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AO-AO90 33b AIR FORCE GEOPHYSICS LAB HANSCOM, AFB 14A F/G 22/1GR04*IOGASEO PREDICTORS OF SPACECRAFT CHARGING DURING ECLIPSE PA--ETC(U)RAR S0 A H WEND~EL
UNCLASSIFIED AFG-TR80-0088 M
i~n101=8 115.ILI141 ' W
11.6 M22
1111 L o 12.0
11111125 AMIIICI RESOLUTIO TET iii1
MICROCOPY RESOLUTION TEST CHART
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UnclassifiedSECURITY CLASSIFICATION OF THIS PAGE (When, DO. Entered)
REPORT DOCUMAENTATION PAGE BEFORE COMPLETrING FORM
Iffro - GOT ACESSION NO 3. RECIPIENT'S CATALOG NUMBER
rRQUND-13ASED .'REDICTORS OF Scientific. Interim.
ECLISE ASS-rEL NG 1 RIG.0. PERFORMING ORG. REPORT NUMBER
UT C)NT NUMBER(*)
rthur H Wendel 2/ILt, USAF IAPERFORMING ORGANIZATION NAME AND ADDRESS 110: PROCRAM ELEMENT. PROJECT. TASK
Air Force Geophysics Laboratory (PHG) ARA&WOK0I1UMFR
Hanscomi AFB 611Bedford, Massachusetts 01731 76610803
It. CONTROLLING OFFICE NAME AND jADRES1.RPR A
Apredfd forsapublcselese ditrbtinuniitd
14. DOITRIUTION NSTATE ME &o thDE SSi if~ee nt Mn Lo 20. (of this~n report),.f
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magnetometer readings near the base of the geomagnetic field line nearest thesatellite.
D ,ON70 F I JA 71 IS OSOLETE UnclassifiedSECURITY CLASSIFICATION OF THIS PAGE (IShi Dot& Ente.d)
Contents
1. INTRODUCTION 51.1 Satellite Relationship to Plasma 51. 2 Spacecraft Charging 6
2. PROCEDURE 6
2. 1 Selection and Analysis 62.2 Magnetometer Tracings 6
3. DATA BASE 7
3.1 Satellite 73.2 Environment 73. 3 Indices and Magnetometer Data 9
4. RESULTS 10
4.1 Estimates of Potential 104. 2 Geomagnetic Indices and Magnetometer Tracings 134.3 In Situ Plasma Measurements 17
5. CONCLUSION 19
REFERENCES AcceSS of 7or - 21TITIS C'DTIC T'JB
SUflal12.L A f :Just if:!C.nt i..
1)---S-t- T-, J 1-1, t ' C,:o
Distr v ...
Ave, 11f.Diet
Illustrations
1. Distribution of RMS Electron Temperatures atGeosynchronous Orbit Measured on ATS-5, 1969-1971 12
2. Distribution of RMS Ion Temperatures at GeosynchronousOrbit Measured on ATS-5, 1969-1971 15
3. Distribution of 10-min Averages of Spacecraft GroundPotentials During Eclipse for ATS-5 16
4. Distribution of Peak Spacecraft Ground PotentialsDuring Eclipse for ATS-5 17
5. Distribution of Eclipse Charging Currents for ATS-5 19
6. Dependence of ATS-5 Ground Potential on RMS PlasmaElectron Temperature 20
Tables
1. Current-Dependent Potential Estimators 13
2. Correlation of Geomagnetic Indices With Spacecraft Potential 14
3. Correlation of Geomagnetic Indices With Spacecraft Potential(only eclipses in which charging occurred) 16
4
Ground-Based Predictors of SpacecraftCharging During Eclipse Passage
1. INTRODUCTION
1.1 Satellite Relationship to Plasma
A geosynchronous satellite in the plasmasphere remains at the plasma potential
or perhaps a few volts positive potential with respect to the surrounding plasma.1
When the satellite passes into the earth's shadow, however, the cessation of the
emission of photoelectrons results in the sudden buildup of a negative charge
until the potential of the satellite with respect to the plasma is up to several thou-
sand volts negative. With such sudden high potentials, the possibility of satellite
operational anomalies or physical damage due to electric arcing becomes significant.
The purpose of this paper is to show relationships between the estimated potentials
of the satellite just prior to, during, and after eclipse; various characteristics of
the space plasma environment measured by the satellite; and diverse ground-based
measurements of geomagnetic activity and local variations in the earth's magnetic
field.
(Received for publication 3 March 1980)
1. Garrett, H. B., Pavel, A. L., and Hardy, D. A. (1977) Rapid Variations inSpacecraft Potential, AFGL-TR-77-0132, AD A046350, p. 11.
5
1.2 Spacecraft Charging
Spacecraft charging can be described in terms of the particle fluxes the space-
craft experiences while moving through the space plasma. In a plasma with electrons
and positive ions (mainly protons) of roughly equal energies, the average velocity
of the electrons is forty times that of protons. More electrons than protons pene-
trate the plasma sheath (which forms around any probe in a plasma) and impinge
upon the spacecraft, resulting in an incident electron current about 100 times
larger than the incident ion current. Some of the impacting electrons and ions
result in the secondary emission of more electrons from the satellite, while in-
cident electrons are also backscattered to a degree. Only the emission of large
numbers of photoelectrons can counterbalance the high flux rate of these high-
velocity electrons. Thus, when the spacecraft is in sunlight, the photoelectron
current is responsible for the spacecraft's slight positive charge. When a space-
craft becomes shadowed, its surface no longer emits photoelectrons and becomes
negatively charged to a high degree; this also holds true for the surfaces of non-
rotating spacecraft permanently shadowed.
2. PROCEDURE
2.1 Selection and Analysis
Our procedure was, first, to select plasma spectrograms from which satellite
potential could be estimated; that is, from those produced from data taken during
the eclipse seasons (near the spring and fall equinoxes) of 1970 and 1971. Next,
the first four velocity moments of the plasma energy distribution were obtained,
calibrated for instrument degradation, and utilized to calculate single and two-
Maxwellian charged particle currents and temperlatures experienced by ATS-5.
These temperatures and currE ts were then employed in charging models of vary-
ing complexity, yielding further estimates of satellite potential.
2.2 Magnetomieter Tracings
Magnetometer tracings made at these times near the base of the geomnagnetic
field line associated with the satellite's position were obtained, scaled, and com-
pared with the spacecraft's potential. All of these parameters were then sub-
jected to multiple regression analysis by means of a straightforward matrix-
inversion FORTRAN program.
6
3. DATA BASE
3.1 Satellite
This study is based upon data gathered by the ATS-5 satellite during 1970 and
1971. This satellite was in geosynchronous orbit near 105°W longitude with an
orbital inclination of 2.300, spinning about an axis parallel to the earth's
rotational axis with a period of 0. 79 sec. 2 The four spectrometers of the University
of California, San Diego plasma experiment supplied the data upon which spacecraft
potential estimates and plasma temperature calculations were based. Two pairs
of electron and proton analyzers are described: one pair directed perpendicular
to, and the other aligned parallel with the spacecraft spin axis. Both directional
orientations yielded, roughly, the same results for the purpose of this study and,
consequently, the average of both orientations was used.
3.2 Environment
Plasma characteristics were calculated from 10-min averages of the first
four moments of the plasma distribution function described by Garrett 3 as:
the number density, (n i > = 4 rr O(Vo) fi V 2 dV n
So ~1 ) f dV . (2kT.1/2the number flux, <NF?= 1O (V 1) fi V 2 dV = i (2 kTi)II2n n m.
the pressure, (Pi) = 4 (1 m i ) (V 2) fi V 2 dV = n kT311
r co(V3)f. V min (2kT.)3/2and the energy flux, (EF = (1 m.) JO (V3 ) f dV = mini (2 k
2 2 rr m
for particle species i.
Electron and ion currents were obtained by multiplying the appropriate number of
fluxes by the charge of a single electron or proton.
Typical values were selected for these four moments as close to the entry
and as close to the exit of the satellite into eclipse as possible. Sudden large
particle fluxes or detector malfunctions occasionally precluded the selection of
the immediately preceding or the immediately following values.
2. DeForest, S. E., and Mcllwain, C. E. (1971) Plasma clouds in the magneto-sphere, J. Geophys. Res. 76 (No. 16):3587.
3. Garrett, H. B. (1977) Modeling of the Geosynchronous Orbit Plasma Environ-ment - Part I, AFGL-TR-77-0288, AD A053164.
7
A continual decrease in the value of the electron moments was observed. This
is due to the decay of the electrostatic analyzers sensitivity with time. Since elec-
tron fluxes were so much higher than proton fluxes, the electron analyzers deterio-
rated earlier than their counterparts for positively charged particles. By the end
of 1972, these electron detectors were almost totally inoperative.
To compensate for this instrument degradation, the averages of the moments
during each of the four eclipse seasons (spring and fall of 1971 and 1973) were
taken for both the parallel and the perpendicularly oriented devices which decayed
at different rates. The moments were then normalized to yield average values
comparable with the spring of 1970 for the two types of detectors.
The calibrated moments were then used to compute two different types of
plasma "temperatures,"
T "average" = TAVG = pressure divided by number density, and
T "root mean square" = TRMS = energy flux divided by twice the number
flux.
As Garrett has shown, these temperatures will be equal, and have meaning as
temperatures, if and only if the plasma is a Maxwellian plasma; that is, repre-
sentable by the equation:
3/2 -m V. 2/2 kT.fi.(V) =n i m. e 1 1
T kT.1
whereni = number density of species i
mi = mass of species i
Ti = temperature of species i
Vi = velocity of species i
K = Boltzmann constant
f = distribution in sec 3 /km 6
The marked difference between TAVG and TRMS as determined from ATS-5
data is a direct result of the absence of a Maxwellian plasma at geosynchronous
orbit. 4
4. Garrett, H. B. (1978) Modeling of the Geosynchronous Plasma Environment,Proc. of the Spacecraft Charging Technology Conference-1978, AFGL-TR-79-0082/NASA Conference Publication 2071, pp. 12-14.
8
As further demonstrated by Garrett, 3 the actual plasma distribution function
can be more successfully approximated by employing the sum of two components
with different temperatures and number densities, each with a Maxwellian distri-
bution. The electron and ion number densities and temperatures were also cal-
culated for both of these components from the four plasma moments.
3.3 Indices and Magnetometer Data
The various geomagnetic indices considered in this study included K, A p
C9, AE, AL, AU, and Dst. Most of these are described in a review paper by
Rostoker. 5 The following brief description is based primarily upon this reference.
These indices are based on the averages of certain values derived from
magnetometer tracings of selected stations around the world. One objective of
this study was to determine whether these averaged indices or whether magnetom-
eter tracings at the foot of the field line provided better indication of spacecraft
charging.
The popular Kp index is a measure of worldwide geomagnetic activity over
3-hr periods. First, each individual station determines a K value based on the
deviation between the minimum and maximum magnetometer readings along all
three axes of measurement and the geomagnetic latitude of the station. The K
index is derived by adjusting the K value to compensate for season-dependent
diurnal variations in accordance with tables devised for each station. Finally,
the K s indices of a number 5 of carefully selected observatories at subauroral
latitudes are averaged to yield the K index.
The Ap index is derived directly from Kp on a linear scale as opposed to
Kp's quasi-logarithmic variation. There is a formula relating A to Kp for all
but the highest values of Kp for which certain values are more or less arbitrarily
selected.
The AL, AU and AE indices are indicators of auroral-zone magnetic activity
and are calculated from magnetometer measurements of the H component from the
auroral latitudes. The readings of the various stations, corrected for average
value, season, and local time variation are superimposed on a single magnetogram
style chart. The upper envelope of such a chart is known as AL and is determined
by the maximum positive H reading for all magnetometer stations used; AU is
similarly the maximum negative value of H for all observatories during a parti-
cular 2. 5-min interval; AE is just the difference between these two quantities.
Further, Dst is used as an indicator of ring current strength alone and is
derived from measurements of the H component of magnetic variation at several
low-latitude observatories. It is basically the deviation of the H component of
5. Rostoker, G. (1972) Geomagnetic indices, Rev. Geophys. 10 (No. 4):935-950.
the geomagnetic field from a baseline value, adjusted for seasonal variations as
well as daily solar influences.
Finally, the magnetometer tracings recorded at the Fort Churchill Observa-
tory, Canada, located at 94. 100 W longitude, 58. 80°N latitude, were also used in
this study. This position compares favorably with the approximate coordinates of
the intersection of the geomagnetic field line nearest the ATS-5 satellite and a
spherical surface 100 km above the earth: 98. 80 W longitude, 55. 400N latitude.
The field line tracings, made during the spring and fall of 1970 (none being avail-
able for 1971), were scaled so as to obtain an "average" level of geomagnetic
activity as well as the magnitude of magnetic fluctuations in the X-direction during
each of the three hours before local midnight. The average level was estimated
visually, smoothing out fluctuations of short duration, and the amount of fluctuation
was determined by the peak-to-peak distance between the minimum and maximum
deflections within a particular hour. The average magnetometer measurements
during hours 0400, 0500, and 0600 (UT) and the measurement of variation during
the same hours will be referred to subsequently as M4, M5, M6, 6M4, AM5, and
AM6, respectively.
When considering various geomagnetic indices as indicators for the possibility
of the accumulation of potential, one finds it useful to remember Rostoker's caveat
regarding their use. 5 Due to the nonuniform distribution of magnetic observatories,
and resulting lack of coverage, indices can define only the lower limit of geomag-
netic activity at any particular point in time. This would be consistent with the data
analyzed for ATS-5 in that charging almost always occurred when the indices
reached higher levels, although not ruled out by low index values.
4. RESULTS
4.1 Estimates of Potential
Various approximate relationships between electron and ion currents measured
by the satellite in sunlight, between electron and ion currents in eclipse, between
electron and ion TAVG and TRMS , and the potential of the satellite were used to
make additional estimates or predictions. These were derived from prior observa-
tion and application of elementary plasma theory. As observed, the natural loga-
rithm of, roughly, one-tenth the electron-to-ion current ratios before eclipse was
proportional to the potential buildup when multiplied by the electron temperature.
This relationship was then tested with both the AVG and RMS electron temperatures
as follows:
10
V 1 T AVGE x In (J /10 J))
V 2 = TRMS x In (JEo/ 10 Ji )
where J and J are the electron and ion currents, respectivIy, before the
satellite enters the earth's shadow.
Garrett and Rubin also observed that, according to probe theory, when
either a thick plasma sheath or no plasma sheath surrounds a spherical probe,
it is negatively charged as:
- qV /TE
JE =JE e (1)0
JI =JI (1I + T I (2)0
where electron current to shadowed spacecraft
il ion current to shadowed spacecraft
JE = electron current to spacecraft before eclipse0
J I = ion current to spacedraft before eclipse0
V spacecraft potential in volts
q unit charge such that qV is in electron volts
Solving Eq. (1) for the potential buildup and trying both TAVG and TRMS,
one derives the following estimates.
V3 TAVG x In PJE/JE0
V 4 TRMS x In PJE/J %
Equation (2) was solved for the spacecraft potential in terms of the ratio of the
ion currents before and during eclipse. Again, both AVG and RMS temperatures
were tested:
V 5 = TAVG x( I /JI
V 6 = TRMSI x (- Ji/Ji I
11
An example of the relationship observed between these predictors and the actual
potentials is given in Figure 1.
When considering the usefulness of these various formulas in predicting
potential buildup, one finds it useful to distinguish between two different questions
which are applicable: (1) Is the potential zero or nonzero? (2) How high is the
potential, given that it is nonzero? It appears that VV. V 2 ' V 3 , and V4 can be
applied successfully in answering both questions, whereas V5 and V6 are very
useful in answering the first question only. In all cases, the predictors using
the electron temperature data (V1 through V4 ) generally yield better results
than V 5 and V6 ' based on ion temperatures; V1 and V2 ' calculated using ion-to-
electron current ratios before and after eclipse, seem to be better in predicting
the occurrence of charging than V3 and V4 . but are of roughly the same value
in foretelling the magnitude of charging. For purposes of real-time warnings of
possible charging conditions, the utilization of particle current data measured
before eclipse as in V 1 and V2 is a clear advantage, even further enhancing the
usefulness of these estimates.
8,000
X X
X6,000 - X
x- X X
4,000 -
x X X
2,000 0k 0 0 0
020 40 s0 so 100 120V2
(ZERO-POTENTIAL PASSAGES SUPPRESSED)
Figure 1. Distribution of RMS Electron Temperatures at Geosynchronous OrbitMeasured on ATS-5, 1969-1971
12
Utilizing RMS, as opposed to average temperatures, for these indicators re-
sulted in better estimates of the extent of charge accumulation only in the case of
V1 and V2 ; it did not improve results for the charging/no charging determination
for any of the estimates.
Finally, whereas V5 and V 6 did not too accurately indicate the precise magni-
tude of the potential buildup, they appear very useful in demonstrating "safe" con-
ditions. When these estimates yielded values of exactly zero,(that is, when there
was no charge in the ion current as the satellite passed into eclipse), there was,
in fact, no spacecraft charging.
One can take the technique of potential estimation by means of current balance
equation C JE - + JSE + JSI + JBS) = 0] relating the incident electron (J.) andion (JI) currents with the voltage-dependent estimates for backscattered electrons
(JBs) as well as secondary electrons resulting from both electron (JSE) and ion
(JsI) impacts. The photoelectron current may also be included; this was, of
course, zero for the eclipse passages described here. The following table indi-cates the improvement in correlation with use of the more complex current-
balancing estimate. This was most dramatic when the probable skewing influence
of the 31 data points at zero potential was removed.
Table 1. Current-Dependent Potential Estimators
With Zero-potential Eclipse Without Zero-potential EclipseEstimator Correlation Uncertainty Correlation Uncertainty
Single -MaximumCurrent 0.79 *.06 0.70 :.08
V 1 0.70 *.07 0.56 ±.09V2 0.73 :.06 0.64 +.08
V3 0.63 -. 07 0.62 :k. 09V4 0.62 .07 0.64 ±.08
V 5 0.49 +.08 0.26 . 11
V6 0.46 .08 0.24 -.11
4.2 Geomagnetic Indices and Magnetometer Tracings
The indices considered and analyzed in the multiple regression were Kp, the
associated Ap, and C9* for the second 3-hr period of the days (0300 to 0600 GMT)
as well as AE, AL, AU, AO, and Dst for hours 0300 to 9400, 0400 to 0500, and
0500 to 0600 GMT. Also, the hourly change in Dst was explored.
*An obscure, arcane index roughly comparable to Kp.
13
These indices appear to divide themselves neatly into two groups: AL, Dst,
and fluctuations in Dst, which showed almost no correlation with the potential, and
Kp, Ap, C9, AE, and AU, which all showed moderate correlation (see Table 1).
The former group of indices thus was eliminated from further calculations and
intercomparisons carried out with the remaining indicators.
Again, when speaking of the relative usefulness of these indices which were
considered more extensively (Kp, Ap, C9, AE4, AE5, AE6, AU4, AU5, and AU6),
one can ask which are better for determining whether or not charging will occur.
Further, given the occurrence of charging, one can consider which indices are
better for predicting the extent of voltage buildup. For predicting the occurrence
of charging, AE6, AE5, AU6, and AU5 seem to be the best indicators (with corre-
lation coefficients of about 0. 50 + 0. 08) while AE4 and AU4 and K have correla-
tion coefficients which are 20 percent lower; Kp proved to be only fair as an indi-
cator when it assumed large values, while extremely low values of Kp (0, 1-, 1+)
almost always corresponded with little or no potential buildup as shown in Figure 2.
As before, the eclipses during which there was no charge buildup were elimi-
nated, and the relationship between the potential and the indices was examined. In
this case AE6 and AE5 were first; AE4, AU6, AU5, and AU4 second, and then K P
and C9. Figures 3 and 4 show potential as a function of AE6 and AU6, respectively.
It is apparent that the correlation coefficients of the sample (including eclipses
without potential buildup) are significantly higher than the coefficients in the sample
with the zero-potential events removed. This may be due to the uneven distribu-
tion (31 points at 0 volts).
Table 2. Correlation of Geomagnetic Indices With Spacecraft Potential
Parameter (All Eclipses) Uncertainty
AE6 0.57 *.10AU6 0.55 +.10M6 0.51 ±.10AE5 0.48 +.11AU5 0.48 -.11AU4 0.47 :.11
AM6 0.47 *.11K 0.46 .11
pAE4 0.44 .11M5 0.43 . 11C9 0.41 . 11
AM5 0.40 .11
14
.. .. ..
8,000
x X
xx
6,000
x .
x
x x
0 x> x x x
x x
IL x
-j 4, 0 x
X xx x x
2,000 x X XSxx xx
XxX
X X x x2x 0x
U X
x X X xK
xKp
Figure 2. Distribution of RMS Ion Temperatures at Geosynchronous Orbit Meas-ured on ATS-5, 1969-1971
15
8,000
x X
6,000 x
x x
4,0 0 0 - X XXK
J x X.4
W x x
x x
X X X x
0 10 20 30 40 50 60 70AE AT HOUR 6
Solid line shows tendency for little or no charging when AE is low.Figure 3. Distribution of 10-min Averages of Spacecraft Ground Potentials DuringEclipse for ATS-5
When the eclipse passages without potential buildup were removed, the resultswere not as conclusive, owing to the small sample size. Several interesting results
were noted, however, including the clear superiority of M6, followed by AU6 and
AE6, the early-warning capability of AU4 (since the measurement is taken 2 hoursbefore eclipse passage), and the relatively low correlations of potential with Kpand M4.
Table 3. Correlation of Geomagnetic Indices With Spacecraft Potential(only eclipses in which charging occurred)
Correlation WithParameter Spacecraft Potential Uncertainty
M6 0.45 *. 13AU6 0.41 ±. 13AE6 0.41 +. 13AU4 0.39 +.14M5 0.36 ±.14AU5 0.35 ±.14AE5 0.33 *. 14AE4 0.31 .14C9 0.31 .14M4 0.29 .14Kp 0.26 ±.14
16
6,000
I.-.-Jo 4,000
-I> x
zX. X
a.Sxx
XX
ee 2,000
x
4
x
0xUiX X X
XX
-I* I I I I I I I I I I I I I I I I0 3 6 9 12 15 Is
K P
Figure 4. Distribution of Peak Spacecraft Ground Potentials During Eclipse for
ATS-5
4.3 In Situ Plasma Measurements
Garrett and Rubin 6 have already considered the relationship between plasma
measurements and spacecraft potential on ATS-5. In this study, close correlation
was found between electron energy flux, pressure, and AVG RMS temperatures and
spacecraft potential. All ion measurements showed an almost total lack of corre-
lation. Again, the multiple regression analysis was performed both with and with-
out zero-potential eclipse passages.
6. Garrett, H. B., and Rubin, A. (1978) Spacecraft charging at geosynchronousorbit - generalized solution for eclipse passage, Geophys. Res. Letters 5(No. 10):865-878.
17
When the analysis was made with the zero-potential events, electron energy
flux, AVG electron temperature, electron pressure, and RMS electron tempera-
ture were in good agreement with spectrogram -estimated spacecraft potential, and
with electron number flux showing somewhat less correlation. Use of only the more
reliable 1970 data resulted in a 10 percent increase in the correlation coefficients,
but no change in the rank order of the variables as shown:
All Data (115 points) 1970 Only (69 points)Variable Multiple Correlation Multiple Correlation
(Electron) Coefficients Uncertainty Coefficients Uncertainty
Energy flux 0.74 .06 0.82 .07TAVG 0.73 *06 0.79 .07Pressure 0.67 +.07 0.76 ±.08TRMS 0.64 ±.07 0.67 +.09Number flux 0. 52 ±. 09 0. 38 ~ .11
Number density 0.29 ±.09 0.38 .11
When the zero-potential events are eliminated, the correlation coefficients
remain relatively high, warranting greater confidence in the results. The RMS
temperature is now a better correlator than the AVG temperature.
All Data (84 points) 1970 Only (46 points)Variable Multiple Correlation Multiple Correlation(Electron) Coefficients Uncertainty Coefficients Uncertainty
TRMS 0.73 ±..09 0.77 ±.09Energy flux 0. 67 +. 08 0.78 ±. 09TAVG 0.62 ±.09 0.72 ~ .10
Pressure 0.56 .09 0.70 11lNumber flux 0.38 ±. 10 0.51 ~ .13
Number density 0. 20 ±. 11 0. 28 ~ .14
The relatively low correlation coefficients of the number density were included
to show an interesting tendency. We noticed that the number flux, the second
velocity moment of the plasma distribution function, showed a higher correlation
with potential than the first moment (number density). The third moment or "pres-
sure" (compared with potential in Figure 5) showed greater correlation than the
second, whereas the fourth moment (energy flux, as shown in Figure 6) was con-
sistently the best correlator of the four moments. It is suggested that this is a re-
sult of the higher moments ability to measure effectively the higher energy compo-
18
A ._ _ ....
10,000
6,000-
X X
U, X
06000 X X
X X
X XZ000 - X X
XX X
XX X
00.00 0.00 0.16 0.24 0.32 0.40
ELECTRON PRESSURE (x 10)ALL POINTS (115)
Figure 5. Distribution of Eclipse Charging Currents for ATS-5
nents of the plasma thought to be more directly responsibie for spacecraft charging7
effects than those of lower energy. 7
5. CONCLUSION
Clearly, among the data examined here, in situ measurements of electrons
near the time of eclipse seem to be the most closely related to spacecraft charg-
ing; in fact, so much so as to make them suitable for prediction purposes. As far
as the more convenient, readily available ground-based indicators are concerned,
7. Garrett, H. B., and Rubin, A. (1978) Spacecraft Charging at GeosynchronousOrbit - Solution for Eclipse Passage, AFGL-TR-78-0122, AD A058983.
19
Q' N N
10,000
6,000-
U, x. X
x-I K X
_j x xz 4,00o0- x x
W. e
X X X
2,000 PA
IkY*C XX
-. .. .. I ..a I I I I I I
0 4 6 12 1S 20ENERGY FLUX
Figure 6. Dependence of ATS-5 Ground Potential on RMS Plasma ElectronTemperature
AE, AU, and the local magnetometer readings at the base of the field line seem tobe the best candidates. While the relationship between these parameters andpotential buildup was not as direct as the in situ measurements, further compari-sons will be made using the superior ATS-6 and SCATHA data which will, it ishoped, show a more direct correlation.
Furthermore, higher moments of the plasma distribution function will becalculated from this more recent data, in order to determine if the higher energyplasma components thus isolated are more directly related to spacecraft charging.
20
References
1. Garrett, H. B., Pavel, A. L., and Hardy, D. A. (1977) Rapid Variationsin Spacecraft Potential, AFGL-TR-77, AD A046350, p. 1 1
2. DeForest, S. E., and Mcllwain, C. E. (1971) Plasma clouds in the magneto-sphere, J. Geophys. Res. 76 (No. 16):3587.
3. Garrett, H. B. (1977) Modeling of the Geosynchronous Orbit Plasma Environ-ment - Part I, AFGL-TR-0288, AD A053164, p. 10.
4. Garrett, H. B. (1978) Modeling of the Geosynchronous Plasma Environment,Proc. of the Spacecraft Charging Technology Conference - 1978, AFGL-TR-79-0082/NASA Conference Publication 2071, pp. 12-14.
5. Rostoker, G. (1972) Geomagnetic indices, Rev. Geophys. 10 (No. 4):935-950.
6. Garrett, H. B., and Rubin, A. (1978) Spacecraft charging at geosynchronousorbit - generalized solution for eclipse passage, Geophys. Res. Letters 5(No. 10):865-878.
7. Garrett, H. B., and Rubin, A. (1978) Spacecraft Charging at GeosynchronousOrbit - Solution for Eclipse Passage, AFUGL-TR-78-012, AD A058983.
21
