33RD INTERNATIONAL COSMIC RAY CONFERENCE, RIO DE JANEIRO 2013THE ASTROPARTICLE PHYSICS CONFERENCE

A catalog of Forbush decreases of the cosmic radiation for the period 1997-2007O.O MUSALEM-RAMIREZ1, J.F. VALDES-GALICIA1, G. MUNOZ2 , E. HUTTUNEN3,1 Instituto de Geofısica, Universidad Nacional Autonoma de Mexico, Mexico D.F. MEXICO.2 Escuela Superior de Ingeniera Mecnica y Elctrica, Instituto Politcnico Nacional, Mxico.3 Faculty of Science, Department of Physical Sciences, University of Helsinki, Helsinki, Finland.

omar@geofisica.unam.mx

Abstract: We developed a catalog of Forbush decreases (Fd), comprising the major part of solar cycle 23 (1996-2008). With the available data (interplanetary data kindly provided by R. Schwenn) we identified three differentinterplanetary phenomena that could be associated with the Fd: Interplanetary Coronal Mass Ejections (ICME),Corrotating interaction regions (CIR), and Interplanetary shocks (IS); additionally there was a set of Fd notassociated with any of the three phenomena identified. A discussion of plausible causes for this fourth set willbe presented.To analyze the Fds, we used data from three neutron monitors representing low, medium and highcut-off: Oulu (Finland), Moscow (Russia) and Mexico City Images of the catalog contain solar wind data plots(magnetic field and plasma data), the Kp index and plots of the cosmic ray intensity in the three stations mentioned.

Keywords: Cosmic ray, Forbush decrease

1 IntroductionThe Fd is surely the most spectacular cosmic ray time varia-tion. In the span of a few hours, the intensity recorded at aparticular station can be reduced from 5% up to 20 or 30%in extreme cases. After reaching a minimum, the intensitystarts to recover slowly, recovery that may last from daysto weeks. In contrast to the diurnal variation, one Fd is reg-istered almost simultaneously all over the world. At firstit was thought that Fd were associated to purely terrestrial(geomagnetic field variations, geomagnetic storms, etc..).In general, Fd are associated with the passage of transientstructures that come from the Sun. We can roughly identifythree stages of a Fd:

• Start. - Beginnings are almost simultaneous world-wide. This simultaneity is also observed in magneticstorms sudden onset of SSC (Sudden Storm Com-mencement), occurring almost always just before oraround the start.

• Amplitude. - The amplitude of the Fd is related tothe rigidity threshold atmospheric depth and type ofdetector of a particular station.

• Recovery. - The recovery phase lasts several daysand sometimes weeks. It may or may not be regular,depending on whether they have small variations.The motivation for this work, was to have a list of Fdduring a solar cycle, to determine their behavior andits relationships with solar-interplanetary phenomena.

2 Database Forbush decreasesWe developed a database of Fd,s that occurred during theperiod 1997 to 2007. Three observatories were chosen,located at different latitudes, so that they have differentmagnetic rigidity and cones of acceptance, to observedifferences between the magnitude and profiles of Fd. Theobservatories were:

1. Oulu, located in Finland, with a geographical locationof 65.05 N, 25.47 E and a cutoff rigidity 0.8 GeV.Detector type: 24-NM64. [20]

2. Moscow, located in Russia, with a geographicallocation of 55.47 N 37.32 E and a cutoff rigidity of2.43 GeV. Detector type: 6-NM64. [21]

3. Observatory ”Dr. Javier Otaola Lizarzaburu ”, locatedin Mexico City, with a geographical location of 19.19N and 99.11 W and a cutoff rigidity of 8.2 GeV.Detector type: 6-NM64. [22]

We proceeded to analize the graphs that these observa-tories had on internet. As a base, to define the existence ofan FD, we took the data from Oulu, since it has the low-est cutoff rigidity, making it more sensitive to interplan-etary perturbations. We analyzed day by day from 1997to 2007 and gathered information from the Fd, their starttimes, magnitude of the decrease and approximate recoverytime. For each of the Fd, corrected hourly data were down-loaded from the observatories. All data, were normalized tothe average intensity of the corrected data during the monthof June 1989. For the normalization, we only consider thecounts, because it was only made to adjust the data intoa single graph. At this stage we are not going to considertypes of detectors or other factors. These values are:

• Oulu: 5445.65 counts/hour, at 101497.2 Pa.

• Moscow: 7810.33 counts/hour, at 98893.2 Pa.

• Mexico: 76487 counts/hour, at 77817.6 Pa.

To proceed with the analysis, we used the IDL protocol.With the program generated, we obtained plots similar tothat shown on Figure 1 1.

The second step was to obtain information on the behav-ior of the interplanetary medium in the days before, duringand after the Fd, in order to identify the physical phenom-ena associated with them. In addition, we obtained infor-mation on the status of the Earth’s magnetosphere through

Catalog of Forbush decreases33RD INTERNATIONAL COSMIC RAY CONFERENCE, RIO DE JANEIRO 2013

Figure 1: Fd on 17/Jul/2002 starting at 15:00. Data from thethree observatories are plotted on a single graph. Moscowdata were multiplied by 0.97 to avoid overlapping with thegraph of Oulu. Data was plotted using IDL.

the Kp index. We also looked for geomagnetic storms withSSC, to study the relationship of these phenomena with dFand indicate on the graphs when submitted. We proceededto contrast our Fd results, with the database of Huttunen[11]. This database contains information on the time of ar-rival to Earth, plasma velocity and some other parametersrelated to the passage of ICME, CIR and shocks, observedby different satellites in nearby orbits around our planet.The database of interplanetary phenomena, provided thebasis for associating Fd, with extreme events of the inter-planetary medium. The criterion used for the associationwas that it had observed the passage of any ICME, CIRor an interplanetary shock within 12 hours before or afterthe start of a Fd. Thus, from the 123 Fd detected between1997 and 2007, 78 could be associated with ICMEs , 18with CIRs and 17 with shocks. The remaining 10 could notbe associated with any of the phenomena considered withthe established criteria. Once the information was collectedand organized, the data on the intensity of cosmic rays atthe three stations mentioned, the parameters of the inter-planetary medium, Kp index and SSC, are joined togetheron a single plot like that shown on Fig 3. Additionally acatalog of Fds was integrated where the starting time anddepth of the decrease are shown, together with the impor-tant times of the interplanetary phenomena to which theyare associated. The catalog consists of four different tablesas mentioned above; an example is presented in Figure 2 2

3 Results and conclusionsThe purpose of this work was to develop a catalog ofForbush decreases from 1997 to 2007, comprising almost allthe solar cycle 23. In the listings of the catalog, we used onlyinformation from Oulu and Mexico, as they have the lowestand highest cutoff rigidity respectively. As conclusions ofthis work, we can enunciate the following: The Fd detectedat a lower cutoff rigidity (Oulu) were deeper than thosedetected in higher cutoff (Mexico), as expected. The twolargest decreases detected: July 13, 2000 (Bastille Dayevent) and October 29, 2003 (Halloween event) are relatedto ICME’s. This is also true for most of the largest Fdsobserved in the period. More Fds associated with ICME’s,were observed during solar maximum (2000-2001), with 14

Figure 2: Part of the Fd catalog. Showing the first page ofFd list, associated with ICMEs.

Figure 3: Image of the Df from 6/Nov/2001 at 0:00, associ-ated to an ICME. In the upper part is the graph of the Fd. Inthe lower-left the Kp index. In the lower-mid the interplane-tary magnetic field (Magnetic field strength and the threecomponents of B in GSE). In the lower-right the solar wind(proton density, velocity, temperature and pressure).

Catalog of Forbush decreases33RD INTERNATIONAL COSMIC RAY CONFERENCE, RIO DE JANEIRO 2013

in 2000 and 16 in 2001, that in the minimum phases (1997and 2007), with 5 in 1997 and 1 in 2007. A greater numberof ICME’s from the Sun during the maximum period,consistent with what was theoretically established accordingto the ratio of the number of ICME with respect to the phaseof the solar cycle. We have a total of 78 Fd observed in Ouluassociated with an ICME, , of those, 70 occurred beforethe passage of the ICME and 8 after. For Mexico, we havea total of 75, of which 66 occurred before the passage ofthe ICME and 9 after. For Fd associated with CIR, we notethat the number of these increased in the descending phaseof the solar cycle (2004-2007), agreeing with the generalbehavior of the Sun so that the CIR tend to be produced inlarger numbers towards the the minimim . Very few CIRdeveloped shocks. Oulu had detected 18 of these Fd, ofwhich 5 ocurred before the passage of the CIR and 13 after.For Mexico it had 13 Fd, of which 4 ocurred before and9 after. For Fd associated with interplanetary shocks, wenote that 88% of cases had at least one SSC associated. ForFd associated with interplanetary shocks, we have a totalof 18, in all three stations. For Fd that were not associatedwith any event, we see that their decreases are very low (¡3% in Oulu). The analysis of these Fd is very interesting,since there is no large-scale solar-interplanetary phenomenaassociated to the CR decrease. A detailed analysis of theinterplanetary medium, to try to identify the possible causesof these Fd is underway. We can see that 91% of Fd areassociated with some interplanetary phenomena caused bythe Sun, and of these, 69% are associated with ICME’s. It isconcluded that most of the Fd detected on Earth are causedby some kind of solar activity, ICME’s are the phenomenathat cause the greatest number of Fd and also those withlarger decrease.

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