The Long Duration Exposure Facility (LDEF) spacecraft flew in a 28.5 deg inclination circular orbit with an altitude in the range from 319.4 to 478.7 km. For this orbital altitude and inclination, two components contribute most of the penetrating charge particle radiation encountered - the galactic cosmic rays and the geomagnetically trapped Van Allen protons. Where shielding is less than 1.0 g/sq cm geomagnetically trapped electrons make a significant contribution. The 'Vette' models together with the associated magnetic field models and the solar conditions were used to obtain the trapped electron and proton omnidirectional fluences reported previously. Results for directional proton spectra using the MSFC anisotropy model for solar minimum and 463 km altitude (representative for the LDEF mission) were also reported. The directional trapped proton flux as a function of mission time is presented considering altitude and solar activity variation during the mission. These additional results represent an extension of previous calculations to provide a more definitive description of the LDEF trapped proton exposure

Armstrong, T. W.

Colborn, B. L.

Watts, John W.

English

NASA Technical Reports Server

N93-29633
REVISED PREDICTION OF LDEF EXPOSURE TO TRAPPED PROTONS
John W. Watts
ES62, NASA/Marshall Space Flight Center
AL 35812
Phone: 205/544-7696; Fax: 205/544-7754
T. W. Armstrong and B. L. Colborn
Science Applications International Corporation
Route 2, Prospect, TN 38477
Phone: 615/468-2603; Fax: 615/268-2676
SUMMARY
The LDEF spacecraft flew in a 28.5 ° inclination circular orbit with an altitude in the range
from 319.4 to 478.7 km. For this orbital altitude and inclination two components contribute most
of the penetrating charge particle radiation encountered--the galactic cosmic rays and the
geomagnetically trapped Van Allen protons. Where shielding is less than 1.0 g/cm 2
geomagnetically trapped electrons make a significant contribution. The "Vette" models (refs. 1-3)
together with the associated magnetic field models (ref. 4) and the solar conditions were used to
obtain the trapped electron and proton omnidirectional fluences reported previously (ref. 5).
Results for directional proton spectra using the MSFC anisotropy model for solar minimum and
463 km altitude (representative for the LDEF mission) were also reported (ref. 6). Here the
directional trapped proton flux as a function of mission time is presented considering altitude and
solar activity variation during the mission. These additional results represent an extension of
previous calculations to provide a more definitive description of the LDEF trapped proton
exposure.
INTRODUCTION
The LDEF spacecraft flew in a 28.5 ° inclination circular orbit with an altitude in the range
from 319.4 to 478.7 km. It was gravity-gradient stabilized and oriented so that one side always
pointed along the velocity vector. For this orbital altitude and inclination two components
contribute most of the penetrating charge particle radiation encountered--the galactic cosmic rays
and the geomagnetically trapped Van Allen protons. Where shielding is less than 1.0 g/cm 2
geomagnetically trapped electrons make a significant contribution. All three sources are strongly
modulated by the Earth's magnetic field with the trapped flux being anisotropic with most of the
flux arriving from a narrow band perpendicular to the local geomagnetic field direction. A model
for predicting the trapped proton angular distribution has been developed (ref. 7) including both the
pitch angle and east-west effects. Since trapped protons produce most of the spacecraft activation
except at heavily shielded locations and almost all of the dose at most LDEF measurement
locations, a large part of calculational effort (refs. 6 and 8) of the LDEF Ionizing Radiation Special
Interest Group has been directed toward testing the predictions of this model and the Vette
137
https://ntrs.nasa.gov/search.jsp?R=19930020444 2020-03-17T04:29:40+00:00Z
o'rnnidirectlonalflux model(ref. 1)againstLDEF measureme,_ts(refs.9-12). Hereis presented
furtherrefinementof thetrappedprotonexposureover theLDEF mission.Improvementsinclude
detailedconsiderationof solarcyclemodulationof theflux, irnprovedtimeresolutionnearthe
missionendwherethealtitudewasclLangingrapidly,directionalflux calculationsoverthewhole
mission,andmodificatio,ato theB and L calculations.
DISCUSSION
Previous predictions of the LDEF mission fluences (refs. 5 and 6) were obtained by
calculating long-term average fluxes for five circular orbits at 478.7,472.3,462.8,426.0, and
319.4 km altitudes which occurred on mission days 0, 550, 1450, 1950, and 2105, respectively,
and performing a numerical integration over time assuming a straight line between time points.
The solar F10.7 crn radio llux which characterizes solar activity exceeded 150 about mission day
1540 (June 27, t988). The environment models used for solar rninirnum (the first three times)
were APgMIN (ref. 1) and the magnetic field rnodel was the IGRF 1965.0 80-term model (ref. 4)
projected to 1964,:the epoch of the environme,_tal model. The environment models used for solar
maximum (the last two times) were APgMAX (ref. i) and the magnetic field model was the
Hurwitz USGS 1970 168-534M model (ref. 4) for 1970, the epoch of the environmental model.
Both rnagnetic field calculations used a fixed constant magnetic moment of 0.311653 which was
built into the ALLMAG package lk_r calculating B and L magnetic parameters.
For low ahitude orbits such as that of LDEF the flux retrieved from the Vette model is very
sensitive I_0the input calculated B and L values. For the current calculation the constant magnetic
moment in the ALLMAG package was replaced by a moment calculated from the magnetic model
expansion COefficienk,; at the epoch of the model. At the highest altitudes this change reduced the
fluxes by about 5%. At the lowest altitudes near the mission end fluxes were reduced by a factor
of 2.
The goals for improving the flux model were better representation of the solar cycle
modulation and better time resolution near mission end. In Figure 1 solar activity as defined by the
solar F10.7 cm l-lux (ref. 13) and orbital altitude over the LDEF mission period are shown versus
mission day. The LDEF mission began near the end of the last cycle and ended near the maximum
of the current one with the orbital altitude changing slowly over the first 1500 days but rapidly
during the last 500 days of the mission. From the data represented in Figure 1 themean FI0.7
value at the last solar rninirnum F,nin was 67 and the mean value at solar maximum FnuTx was
183. Rather than an abrupt switch from APgMIN tO APgMAX a pa,arneter .....
i _ :
a ll, ha(O=( F(O-F,i,i,O/(F,,,ax-F,,,i,_)
was defined where F(t) is the Fi0.7 flux at time t.
given as ii mixfui"_' :=
Then the proton IiUXl ¢_ (t) at time t, was
¢_ (t) = _APS,_41N (l - Alpha(t)) + _AP8MAxAlpha(t)
m
Z
!
!
i|
138
where ¢)AP8MIN and CAP8MAX are the Vette model proton fuxes for solar minimum and solar
maximum, respectively. Table 1 shows the mission times for the current model, the orbital
altitude, the F10.7 value, Alpha and the scale height parameter for solar minimum (min) and solar
maximum (max) (used in the directional flux calculation).
Table 1. Model Inputs
Mission Altitude F10.7 Alpha Scale Height
Day (km) Flux (km)
min max
0 478.7 95 0.24 116.6 127.2
300 475.8 67 0 115.7 ---
1000 469.1 67 0 113.7 ---
1300 466.2 87 0.17 112.8 123.4
1500 461.5 118 0.44 111.4 122.0
1700 449.5 158 0.78 108.0 118.5
1800 433.6 171 0.90 103.6 114.0
1900 412.8 183 1.00 --- 108.4
2000 388.8 183 1.00 --- 102.3
2050 368.0 183 1.00 --- 97.2
2105 319.4 183 1.00 --- 86.4
Note that about half the points are distributed during the last 500 days of the mission. In Figures 2
and 3 the current model flux is compared to the pure AP8MAX and AP8MIN model fluxes and the
previous model fluxes, respectively, over the mission period. Note the transition near 1500 days
in the previous model curve due to switching from AP8MIN to AP8MAX. In Figure 4 the
predicted mission fluences from the current and previous model are compared. The current
fluences are about 20% lower.
For the previously directional flux calculation the AP8MIN model and a fixed orbital
altitude of 463 km were used. The current model has now been used to calculate directional fluxes
at each of the time points in Table 1 as input for dose and activation calculations using a complex
geometrical model of LDEF (ref. 14). In Figure 5 the cumulative mission fluence and the ratio of
eastward to westward traveling flux are shown as a function of mission time. Note that the proton
flux is much more directional near the mission end. Short half-life isotopes produced by activation
might be expected to reflect this greater directionality by greater ratios in abundance on the west
side versus the east side of LDEF.
CONCLUSIONS
Predictions of the LDEF mission's trapped proton exposure have been made using the
currently accepted models with improved resolution near mission end and better modeling of solar
cycle effects. Mission fluences are reduced by 20% from previously reported results. The LDEF
experimental measurements are providing an opportunity to validate the model predictions.
139
REFERENCES
° Sawyer, Donald M. and Vette, James I.: AP-8 Trapped Proton Environment for Solar
Maximum and Solar Minimum. National Science Data Center, Goddard Space Flight
Center, NSSDC/WDC-A-R&S 76-06, 1976.
2. Teague, Michael J. and Vette, James I: A Model of the Trapped Electron Population for
Solar Minimum. NationalScience Data Center, Goddard Space Flight CenterlNSSDC 74-
03, 1974.
3. Vette, James I.: The AE8 Trapped Electron Model Environment. National Space Science
Data Center, Goddard Space Flight Center, NSSDCAVDC-A-R&S 9 i-24, 199 !,
4. Stassinopoulos,E. G. and Mead, Gilbert D.: ALLMAG, GDALMG, LINTRA: Computer
Programs for Geomagnetic Field and Field-Line Calculations, National- Space Science Data
Center, Goddard Space Flight Center, NSSDC 72-12, 1972i
5. Watts, J. W.; Parnell, T. A.; Derrickson, J. H.; Armstrong, T. W.,: and Benton, E. V.:
Prediction of LDEF Ionizing Radiation Environment. First LDEF Post-Re_eval
Symposium, NASA CP-3134, 1991, pp. 213-224.
6. Armstrong, TI W.,, Colborn, B. L. and Watts, J. W.: Radiation Calculations and
Comparisons with LDEF Data. First LDEF Post-Retrieval Symposium, NASA CP-3134,
1991, PP. 347-360.
7. Watts, J. W.; Parnell, T. A.; and Heckman, H.H.: Approximate Angt_!ar Distribution and
Spectra for Geomagnetically Trapped Protons in Low-Earth Orbit. High Energy Radiation
Background Space, Proceedings of AIP Conference, Sanibel Island, Florida, Vol. 186,
1989, pp. 75-85.
8. Armstrong, T. W. and Colb0rn, B. L.. Radiation ModelPredictions and Validation Using
LDEF Satellite Data. Second LDEF Post-Retrieval Symposium, NASA CP-3194,
1993.
9. Benton, E. V.; Frank, A. L.; Benton, E. R.; Csige, I.; Parnell, T. A. and Watts, J. W.:
Radiation Exposure of LDEF: Initial Results. First LDEF Post-Retrieval Symposium,
NASA CP-3134, 1991, pp. 325-338.
10. Frank, A. L.; Benton, E. V.; Armstrong, T., W. and Colbom, B. L.: Absorbed Dose and
Predictions on LDEF. Second LDEF Post-Retrieval Sym0osium, NASA CP-3194,
1993.
i 1. Harmon, B. AI; F_shman, G. J'; Parnell, T. A.; and Laird, C. E.: induCed Radiaoactivity in
LDEF Components. First LDEF Post-Retrieval Symposium, NASA CP-3134, 1991, pp.
301-312.
140
12.
13.
14.
Harmon,B. A.; Fishman,G. J.;Parnell,T. A.; andLaird, C. E.: InducedActivation Study
of LDEF. SecondLDEF Post-RetrievalSymposium,NASA CP-3194, 1993.
AdjustedOttawa2800MHz SolarFlux. HerzbergAstrophysicsInstitute,Ottawa,Ontario,
CanadaNationalGeophysicalDataCenter,Boulder,CO, 1991.
Colborn,B. L. andArmstrong,T.W.; DevelopmentandApplicationof a3-D
Geometry/MassModel for LDEF SatelliteIonizingRadiationAssessments,SecondLDEF
Post-RetrievalSymposium,NASA CP-3194, 1993.
141
Altitude
(km)
Figure 1.
500
450
400
350
300
Solar Variation During LDEF Mission
--,-,-,-,-+---,-,-,-I-,-,-_-t-,-,-,-, t r--,--,-_ -I-,-_-_--_--
Altitude
0
_oco_o o]oO: o
(]D _ FI0.7 0 1_.2_ U I
250
200
Fma x - 183
150
100
--F_n-67
5O
F10.7
Solar
Flux
-500 0 500 1000 1500 2000 2500
Mission Day
Variati, m ,,f the solar F,o.: cm flux (t'*'f. 13) in units of 10-2:a.]/(s-m-'-Hz) an,[ LD[;F
orbital altitude during the peric}d of the LDEF mission.
Figm_" 2.
>
O
O
^
x
u_
9 "o
n E
,-_ o
2
I'..-
v
m
t-
O
O
"ID
e-
E
O
107
1 0 6
105
10 4
Influence of Solar Cycle Variation on LDEF Trapped Proton Exposure
(Example for Protons > 100 MeV)
,,, I,, , I 1 I
FluxatSolarMinimum
" "_...................... 2]a; .......... "'"_2.*'- . .._ LDEF Exposure
.... I .... I .... I .... I ....
0 500 1000 1500 2000 2500
LDEF Mission Day
Curr_'nt mod_.l pr_,t,,n flux > 100 XI_'X," compared to APSMIN mid APSMAX m_l_'l pr,,-
dictions.
142
ii
i
J
[
I
_i
>
:= A
¢.,I I
[21 ¢.a
O
O
1.0E+07
-°- Previous Model -_ Current Model
.............................. [ .........
1.0E+06
1.0IE+05
1.0E+04 t I I I I II I I I I1 I I I I1 1
0 5O0
')
T
T
I
I
m
I II I I I II I [ I I I II I I I I I I I I II I I
1000 1500 2000 2500
Figure 3.
LDEF Mission Day
Current model proton flux > 100 MeV comparcd to previous model
predictions (ref. 5).
143
_I I
1.0E+09
1.0E+08
1.0E+07
..... I....... 't -t_[' ".... !' z-' -i
---l--r--I F-I -I -I '_3- - ]- I ii',l -__ 71/_ i i t! _i__-. .i!IL .-_,_L_L....... t __ t I
.............. f ......... [ - -- _- 7 ....
i k k
...... _ .... L ...... 7 ......... i ...............
.... • __1 .... L,- _c - __- i-- _ i j _L ......
.... r L_ _-k_, _m........ i_I _._ ._
- _ ....... _ _ rli 7.... _ _--' -_--_ ',L r_-
/
1,0E+06 --- _ ............................
............ ! f
i:
i J
144
1.0E+05
1.0E+04
0.01
Figure 4.
-:- Previous Model -'- Current Model
O. 10 1.00 10.00 100.00 1000.00
Energy (MeV)
Current model proton mission fluence compared to previous
model predictions (refi 5).
2.0E+09 10
--'- Fluence -° Ratio
1 .SE+ 0_) I
/ j
........ 71 #I#I
5.0E+08
O.OE+O0 ,,,,,,,,,I,,,,_,_,I,,,,,,_,,I,,_,,,,,,I,,,,,,,,,
0 500 1000 1500 2000
i_ 1.0E+09
._ 1=1
(..)
8
,..., Lr.._
,--..,
¢_ 4--,
0
2
0
2500
LDEF Mission Day
Figure 5. Cumulative mission fluence and ratio of eastward/westward directed
integral flux > 100 MeV versus mission day.
145
2


Revised prediction of LDEF exposure to trapped protons

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