) during the first polar pass (0158 - 0220 UT) that registered the May 17, 2012 event [3], for different values of particle rigidity (color code). The spacecraft position is indicated by the grey curve. The contour curves represent values of constant pitch angle with respect to the IMF direction, denoted with crosses. In this case the IMF direction is almost perpendicular to the sunward

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direction. As PAMELA is moving (eastward) and changing its orientation along the orbit, observed asymptotic directions rapidly vary performing a (clockwise) loop over the region above Brazil. PAMELA data can be combined with data from NMs and other space-based detectors, in order to model the directional distribution of solar events, estimating the omnidirectional density and weighted anisotropy.

[1] A. Bruno et al. (2015), Proc. 34th Intl. Cosmic Ray Conf., PoS(ICRC2015)085.[2] N. A. Tsyganenko & M. I. Sitnov ( 2007), J. Geophys. Res., 112, A06225.[3] O. Adrian et al. (2015), ApJ 801 L3.

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SEP MEASUREMENTS IN LEO BY THE PAMELA EXPERIMENTDr. Alessandro Bruno - INFN and University of Bari, Italy

The PAMELA space experiment is providing first direct observations of Solar Energetic Particles (SEPs) with energies from about 80 MeV to several GeV in near-Earth orbit, bridging the low energy measurements by other spacecrafts and the GLE data by the worldwide network of neutron monitors. Its unique observational capabilities include the possibility of measuring the flux angular distribution and thus investigating possible anisotropies associated to SEP events. The analysis is supported by an accurate back-tracing simulation based on a realistic description of the Earth's magnetosphere, which is exploited to estimate the SEP energy spectra as a function of the asymptotic direction of arrival with respect to the IMF. Fig.1 reports the results for the May 17, 2012 event [1]. Proton fluxes are

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Fig1: Time variations in the intensity (1.57 - 5.70 GV) of protons, He nuclei, electrons and positrons, during the Forbush Decrease event associated with the 13 Dec 2006 CME [5].

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averaged over the first PAMELA’s polar pass (0158-0220 UT) which registered the event. Two populations with very different pitch angle distributions can be noted: a low-energy component (<1 GV) confined to pitch angles <90 deg and exhibiting significant scattering or redistribution; and a high-energy component (1-2 GV) that is beamed with pitch angles <30 deg and relatively unaffected by dispersive transport effects, consistent with neutron monitor observations. The presence of these simultaneous populations can be explained by postulating a local scattering/redistribution in the Earth's magnetosheath. The quasi-perpendicular orientation of the IMF may be a key factor in the anisotropy effect observed in the particle intensities because entry into the magnetosphere on the flank significantly increases the diffusive volume compared to the nominal 45 deg of the Archimedes spiral. This is the first time that we observe distinct effects of the magnetosphere in the transport of SEPs. This type of analysis is only possible with the unique capability offered by the PAMELA instrument.

[1] O. Adrian et al. (2015), ApJ 801 L3.

Boezio

LOW EARTH ORBITING COSMIC RAY MISSIONSDr. Mirko Boezio - INFN and University of Trieste, Italy

The PAMELA [1] and the AMS-02 [2] space experiments represent the state-of-the-art of the investigation of the charged Cosmic-Ray (CR) radiation in the near-Earth environment. The former was launched into a semi-polar (70 deg inclination) and elliptical (350–610 km altitude) orbit on June 2006 onboard of the Resurs-DK1 Russian satellite; the latter was installed in May 2011 on the ISS. Both the instruments are composed by several subdetectors, with the core constituted by a magnetic spectrometer, providing accurate particle identification and rigidity measurement. While the mission temporal coverage and geometric factor are limited in comparison to ground-based detectors, PAMELA and AMS-02 are able to directly measure the spectral shape and the composition of CR fluxes. The high-precision data collected at low energies are significantly improving our understanding of the solar modulation effects on CRs, allowing the investigation of the long- and short-term CR variations between solar cycles 23 and 24 [3]. In particular, PAMELA measured the temporal evolution of different CR species (p, He, e-, e+), founding evidence of particle charge-sign

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dependent modulation effects. In addition, PAMELA is providing comprehensive observations of SEP events during the solar cycles 23 and 24, including energetic spectra and pitch angle distributions in a wide interval (>80 MeV), bridging the low energy data by in-situ spacecrafts and the GLE data by the worldwide network of neutron monitors. Major PAMELA’s results include the first direct evidence of magnetosheath effects on SEPs [4]. Similar results are being achieved by AM-02 experiment at relatively higher energies, due to the higher geomagnetic cutoff related to the ISS orbit. Space- and ground-based measurements can be combined with data from NMs in order to model the directional distribution of solar events, estimating the omnidirectional density and weighted anisotropy. Finally, PAMELA and AMS-02 are performing detailed observations of geomagnetic storms and Forbush Decrease (FD) effects induced by CME events. Complementing the integrated fluxes measured by NMs, PAMELA and AMS-02 provide information on the dependency of FD effects on particle composition and energy. As an example, Fig.1 reports the variations in the intensity of the different CR species (1.57 - 5.70 GV), during the FD event associated with the 13 December 2006 CME [5].

[1] O. Adriani et al. (2014), Physics Reports, 544, 4, 323–370.

[2] M. Aguilar et al. (2013), Phys. Rev. Lett., 110, 141102.

[3] M. Boezio et al. (2015), Proc. 34rd Intl. Cosmic Ray Conf., PoS(ICRC2015)037.

[4] O. Adrian et al. (2015), ApJ 801 L3.

[5] M. Mergé et al. (2013), Proc. 33rd Intl. Cosmic Ray Conf., 1215.

Spectral Information from Neutron Monitor Multiplicity Measurement

A new generation of electronics recently developed by Bartol Research Institute, also records the time between pulses. The average number of evaporated neutrons produced in an inelastic interaction is energy dependent and can be roughly described as a power law, consequently spectral information can be in principle extracted this timing data. An example of this new system as a resource utility was demonstrated during a GLE. Over a 6-minute time period on January 20, 2005, the neutron monitor rate at the sea level station of McMurdo, Antarctica increased by a factor of 30, while the rate at the high-altitude (2820 m) station of South Pole increased by a factor of 56. For a number of years Bartol Research Institute and University of Tasmania have conducted an annual latitude survey with a portable monitor aboard a U.S. Coast Guard icebreaker. At the time of the January 20, 2005 GLE, this instrument was in McMurdo Sound and therefore recorded essentially the same primary flux as the stationary monitor at McMurdo. Unlike the stationary monitor, however, the survey instrument was equipped with the new electronics to record the distribution of elapsed times δt between successive counts in a single detector tube.

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Figure 6 displays δt distributions for an hour preceding the GLE (i.e., for a pure Galactic spectrum) and for an hour that includes the peak of the GLE. Only the first 12 δt values in each second are accumulated in the distribution. Hence the majority of δt values are discarded during the high count rate interval of the GLE peak, but the values that are recorded should represent an unbiased sample of neutron multiplicities.

During quiet periods the Galactic δt distribution has two populations. For time intervals above ~2 ms, the distribution is a flat, smooth exponential representing single uncorrelated counts with the slope corresponding to the count rate. For time intervals less than ~2 ms, there is an additional sharp spike representing multiplicity events, in which multiple evaporation neutrons are counted from a single incident particle. As shown in Figure 5, the Galactic distribution (black curve) clearly reveals these features.

The GLE distribution (red curve), however, displays a number of differences. First, the number of uncorrelated counts has increased relative to the multiplicity spike, because the solar particles have a lower energy on average than Galactic cosmic rays. Second, the GLE distribution displays a different slope in the uncorrelated region, owing to the higher count rate. Third, the GLE distribution deviates from a pure uncorrelated region, owing to the higher count rate. Third, the GLE distribution deviates from a pure exponential below 50 ms reflecting rapidly changing count rates. Work is ongoing to model these differences, thereby gaining new information on the energy spectrum of GLEs.

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Figure 6. Left On January 20, 2005 the Sun emitted cosmic rays of sufficient energy and intensity to increase radiation levels on Earth’s surface. The GLE was especially intense at South Pole (highest peak) and McMurdo, Antarctica (secondhighest), where radiation levels increased by factors of 56 and 30, respectively, in a period of 6 minutes. The McMurdoincrease was the largest observed at sea level since 1956. Right Histogram of time intervals between successive neutron counts, as measured by new electronics interface instrumentation operated during the 2004-2005 Bartol/Tasmania latitude survey. The black (flatter) curve represents a Galactic spectrum, while the red curve includes the peak of the GLE. By modeling these time histograms, we hope to gain new information on the spectrum of the GLE.

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Chapter 3Space Weather

In contrast to the first major deployment of neutron monitors, space weather concerns now are a major consideration in the number and placement of stations. During the IGY, the objective of the neutron monitor network was research and the advancement of our knowledge of the Earth’s environment. Space Weather, however, is a practical concern, that is, understanding, predicting and mitigating effects of transient space events on society has tangible, financial and security factors. One of the phenomena that drives this interest is ionizing radiation, coming from galactic cosmic rays and solar energetic particles. Intense high-energy events, as manifested in ground level enhancements, affect communications at high latitudes and pose radiation hazards for personnel and avionics at aircraft altitudes and orbiting platforms. A network of ground-based network of neutron monitors offers a stable, isotropic and uniform system for the detection and registration of energetic particle events, immune to the operational hiccups that can plague a space-based network in a time of need. Additionally, we would be able to continue building the long term cosmic-ray space climate data base that now extends from the mid 1950s to the present.

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Polar route usage has become a cornerstone of international aviation over the last decade plus. The High Frequency (HF) impacts from Solar Flares and Solar Radiation Storms are fairly repeatable and reasonably well understood. To work around the communication impacts Iridium use is being introduced. The high latitude radiation exposure issue hasn’t been well-addressed to date by all which both an issue of actual risk and perceived risk. Information is needed and that is coming through ICAO

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The importance of this was articulated in the National Space Weather Action Plan of October 2015, where it states:1.2…Changes in the near-Earth radiation environment can affect satellite operations, astronauts in space, commercial space activities, and the radiation environment on aircraft at relevant latitudes or altitudes. Understanding the diverse effects of increased radiation is challenging, but the ionizing radiation benchmarks will help address these effects…5.3.8 DOC, DOD, and NSF, in collaboration with academia, the private sector, and international partners, will develop options to sustain or enhance the worldwide ground-based neutron-monitoring network to include real-time reporting of ground-level events to operational space-weather-forecasting centers.Deliverable: Complete plan to ensure a sufficient number of neutron detectors are deployed, worldwide, to adequately characterize the radiation environment and support a real-time alert and warning system.Historically, the global network of monitors was established by international scientific collaboration toward the common goal of the IGY. What and where a country could support a monitor were the main factors in how many and where stations were placed, with scientific considerations secondary—a reasonable strategy for a never-attempted exercise. However, with the potential global impact of a major space weather event, a more judicious plan for the number and location of stations is warranted. The stations should be numerous enough to cover a full range of cutoff rigidities and asymptotic directions so as to be sensitive to beamed SEPs over a wide spectral range. High latitude stations would have the lowest threshold, of course, but by themselves would not provide the spectral information necessary to assess the event’s potential radiological impact.Medium term action: Construct a plan that has as its main priority a global network of monitors that covers the full range of geomagnetic rigidities over a mesh of asymptotic directions with some minimal angular separation determined through the analysis of archival GLEs.Long term action:Solicit international collaboration to deploy and support these stations with potential assistance by the US.Medium term action: Given that traditional IGY or NM-64 BF3 tubes are no longer manufactured, design around commercially available BF3 tubes, an inexpensive neutron monitor kit with a yield function as close as possible to that of the IGY or NM-64 designs. The magnitude of the yield function may be less than the traditional monitors, but should possess a similar spectral response. Keeping it small and inexpensive would facilitate wide deployment.Implementing this plan could be accomplished as budgets permit, working toward the

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ultimate goal of a systematic global array of monitors with a minimum number of blind spots. The implementation of the plan must include electronic networking of the instruments, so that real-time data are immediately available to concerned parties. How these data are used can be left to the particular stakeholders and affected agencies and offices.

Linking neutron monitors together in a realtime network enables a number of space weather forecasting and “nowcasting” applications (Kuwabara et al. 2006a; Mavromichalaki et al. 2011). Backtesting studies have shown that automated detection of a Ground Level Enhancement (GLE) onset can provide ~10-30 minutes earlier warning of a major radiation storm than the earliest proton alert issued by the Space Weather Prediction Center (Kuwabara et al. 2006b; Souvatzoglou et al. 2009). Three such GLE alert systems are currently in operation by the University of Delaware (http://www.bartol.udel.edu/~takao/ neutronm/glealarm/), IZMIRAN (http://cr0.izmiran.ru/GLE-AlertAndProfilesPrognosing/), and the National & Kapodistrian University of Athens (http://cosray.phys.uoa.gr/index.php/ glealertplus). Existing systems could be substantially improved if realtime communications could be implemented at additional remote stations (e.g., the U.S. station at South Pole and the Russian station at Tixie Bay).

Numerous other space weather applications of neutron monitors have been proposed. For instance, the possibility of near-term forecasting the energy spectrum of solar energetic particle events has been demonstrated in principle (Oh et al. 2012), as has the possibility of nowcasting radiation levels on Earth’s surface during GLE (Mavromichalaki et al. 2011; see their Fig. 6). Additional proposed applications include the use of “loss cone” anisotropies to warn of approaching ICME (Leerungnavarat et al. 2003 and references therein), and prediction of the north-south orientation of the IMF (i.e., “BZ”) using transport theory to link cosmic ray fluctuations with IMF fluctuations (Bieber et al. 2013)

Chapter 4Other Science Applications of NM DataIn addition to space weather forecasting, data from neutron monitors are also used in support of other science investigations, beyond studies of cosmic rays and heliospheric physics. One use of neutron monitor data is to determine the flux of neutrons produced by cosmic rays for a broad range of practical applications, including detecting nuclear threats for homeland and national security, calculating the radiation dose to airplane crews and passengers, understanding the rate of single-event upsets (soft errors) in microelectronic devices, measuring soil and snow moisture content, and calculating the production rate of cosmogenic radionuclides used for atmospheric tracers and nuclear treaty verification. In all these applications, neutrons and other secondary particles produced in the atmosphere and surface materials by galactic cosmic rays (and occasionally by solar particles) are either the source of the effect or an important background. Starting from the cosmic-ray local interstellar spectrum or interplanetary spectrum together with

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a model of the geomagnetic field, the flux and energy spectrum of cosmic-ray-produced neutrons can be calculated using Monte Carlo computer codes to model radiation transport through the atmosphere as a function of altitude/air pressure, geomagnetic cutoff rigidity, and local materials, but the effect of solar modulation on the neutron flux near the Earth’s surface at a given time must come from neutron monitor data. Without ongoing data from stable neutron monitors the cosmogenic neutron flux cannot be determined accurately, and all the practical calculations and measurements that depend on knowing the neutron flux will be severely impaired.

The Neutron Monitor Community Workshop that gave rise to this paper included four presentations on practical applications related to the terrestrial and atmospheric cosmogenic neutron flux determined from neutron monitor data by scientists from the Department of Homeland Security (DHS), the Federal Aviation Administration (FAA), IBM Corporation, and (by proxy) the University of Arizona. Below are articles summarizing those presentations. In addition, the editors received a brief summary from the Byrd Polar Research Center of Ohio State University describing their use of neutron measurements and neutron monitor data to determine snow mass on the surface of ice sheets.

Uses of Neutron Data for Homeland and National Security1

Paul Goldhagen U.S. Department of Homeland Security National Urban Security Technology Laboratory

The detonation of a terrorist nuclear device in the U.S. is one of the worst things that could happen to our country, and the Departments of Homeland Security, Energy, and Defense fund programs to improve our ability to detect any such device, or the fissile nuclear materials that could be used to make one, before it could reach its target. One way to detect nuclear threats is by detecting the penetrating radiation they give off. Uranium can be detected by the gamma rays it emits, and plutonium emits both gamma rays and neutrons. Gamma-ray detection is more widely employed, but it suffers from frequent innocent nuisance alarms from nuclear medicine patients and medical and industrial radioactive sources. There are no medical patients or medical sources that emit neutrons, and fewer industrial neutron sources, so there are far fewer nuisance alarms for neutron detection, and a neutron detection alarm is a serious indicator that plutonium may be present. Neutrons are also harder to shield than gamma rays because the required shielding is bulkier. Neutron detection is also used in active interrogation, where pulsed high-energy x rays from an accelerator are used to induce fission in fissile material that might be hidden in a container. So detecting neutrons emitted by nuclear threats is an important addition to gamma detection for homeland security. Neutron detection is also used for verification of nuclear treaties and could be used to make standoff measurements of the operating power of foreign nuclear reactors where direct access is denied and for other national security applications.

The sensitivity of detection measurements with an acceptable false alarm rate is limited by the background count rate and by its variation. In addition to coping with statistical fluctuations, detection systems should not alarm just because the background is higher in one situation than in another. The only significant natural background for neutron radiation comes from cosmic rays. So accurate determination of the flux (and energy-angle distribution) of the cosmogenic neutrons

1 The views and opinions expressed by the author of this section do not necessarily represent those of the U.S. Government or any U.S. Government agency.

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at a given place and time is a significant capability for homeland and national security. This can only be done with the aid of neutron monitor data.

The background count rate in deployed detection systems is continually measured, but we need to understand the background in advance in order to: Design new, better detection systems with improved signal/background ratio Compare developmental detection systems tested at different times in different places Optimize alarm threshold settings Prepare search teams for what changes in background rate to expect Deal with rapidly varying position-dependent background

Mobile standoff detection in cities – varying shielding by buildings Searching ships.

While there are no free neutrons in the primary galactic cosmic rays (GCR), cosmic rays liberate neutrons when they strike the nuclei of atoms in the atmosphere and cause cosmic-ray air showers. The resulting cosmogenic neutron rate depends on altitude (atmospheric depth), location in the geomagnetic field, solar magnetic activity (solar modulation), and the materials nearby. While determining the effects of these complex dependencies is difficult, with one exception they can be predicted from calculations. The exception is the solar modulation. The only practical way to accurately determine the cosmogenic neutron flux on the surface of the Earth at a given time is to use data from neutron monitors.

The Los Alamos National Laboratory (LANL), in collaboration with the University of Delaware and the National Urban Security Technology Laboratory (NUSTL), has calculated the flux and energy distribution of GCR secondary particles, including neutrons, at the surface of the Earth and in the atmosphere on a global grid of locations as a function of date (McKinney et al. 2012; McGrath et al. 2014; McGrath and McKinney 2014). The work was done with support from the Department of Homeland Security Domestic Nuclear Detection Office. The calculations are similar to those done by several groups to determine the radiation dose to airplane crews (see article below by Kyle Copeland) and neutron monitor yield functions, but employ a unique feature of the MCNP6 Monte Carlo radiation transport code (Pelowitz et al. 2014): the ability to transport charged particles curving through a magnetic field while multiple scattering in air. In this case, it allowed the simulated protons in the air shower to bend in the geomagnetic field and on average spread away from vertical incidence. This feature was crucial for accurate transport through the full depth of the atmosphere to sea level. The cosmic source, calculational methods, and results are available within MCNP6, so scientists and engineers in the homeland security community can use them to make further calculations for all sorts of situations – for example, to calculate the production of cosmogenic radionuclides useful for nuclear treaty verification or to calculate the neutron background spectrum and count rate in a given detector in a building or on soil with a given water content or at each layer of containers on a container ship of a particular size.

The count rate in a neutron detector is given by the integral over neutron incident energy and angle of the response of the detector as a function of energy and angle times the flux and energy-angle distribution of the neutrons. So to determine the background count rate, it is not enough to calculate the cosmogenic neutron total flux; we need the energy and angular distribution, too. The MCNP6 calculations provide these, and there are measurements of the energy distribution to test the accuracy of the calculations.

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NUSTL, in collaboration with a series of other laboratories, has made measurements of the flux and energy spectrum of cosmic-ray-produced neutrons on the ground at various locations and elevations (Gordon et al. 2004) and aboard airplanes (Goldhagen, 2000; Goldhagen et al. 2004) and ships, including container ships. The measurements were made using two extended-range Bonner sphere neutron spectrometers (Goldhagen 2011) and covered a wide range of altitudes/elevations and geomagnetic locations. These measurements are being used to benchmark the LANL calculations.

The cosmogenic neutron spectrum covers an extremely wide range of energies—from less than 0.01 eV to more than 10 GeV. In order to show the details of a neutron spectrum covering such a wide range of energy, E, it is useful to plot the energy on a logarithmic scale and the fluence rate, or flux, d/dt or , per ln(E) rather than as d/dE. In this so-called lethargy representation, the characteristic 1/E dependence of d/dE is flattened out because d/d(ln(E) = E d/dE, and a linear scale can be used on the vertical axis. In the lethargy representation, the flux in each energy region is proportional to the area under the curve, allowing the viewer to see at a glance what energy regions have more or less flux. Figure ?.1 shows three cosmogenic neutron spectra, one measured on land and two on different container ships, plotted both ways. Figure ?.1. Three measured cosmogenic neutron spectra plotted in two representations.

Figure ?.2 shows the cosmogenic neutron spectrum measured inside a 40-foot shipping container in a building in Livermore, CA, (green curve) and the spectrum calculated by LANL using MNP6 together with a detailed model of the building and container (black curve). The measured spectrum is the same one shown as the green curve in Figure ?.1. The agreement between the

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calculation and the measurement is excellent. The spectra show three broad peaks: around 100 MeV from knock-on spallation collisions, around 1 MeV from nuclear evaporation that follows spallation, and around 0.025 eV from thermal neutrons. Between the evaporation peak and the thermal peak there is a plateau where scattering neutrons are slowing slow down and d/dE is proportional to1/E. These four features are typical of terrestrial cosmic-ray neutron energy spectra. Cosmogenic spectra in the atmosphere away from the ground do not have a thermal peak because the nitrogen in air absorbs neutrons that slow to thermal energies. Neutron spectra from fission do not have the high-energy peak. Figure ?. 2. Cosmogenic neutron spectrum measured in a shipping container in a building in Livermore, CA, (green curve) and the spectrum calculated by LANL using MNP6 and a detailed model of the building and container (black curve). The three broad peaks and the slowing-down plateau region typical of terrestrial cosmic-ray neutron energy spectra are labeled.

Figure ?.3 shows cosmogenic neutron spectra measured in a shipping container placed in different locations on container ships and a calculated neutron spectrum from a hypothetical model of shielded weapons-grade plutonium (dashed curve). The red curve was measured with the container housing the neutron spectrometer located on the steel deck of the ship with empty steel containers on top of it. The blue curve was measured with the spectrometer container perched on top of the stack of full containers, shielded from the steel of the ship. (These spectra are the same as the red and blue spectra shown in Figure ?.1.) The neutron flux of the red spectrum from the evaporation peak down to the thermal region is about a factor of two higher than for the blue spectrum. This difference is caused by the high-energy cosmogenic neutrons striking the relatively large iron nuclei in the steel of the ship and containers, generating more evaporation neutrons than they do in the smaller nuclei of lower atomic weight materials. The excess of neutron flux in the energy region where search detectors are sensitive is known as the neutron “ship effect.” Most of the enhanced-background region of the background spectrum overlaps the spectrum from shielded plutonium fission.

Figure ?. 3. Cosmogenic neutron spectra measured in a shipping container placed in different locations on container ships (red and blue curves) and a calculated neutron spectrum from a hypothetical model of shielded weapons-grade plutonium (dashed curve).

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In the currently released version, MCNP6.1.1, solar modulation was done using year-averaged solar modulation potentials between 1960 and 2005 determined from neutron monitor data using the method of McKinney et al. (2006). Within that date range, the solar modulation potential was interpolated, and beyond 2005 it was predicted using a sinusoidal fit over several solar cycles. Future versions of MCNP6 will use solar modulation potentials from Usoskin et al. (2011) determined from neutron monitor and ionization chamber data between 1936 and the most recent past year. Beyond the date of the latest included data, the modulation potential is determined by an optimized sinusoidal fit over the last 8 to 14 years that include the two most recent extrema (Liegey et al. 2016). In principle, current daily or even hourly values of the solar modulation parameter could be used, but that can only be done if there is an operating network of neutron monitors with readily accessible real-time online data.

The possible improvement in accuracy that could be gained by using short-term real-time neutron monitor data to determine solar modulation for cosmogenic neutron flux calculations is shown in Figure ?.4 The figure shows graphs of monthly and daily averages of the count rate in the neutron monitor in Newark, Delaware relative to the long term average from 1967 to 2005.

Even for this sea-level neutron monitor on the mid-Atlantic coast, the daily average count rate is often several percent different from the monthly average and it is not rare for the daily average neutron flux to be as much as 10% different from the monthly average. Figure ?. 4. Relative count rate of the neutron monitor in Newark, Delaware. The green curve shows daily average count rates; the blue curve shows monthly averages.

Neutron detection and cosmogenic background neutrons are important not only for detecting hidden nuclear threats within and at the borders of the U.S. homeland, but also for nuclear treaty verification. Accurate knowledge of the cosmogenic neutron flux at a given place and time could improve treaty verification in several ways, all of which would depend on having reliable real-time or recent neutron monitor data.

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A neutron detector called simply “radiation detection equipment” (RDE) was introduced under the Intermediate-Range Nuclear Forces (INF) Treaty between the U.S. and the Soviet Union and later used under the Strategic Arms Reduction Treaty (START) between the U.S. and Russia for on-site inspections to determine the absence or presence and number of nuclear warheads. The RDE is an array of helium-3 proportional counters surrounded by polyethylene, and it was designed to efficiently detect neutrons from fission (McNeilly and Rothstein 1994). At inspection sites, the proper operation and calibration of the RDE was determined using an americium-lithium (Am-Li) neutron source brought with the inspection teams, and the background count rate was separately measured. Transport of the neutron source in its shielding was awkward, and is more so now that radiation detection is employed at the borders that inspection teams must cross. For possible use in future treaties, it has been proposed that proper operation and calibration could be determined using the background count rate instead of the Am-Li neutron source. This can only be done if the cosmogenic neutron flux and energy spectrum is accurately known at the location of the inspection site at the time of the inspection. This could be achieved by experimentally verified calculations like the LANL calculations described above, but it would require real-time neutron monitor data. To be acceptable to each treaty participant, each country would want data from neutron monitors it can confidently rely upon.

The U.S. is not a signatory of the Comprehensive Test Ban Treaty (CTBT), but our country has an interest in ensuring its provisions can be verified. Protocol Part II of the CTBT (Comprehensive Test Ban Treaty Organization 2016) allows for on-site inspections to detect if an underground nuclear test has occurred. Part of such an inspection would include detecting radioactive gasses produced in the ground by fission neutrons. Along with radioactive xenon, radioargon isotopes, particularly 37Ar, are being considered. To understand soil air measurements taken during an on-site inspection, the radioargon background due to cosmic-ray-induced activation must be understood. Johnson et al. (2015) have used the cosmic-ray source feature of MCNP6 to calculate the cosmogenic neutron flux at ground level as a function of date during the solar magnetic activity cycle, latitude of sampling location, geology of the sampling location, and sampling depth. After the cosmic neutron flux was obtained, the rate of radioargon production was calculated. Radioargon production was shown to be highly dependent on the soil composition, particularly calcium content, as well as on latitude and solar magnetic activity. The half-life of 37Ar is 35.04 days. In a real on-site inspection, the background 37Ar level would depend on the cosmogenic neutron fluence over the past few months, so determining it accurately would require neutron monitor data for that period.

Some of the uses of cosmogenic neutron flux determinations using neutron monitor data for homeland and national security applications have been described above. Without ongoing reliable neutron monitor operation, efforts to improve detection of terrorist nuclear threats and nuclear treaty verification will be significantly impaired.

REFERENCES

Comprehensive Test Ban Treaty Organization (2016) “Treaty text.” https://www.ctbto.org/the-treaty/treaty-text/.

Goldhagen, P. (2000), “Overview of aircraft radiation exposure and recent ER-2 measurements.” Health Phys. 79: 526-544.

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Goldhagen, P., J. M. Clem, and J. W. Wilson (2004), “The energy spectrum of cosmic-ray induced neutrons measured on an airplane over a wide range of altitude and latitude.” Radiat. Prot. Dosim. 110: 387-392.

Goldhagen, P. (2011), “An extended-range multisphere neutron spectrometer with high sensitivity and improved resolution.” Nucl. Technol. 175: 81-88.

Gordon, M. S., P. Goldhagen, K. P. Rodbell, T. H. Zabel, H. H. K. Tang, J. M. Clem, and P. Bailey (2004), "Measurement of the flux and energy spectrum of cosmic-ray induced neutrons on the ground," IEEE Trans. Nucl. Sci. 51: 3427-3434.

Johnson, C., H. Armstrong, W. H. Wilson, and S. R. Biegalski (2015), “Examination of radioargon production by cosmic neutron interactions.” J. Env. Radioactivity 140: 123-129.

Liegey, L. R., J. R. Tutt, T. A. Wilcox, and G. W. McKinney (2016), “Predicting future solar modulation and implementation in MCNP6.” LANL report LA-UR-16-20290.

McKinney, G. W., D. J. Lawrence, T. H. Prettyman, R. C. Elphic, W. C. Feldman, and J. J. Hagerty (2006), MCNPX benchmark for cosmic ray interactions with the Moon, J. Geophys. Res., 111: E06004, doi:10.1029/2005JE002551.

McKinney, G. W., H. J. Armstrong, M. R. James, J. M. Clem, and P. Goldhagen (2012), “MCNP6 Cosmic-Source Option,” Proceedings of ANS Annual Meeting, Chicago, IL, June 24-28, 2012, LANL report LA-UR-12-00196.

McMath, G. E., G. W. McKinney, and T. Wilcox (2014), “MCNP6 Cosmic & Terrestrial Background Particle Fluxes – Release 4.” Proceedings of ANS Annual Meeting, Reno, NV, June 15-19, 2014, LANL report LA-UR-14-20090.

McMath, G. E. and G. W. McKinney (2014), “MCNP6 Elevation Scaling of Cosmic Ray Backgrounds.” ANS RPSD 2014 - 18th Topical Meeting of the Radiation Protection & Shielding Division of ANS Knoxville, TN, September 14 – 18, 2014, on CD-ROM, American Nuclear Society, LaGrange Park, IL.

McNeilly, J. H. and B. D. Rothstein (1994), “Radiation detection equipment (RDE) comparative evaluation test program volume 1—point source measurements.” Defense Nuclear Agency Technical Report DNA-TR-93-160-V1, downloaded from http://www.dtic.mil/dtic/tr/fulltext/u2/a283003.pdf.

Pelowitz, D. B., A. J. Fallgren, and G. E. McMath, editors (2014), “MCNP6 User’s Manual, Code Version 6.1.1 beta, June 2014, Manual Rev. 0” LANL report LA-CP-14-00745, Rev. 0. Export controlled; distribution authorized to U.S. Government agencies and their contractors.

Usoskin, I. G., G. A. Bazilevskaya, and G. A. Kovaltsov (2011), “Solar modulation parameter for cosmic rays since 1936 reconstructed from ground-based neutron monitors and ionization chambers, J. Geophys. Res., 116: A02104. [Data after year 2009 available at http://cosmicrays.oulu.fi/phi/Phi_mon.txt .]

Neutron Monitor Uses in Aviation Radiation Safety

Contributed by: Kyle Copeland, Ph.D.*

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*U.S. Federal Aviation Administration, Civil Aerospace Medical Institute, Aerospace Medical Research Division, Protection and Survival Research Laboratory, Numerical Sciences Research Team. Mail Route AAM-630, 6500 S. MacArthur Blvd. Oklahoma City, OK 73169, USA.

Background

Ionizing radiation exposure is an unavoidable part of daily life. Even our bodies are radioactive, due to the radioactive chemicals from which we are made. Ionizing radiation dose in humans is usually reported in gray (Gy) or sievert (Sv). The gray is the unit of absorbed dose, the average energy per unit mass deposited by the radiation. Deterministic effects of radiation exposure, i.e., those effects which are caused by cell killing of large numbers of cells, are traditionally considered in terms of gray. The sievert is the name used when the absorbed dose is weighted for health effects which are probabilistic in nature, i.e., stochastic effects. This weighting can be based on type of radiation, the tissue being irradiated, or both. The most common use of sievert is the quantity effective dose, which attempts to weight all radiation exposures with respect to cancer inductions, genetic effects, and length of life lost, and quality of remaining life (ICRP, 2007).

The global average dose per person from natural sources is about 2.4 mSv (or 2400 Sv: 1000 Sv = 1 mSv) each year, just from living on Earth, with most of the dose coming from radon exposure; for the U.S., the estimate is 3.1 mSv (United Nations Scientific Committee on the Effects of Atomic Radiation [UNSCEAR], 2000; National Council on Radiation Protection and Measurements [NCRP], 2009). Those living at unusually high altitudes or other places with unique living conditions (e.g., in the mountains, a coral atoll, etc.) can get a few times more or less than the average dose. Most of the natural variation is the result of shielding afforded by the Earth's atmosphere to galactic cosmic radiation (GCR) and variation in local radon levels, but radioactivity in the soil can also be very important.

While their doses from manmade sources are low, aircrews are among the highest occupationally exposed populations. Both the NCRP and UNSCEAR rank aircrew as among the most exposed occupations, at 3 mSv per year, almost all from GCR, roughly equaling their off-duty natural background exposure (UNSCEAR, 2000; NCRP, 2009). As aircraft cruise altitudes continue to increase, this dose will rise. According to the U.S. Bureau of Labor Statistics (BLS), there were about 210,000 crewmembers (pilots and flight attendants) in the U.S. labor force in 2014 (BLS, 2016).

There are several potential sources of radiation in aviation, but most of it usually comes from GCR. A decent approximation for USA based commercial flights within the Northern hemisphere of a few hours or more is about 5 Sv per hour of flight time. Doses per hour on shorter flights and on flights away from the poles, such as trans-equatorial flights, are lower. Equivalently scaled doses from other sources are extremely rare:

While a nuclear accident or spillage of radioactive cargo could cause a large exposure to aircrew or passengers. Such nuclear accidents are very rare and few flights carry radioactive cargo. Radioactive cargo is transported in well-shielded packages spaced far apart from each other to minimize exposure to aircraft occupants. Estimated annual dose: < 0.13 mSv per year (Warner-Jones et al., 2003).

Solar ionizing radiation is almost always orders of magnitude weaker than galactic cosmic radiation, the exception being during rare, intense solar particle events (SPEs). Because the even the most intense solar particle spectrum is much softer than the GCR spectrum, they typically only significantly increase doses on high-latitude flights where the ions with rigidities of a few GV or less are permitted access to the atmosphere. A study of the SPEs that occurred from 1986-2007 by Copeland et al. (2008) found that even at 18 km altitude, solar proton events with dose rates at high latitudes that exceed 20 Sv/h have occurred less than once per year, on average. The events roughly followed solar activity in frequency. Events resulting in possible in-flight exposures exceeding the 1 mSv recommended limit for pregnant crewmembers at 12 km (40,000 ft.) and above are extremely rare, occurring about once a decade.

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Lightning and terrestrial gamma-ray flashes (TGFs) are associated with large thunderstorms, which aircraft avoid if it is possible. About 1 in 1000 flights is struck by lightning, mostly at low altitudes, such as during approach maneuvers. While a dose of up to 30 mSv has been postulated (Dwyer et al., 2010) for a worst case exposure from a TGF, they are associated with only a small fraction of lightning strokes (best evidence is for about 1 in 1000 or less) and are thought to occur only above about 10-15 km. Most lightning only produces insignificant amounts of soft X-rays.

GCR is relatively constant. It cannot be practically shielded against and effective dose increases as altitude increases. At the levels of exposure found in commercial aviation today, a few millisieverts of occupational exposure per year is a reasonable estimate of occupational exposure for crewmembers.

As indicated by the threshold doses in Table 1, the potential exposures from all the exposures listed above are well below the dose levels that can cause deterministic effects. It is the stochastic effects (Table 2) that are of concern. Though the odds are low, cancer induction is the most likely effect. It should also be noted that these risk estimates are for large populations, not individuals. There is too much variation in sensitivity and too little knowledge of cancer induction pathways to estimate risk for any particular individual.

Table 1. Low Dose Deterministic Effects

Deterministic Effect Threshold Dose*None Significant <0.1 GyRisks to conceptus (mental retardation, malformation, etc.) 0.1-0.5 GyTransient mild nausea and headache in adults 0.35 Gy* Assumes acute exposures, 1 Gy here is equivalent to 1 Sv in Table 2.

Table 2. Stochastic Effects

Increased lifetime risk*Stochastic Effect Whole population Age group 18-64 yearsGenetic defect in first or second generation (child or grandchild) following irradiation before conception

0.4 in 100,000 per mSv 2.4 in 1,000,000 per mSv

Cancer (non-fatal or fatal) 34 in 100,000 per mSv 23 in 100,000 per mSv

Cancer (fatal only) 8.0 in 100,000 per mSv 6.3 in 100,000 per mSv*Risks assume exposure to high-LET radiation (i.e., no DDREF) (ICRP, 2008).

Table 3. Federal Aviation Administration's Recommended Exposure Limits for Air-Carrier Crewmembers

Crewmember Status Effective Dose LimitPregnant 1 mSv for duration of pregnancy and 0.5 mSv in any one monthOther 100 mSv per 5 years and no more than 50 mSv in any one year

(FAA) recommended exposure limits are shown in Table 3 (FAA, 2008). Pregnant crewmembers are most likely to exceed recommended limits. Galactic cosmic radiation is enough to exceed the limits during pregnancy over the course of a few transcontinental round trips.

Neutron Monitor Uses

Once it reaches the atmosphere, a cosmic ray, whether of solar or galactic origin, creates a shower of secondary particles. The shower may reach all the way through the atmosphere to the ground if the initiating particle has

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enough energy. For purposes of calculating dose rates at altitudes all the way down to sea level, good information about the incoming particle spectrum is required and neutron monitor data currently play a vital role in unfolding the spectrum. As an example, in CARI-7, the FAA’s latest GCR flight dose calculation software, neutron monitor data are used in two places. The data are used both for Forbush modulation, and also within the GCR models to drive solar modulation. Because of the well-known inverse relation between solar activity and GCR flux, using NM data to estimate solar modulation is common practice for GCR model developers (e.g., O'Brien et al., 2003; O'Neill, Golge, and Slaba, 2015). The BO14 GCR models add satellite data and sunspot numbers as additional sources of solar modulation data, while the ISO model is driven by Wolf sunspot numbers and does not capture short-term variations in local GCR flux (O'Neill, Golge, and Slaba, 2015; ISO, 2004). For GCR spectral variation over the course of a day or more a single 18-tube monitor can be used to drive a model with enough accuracy to match in-flight measurements very well. While the HEPAD instruments on NOAA's GOES series satellites could also provide data on these time scales with reasonable accuracy, to capture shorter term variations, such as are needed for studies comparing hourly-averaged instrument measurements to theoretical calculations, only NMs can currently provide the needed short-term accuracy. Multiple monitors/more tubes are useful to improve counting statistics and reliability and are needed to gain detailed spectral information about short term variations such as Forbush decreases and solar particle events (SPEs).

During an SPE, the cosmic ray spectrum incident at any given location in the atmosphere is more difficult to model well. This is because the particle spectrum is constantly changing significantly (even on timescales as short as 1 minute), and because isotropy of the incident flux is often a poor assumption for the first few hours of an event. Data from many monitors, with many tubes each, is needed to maximize accuracy of event assessments. This is particularly true at the start of events, when high-energy particle fluxes are usually highest. For SPEs, a world grid of neutron monitors, in both hemispheres (N and S), is needed to provide a really good picture. Monitors at different altitudes at the same location are also useful. These data from Inuvik, Goose Bay, and Deep River monitors show the problem (Figure 1). Deep River is farthest south and has a slightly higher geomagnetic cutoff, so it could be argued that it's reduced count rate increase relative to that at Inuvik is the result of the softness of the SPE spectrum, but both Goose Bay and Inuvik are near-sea-level stations sited close enough to the magnetic pole that (after correction for local conditions) they would respond essentially identically if event flux were isotropic. Monitors at the same geomagnetic latitude but different longitudes respond differently when SPE flux is highly anisotropic. Using data from multiple high-latitude near-sea-level monitors, along with lower latitude monitors and higher altitude monitors provides information on the shape of the SPE particle spectrum with rigidities above 1 GV, a part of the solar cosmic ray spectrum that satellites do not measure well. Count rates from GOES satellite instruments provide a picture of the low to intermediate energy proton flux, but very limited information the high energy flux spectrum, only enough to make an educated guess at the shape of the spectrum.

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Figure 1. Relative increase in count rate relative to GCR background during the SPE of 24-25 May, 1990, at three northern-latitude neutron monitors (Bartol Research Institute, 2015).

Summary

With regards to Galactic Cosmic Radiation

Neutron monitors are used for observing solar modulation in real time, including Forbush decreases. High resolution (hourly) data covers practically the entire jet age of civilian flight (starting in the late

1950s). Useful for both long term monitoring (solar cycle) and short term variations (Forbush effects). One monitor with many tubes is enough for basic GCR modelling, if statistics are good and magnetic drift

of the cutoff is accounted for, but more monitors add more insight, and may be needed to get adequate statistics if monitors have only a few tubes each.

With regards to Solar Cosmic Radiation

NMs provide the best data for reconstructing SPE particle spectrum anisotropy, which is needed to drive the most accurate and sophisticated SPE flight dose models.

To reconstruct global anisotropic particle flux spectra requires multiple NM at different altitudes and geomagnetic latitudes, both N and S.

NMs are the only data source providing insight into multi-GeV proton and alpha spectra during a SPE; satellite instruments simply do not have enough shielding to discriminate well.

References

Bartol Research Institute (2015). http://neutronm.bartol.udel.edu/listen/main.html#tell, accessed 10 Oct 2015.

Bureau of Labor Statistics (2016). Occupational Outlook Handbook. Available at: www.bls.gov/ooh/, accessed 11 May 2016.

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Copeland, K; Sauer, HH; Duke, FE; Friedberg, W. Cosmic radiation exposure of aircraft occupants on simulated high-latitude flights during solar proton events from 1January 1986 through 1 January 2008. Advan Space Res, 2008, 42(6), 1008-1029.

Dwyer JR, et al. Estimation of the fluence of high-energy electron bursts produced by thunderclouds and the resulting radiation doses received in aircraft. J Geophys Res, 2010; 115: D09206, doi:10.1029/2009JD012039.

Federal Aviation Administration. Order 3900.19B, Chapter 14, Part 1406, Paragraph ‘a’. Washington, DC: Department of Transportation, Federal Aviation Administration, 26 August 2008.

International Commission on Radiological Protection. The 2007 Recommendations of the International Commission on Radiological Protection, Report No. 103. London: Elsevier, 2007.

ISO (International Standards Organization). Space environment (natural and artificial) --Galactic cosmic ray model. ISO 15930:2004. Geneva, Switzerland: ISO; 2004. Available from: www.iso.org/iso/home/store/catalogue_tc/ catalogue_detail.htm?csnumber=37095&commid=46614, accessed 12 June 2013.

National Council on Radiation Protection and Measurements. Ionizing Exposure of the Population of the United States, NCRP Report No. 160. Bethesda, MD, 2009.

O’Brien, K., Smart, D.F., Shea, M.A., Felsberger, E., Schrewe, U., Friedberg, W., Copeland, K. World-wide radiation dosage calculations for air crew members. Advan. Space Res. 31(4), 835-840, 2003.

O'Niel, P.W., Golge, S., and Slaba, T.C. Badhwar - O'Neill 2014 Galactic Cosmic Ray Flux Model Description, NASA/TP-2015-218569. Houston, TX: Johnson Space Center, 2015.

UNSCEAR (United Nations Scientific Committee on the Effects of Atomic Radiation). Sources and Effects of Ionizing Radiation: United Nations Scientific Committee on the Effects of Atomic Radiation: UNSCEAR 2000 Report to the General Assembly, with Scientific Annexes Volume I: Sources. New York, NY: United Nations, 2000.

Warner-Jones, SM; Shaw, KB; Hughes, JS. Survey into the Radiological Impact of the Normal Transport of Radioactive Material by Air. Report NRPB-W39. National Radiation Protection Board (UK); Apr 2003. Available from: www.hpa.org.uk/web/HPAwebFile/HPAweb_C/1194947310807, accessed 29 Jul 2014.

Single-Event Upsets and Microelectronics(Why neutrons matter to the electronics industry)

Michael Gordon, IBM TJ Watson Research Center

Single-Event Upsets (SEU) are a major reliability issue in modern CMOS devices. They are initiated either by the passage of terrestrial neutrons or by the natural alpha particle radiation from contamination within materials near the transistors, e.g. components of the back end of the line (BEOL) and packaging. Nuclear (spallation) reactions occurring between the terrestrial neutrons and the silicon and other chip materials cause the emission of secondary, highly-ionizing particles. The charge deposited by these secondary particle can cause single, or even multiple, bits to flip. Alpha particles from contamination, and ultra-low energy protons, can also cause SEU’s through direct ionization.

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SEU’s in CMOS devices can affect the performance of computer systems (e.g., servers, data clusters, etc.), laptops, and personal electronic devices. Aside from being a nuisance, SEU’s can also have more serious implications as they can also affect medical implanted devices, as well as autonomous vehicles, and passenger airplane avionics.

Semiconductor companies, and end users of high-end chips, often have test campaigns to assess the sensitivity of their devices, circuits, or systems to terrestrial neutrons. The testing methods are usually “accelerated” meaning exposing the devices or systems to an external beam of neutrons (or high energy protons as a proxy for the neutrons), or “life testing”, where typically a computer system, is exposed to the natural terrestrial neutron flux at high elevations, for a period of several months. Subsequently these experiments are repeated underground (with negligible neutron flux) to determine the alpha particle contribution to the SEU rate. The acceleration factors, the ratio of the neutron flux in either the accelerated testing or life testing to the ground-level flux in NY, ranges from ~1E8 to ~ 1.5E1, respectively, as life testing occurs at high elevations.

The chart below, from Autran, et. al., IEEE Radiation Effects Data Workshop, IEEE, 2014, pp. 1-8, shows the neutron component of the SEU for SRAM devices for various scaling nodes from 130 nm, to 40 nm. The units on the vertical axis are FIT/ Mbit where 1 FIT is one bit flip in 1E9 years. The histograms shown in black are the “real-time” or life test data, the lightly shaded histograms are the results of “accelerated” testing using a neutron beam from Triumf (and the yellow histograms are modeling results). The real-time data came from their experiments performed in the French Alps, at the ASTEP facility, 2552 meters above sea level. This facility has real-time neutron monitors (three 3He detectors) used to measure the terrestrial neutron flux during the test campaign. This allows for the real-time assessment of the acceleration factor (~6), rather than an estimate based on geomagnetic rigidity and altitude. This can be important during periods of active sun where the real-time neutron monitors can measure fluctuations in the terrestrial neutron flux.

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IBM has used a similar facility in Leadville, Colorado, where neutron monitors provide real-time neutron flux data during life-time testing campaigns. Leadville is the highest incorporated city in the United States. The acceleration factor is about 13. The neutron monitors, maintained by Jim Ryan at the University of New Hampshire, were recently upgraded to include new electronics and real-time data recording, as well as on-line, web-based, access.

Although an approximate neutron acceleration factor can be estimated for life testing campaigns, based on the geomagnetic rigidity and altitude of the test site, neutron monitors at the test site are essential to provide a more precise real-time determination of the neutron flux- which gives a more accurate neutron-induced SEU rate estimate.

Cosmic-ray neutron method for measuring area-average soil moisture

Marek Zreda, U. Arizona

Rationale for development

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Soil moisture is the most important part of the water and energy cycle (Fig. 1). It plays a critical role in weather and seasonal climate forecasting, and in linking water, energy, and biogeochemical cycles over land. It should be measured at the scale that is useful for land-surface processes and hydrology (100 m – 1 km). Conventional methods measure soil moisture either at a point (eg, time-domain reflectometry) or over large areas (eg, satellite microwave instruments). They have to be upscaled or downscaled, respectively, to provide data at the useful scale, which is impractical and unreliable. The recently developed cosmic-ray method (Zreda et al., 2008, 2012; Desilets et al., 2010) has a hectometer footprint (Desilets and Zreda, 2013; Köhli et al., 2015), and is, therefore a good scale integrator of soil moisture for land-surface and hydrological studies.

Physical principle The cosmic-ray method (Zreda et al., 2008, 2012; Desilets et al., 2010) takes advantage of the extraordinary sensitivity of cosmogenic low-energy, moderated neutrons of energy between 1 eV and 1000 eV to hydrogen present in materials at the land surface (Fig. 1). Most neutrons on earth are cosmogenic. Primary cosmic-ray protons collide with atmospheric nuclei and unleash cascades of energetic secondary neutrons that interact with terrestrial nuclei and produce fast (evaporation) neutrons at the land surface. The fast neutrons that are produced in air and soil travel in all directions within the air-soil-vegetation continuum, and in this way an equilibrium concentration of neutrons is established. The

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Figure 1. Left: Water and energy mass balance at the land surface; red labels indicate pools and fluxes of water measurable using cosmic-ray neutrons. Right: Cosmic-ray neutron interactions with air and soil. Tracks of two neutrons are shown in the lower panel. Neutron n1 was absorbed in soil and removed from the pool of neutrons measurable by cosmic-ray probe above the surface; neutron n2 went back to the atmosphere and is measurable there. These tracks are copied onto the left panel (red lines).

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equilibrium is shifted in response to changes in the water present above and below the land surface, for example in soil. Adding water to soil results in more efficient moderation of neutrons by the soil, causing a decrease of fast neutron intensity above the soil surface, where the measurement is made. Removing water from the soil has the opposite effect. The resultant neutron intensity above the land surface is inversely proportional to soil water content (Zreda et al., 2008, 2012).

Instruments

Low-energy cosmogenic neutrons are measured using proportional counters (Knoll, 2000), which are sensitive to thermal neutrons (median energy of 0.025 eV), shielded by a layer of plastic that shifts the energy sensitivity of the counter to neutrons of the desired energy (>1 eV). The cosmic-ray probe (Fig. 2) is powered using a solar panel paired with a rechargeable battery, and is equipped with an Iridium satellite modem or a cellular modem for real-time telemetry. It can be operated almost anywhere in the world, except areas with insufficient day light. A stationary neutron probe of this type is implemented in the Cosmic-ray Soil Moisture Observing System (Fig. 3), or COSMOS (Zreda et al., 2012; cosmos.hwr.arizona.edu); therefore, it is sometimes called “COSMOS probe”. A mobile COSMOS detector is a bigger version of the stationary probe, additionally equipped with a GPS system (Chrisman and Zreda, 2013).

Conversion of neutron data to soil moisture

The measured neutron, normalized for variations in pressure, humidity and incoming neutron intensity (Zreda et al., 2012), is converted to soil moisture using the response

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Figure 2. Cosmic-ray soil moisture probe installed at Marshall Lake, Colorado, USA. For description of the components, see Fig. 9 in Zreda et al. (2012).

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function, such as that developed by Desilets et al. (2010). Other local measurements needed for the conversion are atmospheric pressure, temperature and water vapor. Additionally, the knowledge of temporal variations of the intensity of high-energy cascade neutrons is necessary to assess the strength of the source function for low-energy evaporation neutrons. Those data are generated from measurements with neutron monitors. The feasibility of soil-moisture monitoring using the cosmic-ray method relies on the availability of real-time neutron monitor data.

Funding sources

Most of the work on the development and applications of the cosmic-ray soil moisture method was funded by the US National Science Foundation (NSF). The development work was funded through grants from the Hydrology program between 2001 and 2010 (grants EAR-0126241, EAR-0636110). The installation of COSMOS network in 2009-2013 was supported by a grant from the Mid-size Infrastructure program administerd by the Atmospheric Sciences program (grant ATM-0838491). Additional relevant work on cosmic-ray neutron variations in space and time was funded by the Geochemistry program between 1999 and 2004 (grants EAR-0001191 and EAR-0126209).

ReferencesChrisman B., and Zreda M. (2013): Quantifying mesoscale soil moisture with the cosmic-ray rover. Hydrology and

Earth System Sciences 17, 5097-5108.

Desilets D. et al., 2010. Nature's neutron probe: Land-surface hydrology at an elusive scale with cosmic rays. Water Resources Research 46, W11505.

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Figure 3. The COsmic-ray Soil Moisture Observing System (COSMOS) consists of approximately 100 probes of the type shown in Fig. 2. The Australian probes displayed here belong to the CosmOz network. Not shown here are other networks that either already exist or that are under construction, most notably the TERENO network in Germany and the COSMOS-UK network in the United Kingdom.

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Desilets D., and Zreda M., 2013. Footprint diameter for a cosmic-ray soil moisture probe: Theory and Monte Carlo simulations. Water Resources Research 49, 3566-3575.

Knoll, G.F., 2000. Radiation detection and measurement. Wiley, New York, 802 p.

Köhli M. et al., 2015. Footprint characteristics revised for field-scale soil moisture monitoring with cosmic-ray neutrons. Water Resources Research 51, 5772–5790.

Zreda M. et al., 2008. Measuring soil moisture content non-invasively at intermediate spatial scale using cosmic-ray neutrons. Geophysical Research Letters 35, L21402.

Zreda, M. et al., 2012. COSMOS: the COsmic-ray Soil Moisture Observing System. Hydrology and Earth System Sciences 16, 4079-4099.

Glacier DynamicsIan Howat

Byrd Polar and Climate Research Center Ohio State University

Direct, continuous measurements of ice sheet surface mass balance are lacking, particularly in the accumulation zone where the surface snow and firn varies in density. Nearly all of our knowledge of surface mass variability comes from snow pit and ice core stratigraphy, providing annual resolution with relatively large uncertainties that are inadequate for constraining meteorological models. Further, little information is available on how the density of the firn layer changes with time, hampering efforts to estimate mass change from altimetry measurements. We are testing cosmic ray sensing technology to obtain the first high-accuracy (mm-scale) and continuous measurements of ice sheet mass balance that can be directly compared with models. The technology is simple in concept: we measure the attenuation of neutrons impacting a sensor as it is buried by accumulating mass. To obtain accurate measurements of mass accumulation, however, this signal must be corrected for variations due to atmospheric pressure and solar fluctuations. For the later, the neutron monitors maintained by the Bartol Research Institute provide a critical reference series. Following the successful pilot deployment of our snow mass balance sensors, we aspire to deploy a large network of sensors in both Greenland and Antarctica. The Bartol monitors at Thule, Labrador, South Pole and McMurdo Station will be critical for this effort.

Evolution of the geomagnetic field and its influence on cosmic radiation measurements

Peggy Shea and Don Smart

As cosmic radiation measurements became more precise, the inadequacy of the geomagnetic latitude or cutoff rigidities derived from the dipole components of the geomagnetic field became increasing apparent in the satisfactory ordering of cosmic ray data. The 1950s saw the advent of digital computers being used for scientific data analysis. Using a computer at MIT, K. G. McCracken developed a computer code to trace the orbits of charged particles through a high order (6th degree simulation) of the geomagnetic field for Epoch 1955 (McCracken, et al., 1962). The original purpose was to investigate charged particle access to neutron monitors for the ground-level solar cosmic ray events (GLEs) in 1960. This trajectory-tracing code was also used to compute the geomagnetic cutoff rigidity for the neutron monitor at Port aux Français,

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Kerguelen Islands to study the GLE increases on 12 and 15 November 1960. Freon and McCracken (1962) found that these trajectory-derived calculations of the vertical cutoff rigidity provided a better fit to the solar proton event data than previous available values. Using this same basic trajectory computer code (and many computer hours), Shea et al. (1965) determined "effective vertical cutoff rigidity values" for neutron monitor locations.

In subsequent years newer models of the geomagnetic field became available, primarily under the auspices of the International Association of Geomagnetism and Aeronomy (IAGA). With improvements in the speed of computers, Shea and Smart continued their cutoff rigidity calculations using these newer field models which were developed from both ground and satellite data. Shea (1971) found a decrease in the vertical cutoff rigidity values in the Latin American region from 1955 to 1970 and predicted that neutron monitors in that area should observe an apparent increase in the galactic cosmic ray intensity from 0.8 to 2.0 percent between successive solar minima. This result was initially viewed with skepticism; however, an apparent increase in the galactic cosmic radiation at Huancayo, Peru between two solar minima was reported by Cooper and Simpson (1979), and this was attributed to a decrease in the geomagnetic cutoff rigidity.

Using neutron monitor data from an aircraft flight between Johannesburg and New York City in October 1976, König and Stoker (1981) reported that the latitude

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Figure 1. The change in the magnitude of the geomagnetic dipole term: 1600-2010.

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curves above and below the cosmic ray equator were displaced when ordered by the geomagnetic cutoff rigidity values for 1965. Calculating the cutoff rigidity values for the Epoch of the flights, Shea and Smart (1990) found that while the vertical cutoff rigidity values had decreased in time in the South Atlantic region, they had markedly increased in the North Atlantic region by as much as one percent per year between 1965 and 1980 (Figure 1). Using the appropriate cutoff rigidity for the Epoch of the measurement resulted in a satisfactory ordering of the cosmic radiation intensity over the latitude survey (Figure 2).

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Figure 3 illustrates the change in the magnitude of the dipole term of the geomagnetic field over 400 years. While the magnitude of the geomagnetic field is decreasing, the higher order components of the field are both increasing and decreasing. The resulting overall changes are not distributed uniformly over the world. For the period 1955-1995, this has the effect of an increase in the vertical cutoff rigidity values in the North Atlantic area (including the east coast of the USA) and a decrease in the Latin American region. As an example, the vertical cutoff rigidity for Mexico City in 1955 was 9.45 GV; in 1995 was 8.02 GV - a decrease of 15.1% over 40 years. For Mt. Washington, NH, the change in vertical cutoff rigidity was an increase over time: 1.25 GV in 1955 and 1.58 GV in 1995 - a 26.4% increase. The cutoff rigidity for the Climax, Colorado neutron monitor remained approximately constant over this time interval, and the cosmic ray intensity at this station has been used by many scientists as a benchmark for the long term time variations of the galactic cosmic radiation.

The rapidly evolving geomagnetic field has several implications for cosmic ray research. The calculation of the cutoff rigidities for the proper Epoch of the geomagnetic field would be essential for studies of the long term galactic cosmic radiation, latitude surveys, relativistic solar proton events and for computing the radiation dose along specific airline flight paths. A world map of vertical cutoff rigidities for Epoch 2010 is shown in Figure 4.

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Figure 3. Contours of averaged annual change of vertical cutoff rigidities (in GV) between 1965.0 and 1980.0 The dashed line indicates the route of the airline flight between South Africa and New York City in October 1976.

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REFERENCES

Cooper, J.F., and J. A. Simpson, Origin of the large-scale difference in the observed magnetic rigidity dependence of the cosmic radiation for two solar cycles - 1954-65 and 1965-1976, 16th International Cosmic Ray Conference, Conf. Pap., 12, 176-181, 1979.

König, P. J. and P. H. Stoker, Displaced isorigidity contours in the North Atlantic for 1975, J. Geophys. Res., 86, 219-233, 1981.

McCracken, K.G., U.R. Rao, and M.A. Shea, The Trajectories of Cosmic Rays in a High Degree Simulation of the Geomagnetic Field, Massachusetts Institute of Technology Technical Report No. 77, NYO-2670, August 1962.

Shea, M.A., Changes in Neutron Monitor Response and Vertical Cutoff Rigidities Resulting from Secular Variations in the Geomagnetic Field, 12th International Conference on Cosmic Rays, Hobart, Conference Papers (University of Tasmania), 3, 859-864, 1971.

Shea, M.A. and D.F. Smart, The Influence of the Changing Geomagnetic Field on Cosmic Ray Measurements, J. Geomag. Geoelectr., 42, 1107-1121, 1990.

Shea, M.A., D.F. Smart, and K.G. McCracken, A Study of Vertical Cutoff Rigidities Using Sixth Degree Simulations of the Geomagnetic Field, J. Geophys. Res., 70, 4117-4130, 1965.

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Figure 4. World map of vertical cutoff rigidity contours (in GV) for Epoch 2010.

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Chapter 5 Current status and outlookInternational perspectives of neutron monitors

Sodankylä Geophysical Observatory, University of Oulu, Finland

The NM network is most useful as a joint network, which has much more abilities for science and applications than a sum of individual detectors. It is a balance between national and international efforts. It is crucially important that the NMs network is as homogeneous as possible without large gaps in coverage. For example, closure of NMs in the North America would result in a dramatic drop of quality of the NM network data in international perspective.

Data from individual NMs are gathered in several international databases:

World Data Center for Cosmic Rays (WDC CR) in Japan [http://center.stelab.nagoya-u.ac.jp/WDCCR], where all hourly-resolution data from NMs are collected in an ftp format;

Neutron Monitor Database (NMDB) in Germany [http://www.nmdb.eu] provides a user-friendly access to many long-operating NMs;

Ground Level Enhancement (GLE) data base [http://gle.oulu.fi] is an international database collecting all available data on GLE events recorded by the NM network.

The NM network is an international effort initiated at International Geophysical Year 1957. It provides a unique global network which is the primary instrument to study ”lower energy” (< 100 GeV/nuc) GCR, strong SEP events (GLE) and interplanetary transients (FD). The homogeneous NM data series exists for 60+ years forming a benchmark data set of cosmic rays and heliospheric/solar studies. The NM network has many advantages: it is a standard device, providing continuous monitoring of cosmic rays with stable operation, providing a homogenous data series; data are ready for real-time analysis; its response is known; important is low cost (distributed among different countries/institutions) and easy maintenance. The only disadvantage is that a NM is an energy integrating device, but this is overcome by the global networking, so that the NM network operates as a giant spectrometer using the Earth's atmosphere and magnetosphere.

The network cannot be replaced by space-borne instruments. The primary interest of many space-borne detectors (e.g., SOHO, ACE, GOES) is in solar transients, interplanetary medium and space weather, they have low energy range and cannot give an estimate for the atmospheric penetration of cosmic rays. Special CR-dedicated missions (PAMELA, AMS-02) are focused on high-energy exotic CR and HE astrophysics. The very complex DAQ system

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makes it difficult for real-time analysis. Because of the low inclined orbit only a fraction of time can be used to study SEP events. These detectors have limited life-time.

The NM network data is used for several fields:

Scienceo GCR variability (heliospheric modulation) – NMN is the primary instrument;o GLE analysis (source direction, anisotropy, spectra) – NMN is the primary

instrument;o Solar neutron events – NMN is an important instrument ;o Interplanetary transients – NMN is a useful instrument.

Practical use o Aircraft crew dosimetry – NMN is the only instrument; o Space weather: storm fore/now-casts – NMN is an important instrument;

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Chapter 6 Recommendations and priorities

Recommendations emerged from the workshop that if implemented would bring the network into a condition to conduct the best science and support operational needs. In order of priority, they are:

Fully restore the scientific functionality and update the existing US network. This will constitute a major step in restoring the global network by restoring coverage provided by the US stations and by making a statement to international funding agencies that the global network is important,

Establish a desired network concept that would fulfill the needs of the science and operational communities. This would involve identifying new key strategic sites to complete the global coverage,

Improve station and data uniformity and accessibility,

Train a new generation of scientists and expand educational outreach,

Given the unavailability of standard detectors, design and deploy a new generation of neutron monitors in the form of inexpensive kits to be widely deployed as part of global strategy for the network, and

Modernize or install new timing electronics to study rapid phenomena, not anticipated in 1957.

Student and Postdoc involvement in Neutron MonitorsThe Newark and Durham Neutron Monitors are located on the campuses at the University of Delaware and University of New Hampshire respectively. These detectors have provided important teaching tools for undergraduates, graduate students and post docs over the years. These local stations gives students the opportunity to gain experience working on advanced instruments and high tech equipment. The operation of the system and detection strategy can be used to help engage students to explore physics and electronics from the basic level to the advance.

Need more

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Neutron Monitors also provide a great opportunity to engage broader audiences. Novices are intrigued that the sun is not the steadily humming stellar engine typically portrayed, and among the science-interested public, there is an increasing awareness and interest in space weather.

Here we describe three ongoing efforts at the University of Wisconsin-River Falls (UWRF) that connects high school students, teachers, and undergraduate students to neutron monitor research. Besides exposing more people to astrophysics, these efforts provide access to new resources to support astrophysics research. These examples or similar programs are expandable to other institutions doing neutron monitor research.

UWRF, working with the Bartol Research Institute at the University of Delaware, has assumed responsibility for maintaining and operating the neutron monitor at the South Pole, and is playing a significant role in moving the neutron monitors on the coast of Antarctica at McMurdo Station to the Korean Jang Bogo base. We have leveraged the intrigue of the extreme Antarctic locale to garner broader interest in the research. To tap into new funding streams, the UWRF National Science Foundation proposal committed to utilize undergraduates, targeting students from two year colleges and underrepresented groups. Three students have deployed, one in the 2014-15 season and two in the 2015-16 season, and they kept blogs (https://i3uwrf.wordpress.com/) describing their experiences. Six students in total have done fully supported research at UWRF during the last three summers (2013-15).

We have also worked with the NSF PolarTrec (https://www.polartrec.com/) program that pairs teachers and researchers who have projects in the Arctic or Antarctic. This is a great way to leverage resources as PolarTrec handles logistical training, helps delineate and define the expectations for both the teacher and researcher, and monitors follow through, including resource development. This usually includes classroom or public outreach activities, areas where teachers excel. We continue to work with high school science teacher Mr. Juan Botella (https://www.polartrec.com/expeditions/cosray-neutron-monitors), though he unfortunately was not able to deploy at the last minute in the 2015-16 season. We have selected high school and two year college teacher Mr. Eric Thuma (https://www.polartrec.com/expeditions/antarctic-neutron-monitors-for-solar-study) to deploy with us in the 2016-17 season.

To reach high school students directly, we also work with the UWRF Upward Bound (https://www.uwrf.edu/AcademicSuccess/Upward-Bound.cfm) program, providing a summer eight-day math and science residential course for a highly diverse group of 9-12 students. Upward Bound (http://www2.ed.gov/programs/trioupbound/index.html) is a nation-wide federally funded program to help low-income high students prepare for success in college. Rather than focus exclusively on astrophysics, we pick a theme each year, and build from familiar accessible experiences to more complex, abstract concepts. We spend at least one day describing the research we do at UWRF that includes undergraduate students, and emphasize that the Upward Bound students could also be part of a research team in a few years.

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Right to Left: Northern Illinois student Robert Zill with UW-River Falls professor Jim Madsen and student Laura Moon atop Observation Hill, near the site of the Cosray neutron monitor, McMurdo Station, Antarctica (Jan. 2016, Robert Zill Photo).

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Table of Acronyms (incomplete)

NM Neutron Monitor

IGY International Geophysical Year 1957

IGY Simpson style Neutron Monitor 1949

NM64 Neutron Monitor designed by Carmichael and Hatton 1964

GLE Ground Level Event

SEP Solar Energetic Particles

IAGA International Association of Geomagnetism and Aeronomy

SWORM Space Weather Operations, Research, and Mitigation SWORM Task Force

SEU Single-Event Upsets

UNSCEAR United Nations Scientific Committee on the Effects of Atomic Radiation

DHS Department of Homeland Security

FAA Federal Aviation Administration

LANL Los Alamos National Laboratory

GCR Galactic Cosmic Rays

NMDB Neutron Monitor Database

START Strategic Arms Reduction Treaty

RDE Radiation Detection Equipment

NCRP National Council on Radiation Protection and Measurements

NUSTL National Urban Security Technology Laboratory