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Adv. Space Res. Vol. 29, No. 10, pp. 1553-1559.2002 0 2002 COSPAR. Published by Elsevier Science Ltd. All rights reserved

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TIGER-PROGRAM FOR THERMOSPHERIC-IONOSPHERIC GEOSPHERIC RESEARCH

Long-term measurement of solar EUVKJV fluxes for thermospheric-ionospheric (T/I) modelling and for space weather investigations

G. Schmidtke’, W. K. Tobiska*, and D. Winningham

IIPM Freiburg, Heidenhofstr. 8, 79110 Freiburg, Germany, (gerhard.schmidtke@ipm.jhg.de) 2Federal Data Corporation/Jet Propulsion Laboratory, Pasadena, CA 91109, U.S.A.,

(ktobiska@popjpl.nasa.gov) 3Southwest Research Institute, 6220 Culebra Road, San Antonio, Texas 78238, U.S.A.

(david@cluster.space.swri.edu)

ABSTRACT

On 18/19 June 1998 the 1st TIGER Symposium was held in Freiburg/Germany. After presentation and discussion of 28 invited and contributed talks, the symposium agreed to establish a longterm TIGER Program within the framework of the SCOSTEP International Solar Cycle Study Working Group 1, Panel 2. This decision is based on the general agreement that, for thermospheric-ionospheric research as well as for a broad range of commercial applications in space, the improvement of existing thermospheric- ionospheric (T/I) models is absolutely necessary to meet scientific goals. There are a number of scientific questions underlying the goal of understanding solar EW/UV variability such as what are the primary mechanisms by which solar ultraviolet (UV), extreme ultraviolet (EW), and soft X-ray (XW) n-radiance variations affect terrestrial global climate change and/or weather and what is their significance? How does solar forcing compare with that from other sources such as increasing concentrations of radiatively-active gases and atmospheric aerosols? How sensitive is the Earth’s climate to changes in solar radiation? What time scales of solar variability are significant to climate ? How might solar variability in these wavelengths affect global warming projections? Are there signatures of solar influences in the upper atmosphere that are distinct from anthropogenic effects?

To meet these goals, coordinated work on the following topics is required and is discussed in detail below:

1. Measurement and modelling of solar E-W/W radiation

2. Measurement and modelling of the solar wind (particles) originated energy T/I influx

3. Measurement of relevant thermospheric-ionospheric parameters

4. Modelling of the thermosphere/ionosphere

5. Fundamental physical investigations of photoenergetic atomic and molecular processes

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1554 G. Schmidtke et al.

To make substantial progress in developing a complete understanding of the T/I processes, it is necessary to envisage solar cycle and longer timescales. This can be done in the global change context by making use of a broad range of worldwide existing resources with respect to manpower, experience, hardware, methods, flight opportunities, and funding resources. The TIGER Program aims to facilitate the coordination of these existing and planned activities and to help define missing links for achieving the scientific goals. 0 2002 COSPAR. Published by Elsevier Science Ltd. All rights reserved.

MEASUREMENT AND MODELLING OF SOLAR EWKJV RADIATION

The most serious disadvantage of the existing T/I models consists in the use of the proxy F10.7 (radio flux at 10.7 cm) instead of the solar extreme ultraviolet (EWW) radiation that is the primary energy source driving the T/I system. Since the solar radio emission F10.7 is generated by a variety of different physical processes localized in the solar photosphere, chromosphere, and corona, the correlation with the solar EWA-JV fluxes is inadequate for modem requirements. Yet, T/I modelers and operations continue to use F10.7 solely for convenience. For special T/I applications there have been numerous other proxies (such as Sun Spot Number) developed and they each have disadvantages as well.

On the other hand, the EWAJV data sets measured in the past do not yet meet the necessary radiometric accuracy due to technological limitations. Though it is not possible to present reliable numbers for the accuracy of the measurements achieved so far, the most quoted estimates range from 20 to 100 %, depending on the wavelength range. Soon-to-be launched instruments will change this situation. With the recent development of better calibrated EW/UV spectrometers, including those using true in-flight calibration capabilities, and with the high potential of reliable, low-cost, broad-band EW detectors to be flown on planned spacecraft, researchers will soon have high-quality EW/W data which meet the requirements of current T/I research and applications.

The TIGER Program can achieve the goal of coordinating higher accuracy, precision measurements within the given framework of existing resources. The EWAJV experiments on the SOHO, SNOE, TIMED, and PHOTON satellite missions plus planned measurements from others like the EW Patrol Mission, the Solar Package (SOVIM, SOLSPEC, and SOL-ACES) for the International Space Station (ISSA), and GOES broadband EW detectors will provide a good opportunity to collect accurate, overlapping solar flux data during the period of an entire solar cycle provided that these missions are well coordinated.

Measurements of Solar EWKJV Radiation

Spectrophotometric satellite survey EWAJV observations were conducted since the 1960’s. Following the end of the Atmospheric Explorer-E measurements in December 1980 (Hinteregger et al. 1973), there have not been daily solar EW n-radiance measurements except for approximately 20 days during the San Marco 5 satellite mission (Schmidtke et al., 1992). Donnelly (1987) refers to this lack of solar EW spectral irradiance measurements as the “solar EW hole” (see Figurel).

However, it is important to note that for wavelengths 4 10 nm no satellite spectrometer has been calibrated in-flight, yet. Cross-calibration by rocket spectrometers was applied after 1972.

In the spectral range of l-200 nm the measurements reveal the highly variable solar EW/IJV radiation that will be measured for an entire solar cycle with a spectral resolution of ~1 mn from l-100 mn and of ~2 nm from 100-200 nm and with a radiometric accuracy of 40 %. This accuracy would mean a fundamental improvement, because it would be measured in-flight. To meet this ambitious goal a combination of two types of instruments will be flown covering the period of 11 years. Some of these include sophisticated EWAJV spectrometers (TIMED/SEE and the ISSABOL-ACES) and low-cost

TIGER-Program 1555

broad-band sensors (SNOE, PHOTON, GOES). Whereas the low-cost instruments can be flown to cover a solar cycle with overlapping periods, the more expensive spectrometers will provide data of higher spectral resolution and of higher radiometric accuracy and will recalibrate the broad-band sensors. There are plans to refly the ISSA-Solar Package including SOL-ACES at different solar activity levels that can only be done aboard the ISS. In addition to the application of true in-flight calibration with this instrument the variable straylight background and higher spectral order efficiencies can be periodically determined by in-flight measurements to achieve the highest possible radiometric accuracy. These calibration measurements are mandatory for future determination of the solar EW/W n-radiance.

t 200

150

E ; 100 ‘n As ; 50 3 EUV hole II

0 1967 1970 1975 1980 1985 1990 1995 1998

time/yaer .-+

Time I wavelength of solar coverage of solar flux measurement from satellites by spectrometers

Fig. 1. Time/wavelength coverage of solar EWKJV spectrophotometric measurements from satellites

A close cooperation between the groups conducting EWKJV experiments is indispensable, especially

l to intercompare the calibration procedures on the ground,

l to apply the same algorithms to the data evaluation,

l to provide standardized data structures (e.g., time steps and wavelength bins),

l to select the same wavelength ranges for the broad-band measurements, etc.

Empirical Modellinn of the Solar EWKJV h-radiance

There are several empirical models that represent solar EW/W fluxes for periods of missing measurements. In the context of the TIGER Program, existing models (see Figure 2) will be intercompared and their experience transferred to an upgraded model that can be used as a primary energy input into T/I models, for example. The new model will be able to incorporate new EWAJV data to continually improve the accuracy of the model, will be able to relate ground-based images to EW emissions, and will be easily accessible via a web interface. Because the spectrometer and broadband instruments will measure EW/W u-radiances at different times with different spectral resolutions, the new model will also be used to intercompare the datasets so as to achieve a common data format.

1556 G. Schmidtke efal.

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.,.. .wf?: :.._: 11.1 1,: ,‘l,., ,‘I 1: 1,: 1,. Fig. 2. Six aeronomically important EW wavelengths modeled for solar cycles 21 and 22 and predicted for solar cycle 23. Solar cycle maximum and minimum values are shown in each panel as are the max/min ratios based on daily-average values. Photon flux is in units of photons cm-* s-’ nm-’ (Tobiska and Eparvier, 1998).

Phvsical Modellinn of Solar EWAJV Emissions

In parallel with the improvement of empirical EW/W models, solar EWKJV radiation is being modelled from first principles of solar physics. Numerical and empirical models will both be improved by comparison with one another and with the solar EWKJV measurements in addition to visible and radio solar observations from the ground.

Introduction of Solar EW Indices

Different applications of the measured and modelled solar EWRJV fluxes require different representation or their weighted combination. One method of representing the flux is with indices in the form of EW, and EW,_, in units of pW rn-* (Schmidtke, 1976). For example, EW,,,, = 572 represents the flux of 572 pW m’* (He II emission at 30.38 nm), whereas EW,,,,, = 3372 quantifies the energy in the range from 103- 16 mn. Any single emission line EW, can be combined with any range EW,,.

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IS0 EUV Standard

Participants in the TIGER program are actively engaged in the development of an International Standards Organization (ISO) EUV standard. This standard EUV (both spectra and model) is being developed through international collaboration at all levels and will result in an internationally recognized standard for use by the aerospace, aeronomy, and astronomy communities.

MODELLING OF THE SOLAR WIND (PARTICLES) ORIGINATED ENERGY T/I INFLUX

The energy originating from the solar wind particles is considered the second energy input source to the T/I system. This energy source can be quantified for the physical modelling of the T/I system. Program members are developing an empirical model of the energetic particle flux. In addition, it may be possible to provide a rough estimate for this energy, derived on a geospherical scale, from satellite optical observations of aurora1 emissions. Similar to the low-cost solar instruments, there are recent developments for low-cost airglow instruments which combine limb scanning and column measuring airglow spectrometers. Participants in the TIGER Program will investigate this and other possibilities taking into account LIARS, SOHO, and other observations.

For example, a particle climatology being built using measurements from the UARS Particle Environment Monitor (PEM) is reported by Sharber et al. (1998, this issue). This climatology is an empirical statistical model built using the PEM particle database from which the user may obtain average spectral characteristics, precipitating particle fluxes, and ionization rate profiles as functions of latitude, local time, and activity level. A unique aspect of this model is the inclusion of differential measurements of high energy fluxes that extend to the -MeV range. The activity levels include both planetary and solar indices. Each plot is presented in invariant latitude (40 deg - 90 deg) and magnetic local time (0 - 24 h). One application of the climatological model is to provide inputs to global circulation and assimilative models. Further details are provided by Sharber et al. (2002).

MEASUREMENT OF RELEVANT THERMOSPHERIC-IONOSPHERIC PARAMETERS

Remote optical sensing from satellites and from other space platforms, GPS occultation, ground observations and many other measurements are powerful tools to derive thermospheric-ionospheric parameters on a geospherical scale. With the availability of low-cost instrumentation to measure relevant geospheric T/I parameters, it is possible to coordinate on-going activities and to accomplish a complete overlap of an entire solar cycle.

MODELLING OF THE THERMOSPHERE/IONOSPHERE

There are several T/I models of which the empirical models are mostly used. The improvement of these models and the further development of first principles T/I models is part of the activity conducted by participants in this program. The T/I models can be applied to forecasts of low and medium altitude spacecraft orbits including MIR and future Space Stations such as ISS, to space communication and navigation, as well as to space weather research.

Emuirical and Phvsical Modellinn of the Thermosnhere/Ionosuhere

All T/I aeronomical models can benefit from more accurate measured or modeled data sets of solar EUV/UV fluxes and of relevant T/I parameters. In addition, physical parameters such as high resolution

1558 G. Schmidtke e-f al.

photoabsorption cross sections, electron impact cross sections, rate coefficients, etc. need to be known to higher accuracy. These can be obtained by laboratory programs which measure absorption cross sections of upper atmosphere species for temperature regimes representing the range of solar minimum to solar maximum. The thermosphere temperature regimes vary with altitude and solar cycle from 200-1000 K in the T/I airglow layers, nominally found between 90 and 180 km. Species of particular importance to T/I energy deposition in the EW/W are molecular oxygen and nitrogen and atomic oxygen.

In special applications, for example navigation, space communications and high altitude RADAR measurements, the column density of electrons (TEC) should be known with high accuracy. Since the TEC strongly depends on the solar EWW flux, improved modeling and analysis of the TEC data can be provided in the context of this program.

_

BASIC PHYSICAL INVESTIGATIONS TO SUPPORT THE PRECEDING TOPICS

Special laboratory and space investigations shall support space measurements atmospheric EW/W emissions and the modelling activities.

Development of Primarv and Secondary EW/W Detector Standards

of the solar and

Ionization chambers and coated silicon diodes are key components to be used as primary and as secondary space detector standards, respectively. In combination with thin film filtering, these techniques need further investigation to improve stability and endurance in space and to support the continuing trend of low-cost instrumentation development.

Effective Absorption Cross Sections: Effect of Radiation Hardening

In the spectral range of about 60-140 nm the temperature-dependent rotational-vibrational structure of the photoabsorption spectra should be taken into account in the physical modelling algorithms. These spectra have to be folded with the half-width of the corresponding solar emissions that change with solar activity. The corresponding effective cross sections have not yet been determined with sufficient accuracy. This can be achieved by using sounding rockets with EW/UV spectrometers to be flown during typical solar activity conditions as well as by laboratory measurements and theoretical work.

Effects Causing Efficiencv Changes in EW/W Instruments

In order to improve the primary components of EWAJV spectrometers (detectors and gratings) so that they show as little degradation as possible, the fundamental physical effects causing these changes should be further investigated. This is seen as an important activity within the scope of this program.

Common Use of Calibration Eauipment/Procedures

In the past, it was quite difficult to explain discrepancies in the EW/W data from different groups. To improve this situation, the calibration procedures on the ground and the algorithms used in data evaluation will be intercompared and modified. This will lead to a standardized data structure, especially if the same wavelength ranges for the broad-band measurements are to be selected.

The prioritization of these topics will be achieved during the course of the TIGER Program.

TIGER-Program 1559

REFERENCES

Donnelly, R.F., in Solar Radiative Output Variation, edited by P. Foukal; pp. 139, Cambridge Research and Instrumentation Inc, Cambridge, Massachusetts (1987).

Hinteregger, H.E., D.E. Bedo, and J.E. Manson, Radio Science, 8,349 (1973). Schmidtke, G., Geophys. Res. Lett., 3, 573 (1976). Schmidtke, G., T.N. Woods, J. Worden, G.J. Rottman, H. Doll, C. Wita, and S.C. Solomon, Geophys.

Rex Lett., 19, 2175 (1992). Sharber, J. R., J. D. Winningham, R. A. Frahm, G. Crowley, A. J. Ridley, and R. Link, Construction of a

particle climatology for the study of the effects of solar particle fluxes on the atmosphere, Advances in Space Research, this issue (2002).

Tobiska, WK., and F.G. Eparvier, Solar Phys, 177, 147 (1998).