- Email: [email protected]
Electron component of the trapped radiation environment at altitudes below 1000 km, according to recent satellite data
Radiation Measurements, Vol. 26, No. 3, pp. 359-361, 1996
Pergamon Pll: S1350-4487(96)00011-X
Copyright © 1996 ElsevierScienceLtd Printed in Great Britain. All rights reserved 1350-4487/96 $15.00+ 0.00
ELECTRON COMPONENT OF THE TRAPPED RADIATION ENVIRONMENT AT ALTITUDES BELOW 1000 km, ACCORDING TO RECENT SATELLITE DATA M. I. PANASYUK, YU. V. MINEEV, E. D. TOLSTAYA and G. I. PUGACHEVA Skobeltsyn Institute of Nuclear Physics, Moscow State University, Moscow 119899, Russia A~traet--Discrepancies between experimentally registered energetic electron fluxes and model predictions at low altitudes are stated. The main factors which could induce such changes in the trapped radiation environment are discussed. Preliminary results on the development of a more modem low-altitude trapped electron model, utilizing relatively recent data registered on 'COSMOS-1686' and 'INTERCOSMOS-19' satellites, are reported and discrepancies with AE-8 predicted fluxes at these altitudes are discussed. Copyright © 1996 Elsevier Science Ltd
I. INTRODUCTION One of the main factors affecting the reliability and operation characteristics of low altitude spacecraft is the impact of energetic electrons and protons of the Earth's radiation belts. The designing of low-orbiting spacecraft is usually made with consideration for such characteristics as the overall radiation dose acquired during the mission, since this factor is critical for degradation of solar panels, electronic components and other spacecraft systems and sub-systems. The estimates of radiation doses for future missions are typically based on such currently available models of the trapped radiation environment as AE-8 (Vette, 1991) and AP-8 (Sawyer and Vette, 1976). However, it has now become obvious that these models need to be improved and updated. Significant discrepancies between electron fluxes, experimentally registered at altitudes < 1000 km, and model fluxes have been reported. At altitudes of 200-500 kin, for example, the predicted doses can exceed experimentally registered values by a factor of 10 and more (Stassinopoulous, 1989). A comparison of recent CRRES data on doses acquired behind shielding of different thicknesses (Mullen et al., 1989) also shows that the models not only overestimate the electron flux levels, but probably also fail to describe adequately the shape of the electron energy spectrum. The AE-8 and AP-8 models were based on experimental data acquired in the 1958-1970 time interval, with some electron data from geostationary satellites extending to 1978. Though some attempts were made, e.g. in AE-5 (Teague et al., 1972), to account for the slowly decaying Starfish electron population, it is still obvious that models based on recent data are needed to provide an accurate description of the electron flux values in the inner zone. It should also be mentioned that the time
period during which the bulk of the data used in AE-8 was acquired corresponds to an extremely weak solar cycle (in fact the weakest of the last six solar cycles). Solar cycle intensity is directly connected with atmospheric density at low altitudes and, therefore, has an extremely significant impact on particle lifetimes and flux values. For example, the atmospheric density for the maximum of cycle 21 at 600 km altitude was greater by a factor of three (approximately) than for the maximum of cycle 20 (Waltersheid, 1989). Secular changes of the terrestrial internal magnetic field will also impact the low altitude trapped particle flux intensities, though these changes are expected to be less than those due to atmospheric density effects. Another problem encountered in low-altitude trapped electron modelling is the choice of an appropriate coordinate system, since the electron flux values at these altitudes display a very steep altitude dependence (small changes of the magnetic field value B result in changes of the flux values by orders of magnitude) and, therefore, cannot be adequately described in terms of the B L coordinate system. Therefore additional parameters need to be introduced. Several useful variables such as longitude, average atmospheric density over the trapped particle trajectory and local time are being discussed, but no final conclusion has yet been reached (Lemaire et al., 1990)
2. L O W ALTITUDE ELECTRON MODEL, E M P L O Y I N G RECENT SATELLITE DATA We are now working on a more modern trapped electron model, employing relatively recent experimental data obtained by identical instruments on board two different spacecraft during 6 months of 1979 around solar maximum and 12 months of 1986 359
M . I . P A N A S Y U K et al.
360
3. C O M P A R I S O N W I T H AE.-8
around solar minimum. These spacecraft are INTERCOSMOS-19 and COSMOS-1686, respectively. INTERCOSMOS-19 was launched into an elliptical orbit in the altitude range of 500-1000 km at a 74 ° inclination. COSMOS-1686 had a circular orbit of 350 km with a 52 ° inclination, and later was raised to a 500 km altitude orbit. The common instrument was composed of a set of two Geiger counters and a semi-conductor spectrometer, registering electrons in the following energy channels (MeV): > 0.04, > 0.1 (Geiger), 0.3-0.6, 0.6-0.9, 0.9-1.2, and 1.2-2.0. The well stabilized INTERCOSMOS-19 carried one such instrument and COSMOS1686, which operated frequently in the spinning mode, had two instruments located perpendicularly. As already mentioned, the B,L (as well as B/Bo,L) is not sufficient for an adequate description of electron flux values at low altitudes. After considering various coordinate systems for arranging the data tables for modelling, we found that the most favourable, from the point of view of statistical error minimization, was the geographical system with a ten-degree grid in latitude at a given altitude. When averaging the data we selected time periods when geomagnetic conditions were quiet for approx, l0 days prior to the averaging time interval. We are hopeful that this technique will allow us to separate atmospheric density and radial diffusion effects at solar maximum. The preliminary version of our static electron model includes synoptic maps in the above-mentioned geographical grid of omnidirectional fluxes for the following threshold energies (MeV): 0.04, 0.1, 0.3, 0.6, 0.9, 1.2 and 2.0 at basic altitudes of 350, 500 and 800 km for both solar maximum and minimum. The 42 tables can be used to interpolate to other altitudes and energies.
-180 90
-150
-120
-90
-60
-30
Having come up with a preliminary version of the model we made a comparison of the electron environment according to our model and AE-8. Figure 1 shows the electron population at 500 k m - the closed lines denote electron fluxes of equal intensity (the numbers indicating the logarithms of the flux intensity values), according to AE-8. Figure 2 shows the electron population at the same altitude according to our model. The epoch corresponds to solar maximum. As can be seen from the two figures, there is a certain shift of the location of the Brazil anomaly region (the position where the highest flux values are observed). This is consistent with the current understanding of the impact of the secular variation of the terrestrial magnetic field. Another important difference is the fact that electrons, according to our model, are actually registered over a much broader area, especially in the southern hemisphere, than can be inferred from AE-8. This is significant, since these electrons are expected to contribute significantly to the overall dose acquired over a mission. Our calculations also show that the averaged trapped electron flux, estimated for the altitude of 500 km, is smaller by a factor o f - 2 for our model than for AE-8, whereas the precipitating electron flux is, on the contrary, somewhat larger. This underestimation of the precipitating electron flux could possibly be attributed to the inefficiency of the B/Bo coordinate system at low altitudes. It is also worth mentioning that our model gives lower electron flux values (compared to AE-8) in the inner radiation belt zone. The possible reason here could be the fact that inner zone flux values were never measured accurately enough prior to the Starfish explosion, and attempts to account for the slow decay of these
0
30
60
90
120
150
180 90
60
60
30 ~-
30
-30
,~ -30
_~ -90 -180
~
~
~
' -150
-120
-90
-
'
-90 -60
-30
0
30
60
90
120
150
180
Fig. 1. Electron fluxes E,0.3 MeV (1 cm-'- s) at 500 km altitude, solar maximum, according to AE8.
THE TRAPPED RADIATION ENVIRONMENT -180 90
-150
-120
-90
-60
-.30
0
30
60
90
361 120
150
180 90
60
60
30
30
0
0
-30
-30
-60
-60
-90 -180
-150
-120
-90
-60
-30
0
30
60
90
120
150
-90 180
Fig. 2. Electron fluxes E~0.3 MeV (1 cm -2 s) at 500 km altitude solar maximum, according to INP MSU model. electrons in the inner zone were difficult to verify. Another important feature which we can mention, but which still needs further verification, is the fact that, according to our model (utilizing data for the very intense 21 st solar cycle), the obtained flux values at altitudes ranging from 350 to 1000 km are lower for solar maximum, than for solar minimum. This situation is quite different from AE-8 predictions, which give, for example, that at 500 km the fluxes for solar maximum should be larger by a factor o f - 3 than for solar minimum. We think that the reason for such a difference between AE-8 and our model is due to atmospheric density increase at low altitudes in powerful solar cycles. 4. CONCLUSIONS The low-altitude trapped electron model, based on relatively recent satellite data which is now being developed at INP MSU, reveals certain changes in the trapped electron environment at altitudes below 1000 km. The major differences are: lower electron flux values in the inner zone, a much broader region with precipitating electrons, and lower flux values during an intensive solar maximum, than for solar minimum. We hope that further work on the model, involving increase of statistics and developing of adequate interpolation procedures, will result in a more accurate description of the current electron
RM 26/3-~D
environment at altitudes below I000 km, permitting us to make more precise estimates of the radiation environment for the broad class of low-altitude operating spacecraft.
REFERENCES Lemaire J. et aL (1990) Study of the Development of Improved Models of the Earth's Radiation Environment, ESA-CR(P)-3126. Final report of ESA contract 8011/88. Mullen E. G., Gussenhoven M. S. and Hardy D. A. (1989) The space radiation environment at 840 km. In: AIP Conference Proceedings 186, Vol. 5, pp. 329-342. American Institute of Physics, New York. Sawyer D. M. and Vette J. I. (1976) AP-8 Trapped Proton Environment for Solar Maximum and Solar Minimum, NSSDC/WDC-A-R and S 76-06, NASA TMX-72605. Goddard Space Flight Centre, Greenbelt, Md. Stassinopoulous E. G. (1989) Charged particle radiation exposure of Geocentric Satellites. In: AlP Conference Proceedings 186, Vol. 5, pp. 3-63. American Institute of Physics, New York. Teague M. J., Stein J. and Vette J. I. (1972) The Use of the Inner Zone Electron Model AE-5 and Associated Computer Programs, NSSDC/WDC-A-R and S 72-11, 9-10. Vette J. I. (1991) The AE-8 Trapped Electron Model, NSSDC/WDC-A-R and S 91-24. Waltersheid R. L. (1989) Solar cycle effects on the upper atmosphere: implications for satellite drag. J. Spacecraft and Rockets 26, 393.
Pergamon Pll: S1350-4487(96)00011-X
Copyright © 1996 ElsevierScienceLtd Printed in Great Britain. All rights reserved 1350-4487/96 $15.00+ 0.00
ELECTRON COMPONENT OF THE TRAPPED RADIATION ENVIRONMENT AT ALTITUDES BELOW 1000 km, ACCORDING TO RECENT SATELLITE DATA M. I. PANASYUK, YU. V. MINEEV, E. D. TOLSTAYA and G. I. PUGACHEVA Skobeltsyn Institute of Nuclear Physics, Moscow State University, Moscow 119899, Russia A~traet--Discrepancies between experimentally registered energetic electron fluxes and model predictions at low altitudes are stated. The main factors which could induce such changes in the trapped radiation environment are discussed. Preliminary results on the development of a more modem low-altitude trapped electron model, utilizing relatively recent data registered on 'COSMOS-1686' and 'INTERCOSMOS-19' satellites, are reported and discrepancies with AE-8 predicted fluxes at these altitudes are discussed. Copyright © 1996 Elsevier Science Ltd
I. INTRODUCTION One of the main factors affecting the reliability and operation characteristics of low altitude spacecraft is the impact of energetic electrons and protons of the Earth's radiation belts. The designing of low-orbiting spacecraft is usually made with consideration for such characteristics as the overall radiation dose acquired during the mission, since this factor is critical for degradation of solar panels, electronic components and other spacecraft systems and sub-systems. The estimates of radiation doses for future missions are typically based on such currently available models of the trapped radiation environment as AE-8 (Vette, 1991) and AP-8 (Sawyer and Vette, 1976). However, it has now become obvious that these models need to be improved and updated. Significant discrepancies between electron fluxes, experimentally registered at altitudes < 1000 km, and model fluxes have been reported. At altitudes of 200-500 kin, for example, the predicted doses can exceed experimentally registered values by a factor of 10 and more (Stassinopoulous, 1989). A comparison of recent CRRES data on doses acquired behind shielding of different thicknesses (Mullen et al., 1989) also shows that the models not only overestimate the electron flux levels, but probably also fail to describe adequately the shape of the electron energy spectrum. The AE-8 and AP-8 models were based on experimental data acquired in the 1958-1970 time interval, with some electron data from geostationary satellites extending to 1978. Though some attempts were made, e.g. in AE-5 (Teague et al., 1972), to account for the slowly decaying Starfish electron population, it is still obvious that models based on recent data are needed to provide an accurate description of the electron flux values in the inner zone. It should also be mentioned that the time
period during which the bulk of the data used in AE-8 was acquired corresponds to an extremely weak solar cycle (in fact the weakest of the last six solar cycles). Solar cycle intensity is directly connected with atmospheric density at low altitudes and, therefore, has an extremely significant impact on particle lifetimes and flux values. For example, the atmospheric density for the maximum of cycle 21 at 600 km altitude was greater by a factor of three (approximately) than for the maximum of cycle 20 (Waltersheid, 1989). Secular changes of the terrestrial internal magnetic field will also impact the low altitude trapped particle flux intensities, though these changes are expected to be less than those due to atmospheric density effects. Another problem encountered in low-altitude trapped electron modelling is the choice of an appropriate coordinate system, since the electron flux values at these altitudes display a very steep altitude dependence (small changes of the magnetic field value B result in changes of the flux values by orders of magnitude) and, therefore, cannot be adequately described in terms of the B L coordinate system. Therefore additional parameters need to be introduced. Several useful variables such as longitude, average atmospheric density over the trapped particle trajectory and local time are being discussed, but no final conclusion has yet been reached (Lemaire et al., 1990)
2. L O W ALTITUDE ELECTRON MODEL, E M P L O Y I N G RECENT SATELLITE DATA We are now working on a more modern trapped electron model, employing relatively recent experimental data obtained by identical instruments on board two different spacecraft during 6 months of 1979 around solar maximum and 12 months of 1986 359
M . I . P A N A S Y U K et al.
360
3. C O M P A R I S O N W I T H AE.-8
around solar minimum. These spacecraft are INTERCOSMOS-19 and COSMOS-1686, respectively. INTERCOSMOS-19 was launched into an elliptical orbit in the altitude range of 500-1000 km at a 74 ° inclination. COSMOS-1686 had a circular orbit of 350 km with a 52 ° inclination, and later was raised to a 500 km altitude orbit. The common instrument was composed of a set of two Geiger counters and a semi-conductor spectrometer, registering electrons in the following energy channels (MeV): > 0.04, > 0.1 (Geiger), 0.3-0.6, 0.6-0.9, 0.9-1.2, and 1.2-2.0. The well stabilized INTERCOSMOS-19 carried one such instrument and COSMOS1686, which operated frequently in the spinning mode, had two instruments located perpendicularly. As already mentioned, the B,L (as well as B/Bo,L) is not sufficient for an adequate description of electron flux values at low altitudes. After considering various coordinate systems for arranging the data tables for modelling, we found that the most favourable, from the point of view of statistical error minimization, was the geographical system with a ten-degree grid in latitude at a given altitude. When averaging the data we selected time periods when geomagnetic conditions were quiet for approx, l0 days prior to the averaging time interval. We are hopeful that this technique will allow us to separate atmospheric density and radial diffusion effects at solar maximum. The preliminary version of our static electron model includes synoptic maps in the above-mentioned geographical grid of omnidirectional fluxes for the following threshold energies (MeV): 0.04, 0.1, 0.3, 0.6, 0.9, 1.2 and 2.0 at basic altitudes of 350, 500 and 800 km for both solar maximum and minimum. The 42 tables can be used to interpolate to other altitudes and energies.
-180 90
-150
-120
-90
-60
-30
Having come up with a preliminary version of the model we made a comparison of the electron environment according to our model and AE-8. Figure 1 shows the electron population at 500 k m - the closed lines denote electron fluxes of equal intensity (the numbers indicating the logarithms of the flux intensity values), according to AE-8. Figure 2 shows the electron population at the same altitude according to our model. The epoch corresponds to solar maximum. As can be seen from the two figures, there is a certain shift of the location of the Brazil anomaly region (the position where the highest flux values are observed). This is consistent with the current understanding of the impact of the secular variation of the terrestrial magnetic field. Another important difference is the fact that electrons, according to our model, are actually registered over a much broader area, especially in the southern hemisphere, than can be inferred from AE-8. This is significant, since these electrons are expected to contribute significantly to the overall dose acquired over a mission. Our calculations also show that the averaged trapped electron flux, estimated for the altitude of 500 km, is smaller by a factor o f - 2 for our model than for AE-8, whereas the precipitating electron flux is, on the contrary, somewhat larger. This underestimation of the precipitating electron flux could possibly be attributed to the inefficiency of the B/Bo coordinate system at low altitudes. It is also worth mentioning that our model gives lower electron flux values (compared to AE-8) in the inner radiation belt zone. The possible reason here could be the fact that inner zone flux values were never measured accurately enough prior to the Starfish explosion, and attempts to account for the slow decay of these
0
30
60
90
120
150
180 90
60
60
30 ~-
30
-30
,~ -30
_~ -90 -180
~
~
~
' -150
-120
-90
-
'
-90 -60
-30
0
30
60
90
120
150
180
Fig. 1. Electron fluxes E,0.3 MeV (1 cm-'- s) at 500 km altitude, solar maximum, according to AE8.
THE TRAPPED RADIATION ENVIRONMENT -180 90
-150
-120
-90
-60
-.30
0
30
60
90
361 120
150
180 90
60
60
30
30
0
0
-30
-30
-60
-60
-90 -180
-150
-120
-90
-60
-30
0
30
60
90
120
150
-90 180
Fig. 2. Electron fluxes E~0.3 MeV (1 cm -2 s) at 500 km altitude solar maximum, according to INP MSU model. electrons in the inner zone were difficult to verify. Another important feature which we can mention, but which still needs further verification, is the fact that, according to our model (utilizing data for the very intense 21 st solar cycle), the obtained flux values at altitudes ranging from 350 to 1000 km are lower for solar maximum, than for solar minimum. This situation is quite different from AE-8 predictions, which give, for example, that at 500 km the fluxes for solar maximum should be larger by a factor o f - 3 than for solar minimum. We think that the reason for such a difference between AE-8 and our model is due to atmospheric density increase at low altitudes in powerful solar cycles. 4. CONCLUSIONS The low-altitude trapped electron model, based on relatively recent satellite data which is now being developed at INP MSU, reveals certain changes in the trapped electron environment at altitudes below 1000 km. The major differences are: lower electron flux values in the inner zone, a much broader region with precipitating electrons, and lower flux values during an intensive solar maximum, than for solar minimum. We hope that further work on the model, involving increase of statistics and developing of adequate interpolation procedures, will result in a more accurate description of the current electron
RM 26/3-~D
environment at altitudes below I000 km, permitting us to make more precise estimates of the radiation environment for the broad class of low-altitude operating spacecraft.
REFERENCES Lemaire J. et aL (1990) Study of the Development of Improved Models of the Earth's Radiation Environment, ESA-CR(P)-3126. Final report of ESA contract 8011/88. Mullen E. G., Gussenhoven M. S. and Hardy D. A. (1989) The space radiation environment at 840 km. In: AIP Conference Proceedings 186, Vol. 5, pp. 329-342. American Institute of Physics, New York. Sawyer D. M. and Vette J. I. (1976) AP-8 Trapped Proton Environment for Solar Maximum and Solar Minimum, NSSDC/WDC-A-R and S 76-06, NASA TMX-72605. Goddard Space Flight Centre, Greenbelt, Md. Stassinopoulous E. G. (1989) Charged particle radiation exposure of Geocentric Satellites. In: AlP Conference Proceedings 186, Vol. 5, pp. 3-63. American Institute of Physics, New York. Teague M. J., Stein J. and Vette J. I. (1972) The Use of the Inner Zone Electron Model AE-5 and Associated Computer Programs, NSSDC/WDC-A-R and S 72-11, 9-10. Vette J. I. (1991) The AE-8 Trapped Electron Model, NSSDC/WDC-A-R and S 91-24. Waltersheid R. L. (1989) Solar cycle effects on the upper atmosphere: implications for satellite drag. J. Spacecraft and Rockets 26, 393.