Two days of data from the ATS-6 1976 eclipse season were analyzed to determine the effects of varying photoelectron flux on spacecraft potential. Particular emphasis was placed on the variation in potential as the satellite entered the earth's penumbra. Measurements from the AE-C satellite of the solar UV radiation were used to construct a model of atmospheric attenuation. This model was found to be consistent with direct measurements of the variations in photoelectron flux as Injun 5 passed into eclipse. Applying the model to the ATS-6 data gave the time dependency of the solar illumination/photoelectron flux as the satellite was eclipsed. This relationship, when combined with the ATS-6 measurements of satellite potential, revealed a nearly linear relation between the solar illumination/photoelectron flux and the logarithm of the satellite potential

Garrett, H. B.

English

NASA Technical Reports Server

Cbdtontb
I. Introduction ID|
2. Observations lg2
3. TheOretiCal Atmlysls 191
4. ConClusions 199
A ckno_vledRmeflts 20 l
E_eterences 201 i
11. Analysis of Penumbral Eclipse Data
H. B. Gotrett, 1/Lt. USAF
Air ForceGeophysicsLaboratory
HartshornAFB
8®d_rd,Mass.
Abst¢act
Two days ot data trom the ATS-6 1976 eclipse season were analyzed to deter-
mine th_ effects ot varying photoelectron flux on spacecraR potential. Particular
emphasis was placed on the vai'iation in potential as the satellite entered the
earth's penumbra. Measurements from the AE-C satellite o£ the solar UV radia-
tion were used to construct a model or atmospheric attenuation. This model was
Found to be consistent with direct measurements oJ_the variations in photoelectron
flux as INJUN V passed into eclipse. Applying the model to the ATS-6 data _ave
the time-dependency of the solar illumination/photoeleCtron flu_t as the satellite
was eclipsed. This relationship, when combined with the ATS-6 measurements of
satellite poteattM. _-evealed a nearly linear relation betweefl the solar illumination/
photoelectron fluk and the logarithm of the sateilite potential.
I. INTRODUCTION
The charging of SpaCecraft at geosyrichronous orbit has become a primary cOn-
cern for englr/zers and sat_liite designers. A major goal of scientists concerned
with the problerfi is to provide to.th_ engineers and deslgn_rs adequate inodels or
ti_is pheflbmeilbn. Aithbugh several models o£ the sheath su/'routiding the spacecraft,
lgi
https://ntrs.nasa.gov/search.jsp?R=19780002199 2020-03-20T12:39:55+00:00Z
of the way in which this ._heath altet'._ the ambient pat,ticle fiuxes, and of the sub-
sequel t inter-action With the spacecraft surface ekist in varyit_g dc_re_s of sophis-
tication, few means exist for verifying these models. I The purpose of this report
will be to present observations of the variation in the ATS-_ spacecraft potential
as one parameter, the photoelectron fiu_0 was eat'led tit a known manner'. This
will be accomplished by detailed analysis ol_data from periods whe_ the sun, as
observed by a satellite, is eclip:_ed by the earth.
i 2. OBSERVVI'IO%S
i" The highestpotentialsyetobserved on.spacecraR have been by the ATS-5 and
i. ATS-6 satellltes as they passed into the earth's shadow.. These potentials are
i onlyobserved when the satellitesare immersed in.thehot plasma associatedWith
geoma_etic storms and plasma injection events. Briefly° as a satellite passes
i intoeclipse,the incidentsolarphotonfluxisdecreased resultingina decrease in
i the photoe1_ctroncurrentemitted,fromthe satellitesurface. Itisthe elimination
i of thi_currentsource thatresultsina currentimbalance which leads toa large
!
i_ negative potential.
i Figure_ 1 and 2 are spectrograms for 28 Febrtlary(Day 59)and 6 March (Day
66)ofthe partlclepopulationsobserved by the ATS-6 sat_lliteduringthe spring
1976 eclipseseason (see2 foran explanationOf thesespectrograms). The intense
bands observed ineach positiveion spectrogram are the coldbackground ion popu-
i lationwhich has been acceleratedby electricfiellaon the spacecraft. These data
i:
can be interpretedas voltagechanges on ATS-6 amounting _ over -I0,000 voltsanti!
i are coincident with the passage of the satellite into eclipse. In FigUre 3. the volt-
age changes as the spacecraft passes into eclipse (exits eclipse) are plotted as[--
, continUoUs(dashed)linesfor Days 59 and 66. Entry and exit(reversedin time}
F_
have been superimposed on each othertoshow theiragreement for each eclipse.
i_' This a_reement is a consequence of the eclipse symmetry and the apparent stability
i ; of ambient condltion_ during each eclipse.
i FigUres 4 _nd $ show theplasma distrlbu_ionfunctionsfor theelectronsand
i ions associated respectively with the eclipses on Day 59 and Day 66. Figure 4, for
i_ Day 59, shows theplasma =ondltlonsbefore antiaftereclipse(dashetlines}arid
during eclipse (solid lines). The spectra immediately ttefore anti after eclips_ are
nearly identical; whereas the spectra measured while the sat_URe was in eclipse
show the effects of chargii_g. Normally these eff_ct,,_ result in a constant, positive
i i,
i": 192
_=_
d '.
00000003-TSA04

Figure 2. Spectrogram ofATS-6 Count Hates as a FunctionotTime and Energy
forDay 66 of 1976. Th_ spacecraftiseclipsedby the Earth between 2100 and
2300 UT
194
.....................'.........., ; ; ;,iiii.....,. ........:i,i ..;::;r.i,o_ LII;:-;_L;L__'-_"G'
00000003-TSA06
_il_
(
, i
i ! l ! ! ]I T •-T-l- I ! ! , | ! !
I 104
i "_ IJII ¢ -'
i -
i ,
i :!
., 4111_.
AY 66
_:i io3
i_ I i I I I I I I i I I ! I ! I I II
-_:. 0 5 I0 15
_: TIME (MINI
!_ Fi_tre 3. ATS-6 Potential as a Ftlttctibn o£
_",_ Time for Day 59 and Day 66. For reference,
, as the time axis is not absolute. ATS-6 had
"_ voltage_ in excess of -10, 000 V for 1060 sec
=_: on Day 5Q and 3i26 _ec on Day 66
iL
_i displacement in energy o£ the ion distribution function that is equal to the potential
i on the satellite anti a corresponding negative displacement in the electron distribu-
_ tlon futlction. The spectra £or Day 6G in Figure 6 sltow this effect, but, forF_
! _ energies above 30. keV, th_ spectra £or Day 59 are not consistent with this behavior.
_
i ! The exact cause o£ the dep_trture is not kno_vn but it may result £rom effects on the
! i.. spaCect-a£t dize to the eclipse rather than to changes in the ambient plasma.
%,i ,
., , ig5
[ "
00000003-TSA07
, , ; I ! _ I i
tO° ELECTRONS IOs PROTONS
o_ F- _, : :.,., ,, ECLIPSE
z _ "_'_, E:C E . = = " ""
LIPS "
crb-
r_
1 I ....J...... L__L_ E__X._L.....±......_1 l /__L_L__
0 I0 20 30 40 50 60 0 I0 20 30 40 50 60 70
ENERGY(KeV) ENERGY(KeV)
Figure 4. Electron and I0n Distribution Functions as Functions of
Energy for Day 59. DaShed lines represent the _pectra before and
after eclipse. Solid lines represent eclipse Spectra
_ I0° l-- tO5 -
._ ELECTRONS PROTONSE
x_o \\ _ ECLIPSE
=
_.162_- iO3 -,
g
I0"
_- ECLIPSE _' 102 -
" __L__I L 1 -I " I I,,, _ I 1 1 I
0 I0 _0 30 40 50 60 0 I0 20 30 40 50 60 70
ENERGY (KeV) ENERGY(KeV)
Fig_Pe 5. Electron and Ion Distrib_ttion Fu_tetions versus Energy
fo/" Day 66. Dashed lines represent spectra before eclipse. Solid
lines represent #ellpse specti'a
196
"" 00000003-TSA08
00 I
• ATS6 DATA _'_ [
". o.,..r )'N
..°o,o,70 l
i • 1...... 1_ t ......... J. l I I
!. s% IO zO 30 40 _o 60 "/0
! TIM¢' (MIN)
i
Figure 6. Duration of Penumbral and Umbral Eclipse
i. Periods as a Functionof Day of the Year for the ATS-6
1976 Spring Eclipse Season. Observed and predicted
i results are plottedr
i, 3. TtlEORETIC,_I[,A._AL_SISi
[.: In the preceding section, the experimental data were presented along with a
_- belef description ot" the plasma corlditions. Based on these data, I feel that the
_: ambient plasma distributionduringeclipseDays 59 and 66 did not change signif-
,_- icantlyexcept for a displacementin thedistributionfunctiondue tocharging effects.
i Sispreviouslydiscussed, itisthe lossof thephotoemission currentas the sun is
! '
_. eclipsed which is the apparent source of the charging. To analyze the charging in
more detail, it is necessary, to accurately model the time variation of the solar
•_ illumination.To accomplish this,data from severalsources willbe used to com-
_ pureand testa model of thepenumbral variation.
i, A simple model of the penumbral region carl be derived by assuming the earth
and sun to be well defined discs. Geometry thbn gives a relation between the pei _-i "
!,: cent&geof solaril'Uminationand time. In Figtlre6, thedurationof eclipseversus
d_y of year rot" the ATS-6 1976 spring ecli[_se season has been plotted. Where it
i ,
was possible to estimate botit, bntry into the penumbral and umbral regions arei
shown connected by a line. The two curves represent the theoretical predictmn of
: _ partial and total eclipse. The data indicate that even this simple theory give_ an
:_ adequate prediction of the duration of partial and total eclipse at synchronous
• orbR. Note that Day 59 is an example of grazing incidence while Day 66 Illustrates
:: direct entry into eclipse.
B - J
_-_ 197
) ,-
00000003-TSA09
The major'objectiontosuch a simple theoryis theexclu._ionof atmo._pheric
effects. For low altitudes(les._thall2 earth radii),_uch offeet,_are clearly
[mportafR. Thls isdCmofistratedin Figure 7 where t_e variationof photoeleciron
flux (dots) versus time as the INJUN V _atellite pas._ed into eclip._e in plotted. At
the time of these data, the INJUN V satellite was at 1.5 earth radii, The _olid
line is a theoretical curve of the percentage of solar illumination versu,_ time
when atmospheric effects are included iri the eclipse model_ Tgnorlng atmosph0ric
effects gives the dashed curve iti Figure 7 (note: the carthts eclipsO boundary
Was placed at 6378 km+ 145 km for this calculation).
Using data from the AE-C satellite, 3 the attentuation, ,, (such that _ _e e'_°
whet, e _ is the observed photon flux), as a function of minimum ray height abov_
sea level, was calculated. At a given wavelength, it was readily found that:
-(x - xo)
,r= e 6 (I)
where
X = heightabove sea level,
X ° : constant,dependenton frequencyof light,and
6 --scaleheight,depetldenton frequencyof light.
As thedetailedvariationofthe photoelectronfluxisdependenton the materialsof
which the spacecraftisconstructedand as the atmosphere itselfvariesintime. it
isdifficultto constructa more exact model. Carefulconsiderationof thephoto-
electronemission versus freqtlencyand the attenuationversus frequencyfrom the
AE-C satellitegave average valuesof:
X --145 km (2)O
6 --23.5-kln . (3)
The INJUN V data of FigUre 7 are fitWitha value of 60 km for6, whereas ATS-6
isbetterfitby 6 - 23.5 kin. The differenceis likelydue to the matvrialsof which
the satellitesare consti-uctedas the INJtJN V detectoris made of tungst,"which is
respotmivetolongerwavelengthsthanaluminum, th_ pi'imaryATS-6 material.
To firstorder, the effectot the Btmosphere isto make the earth appear lar_,cr
(approxifi_ately145 km larger)in radius. Although 6 isnot accuratelyknown, the
AE-C dataimpliesvaluesof less than 100 kin. This uncertaintygeneratestiming
errors inour estimationof theeclipsedui'ationon theorder of30 sec or lessbut
does not greatiyalterthe resultsto be presented. By comparison, emphemeris
198
00000003-TSA 0
• ,.o1.o.
!.o .o!
\.
0 - ' I I | 0
0 I0 20 30 40 _0 6_
TIME (tee)
Figure 7. INJUN V Measurements of the Photo-
electron Flux as a Function of Time in the Penum-
bral Region (Dots). Also shown are two theoret-
ical curves of the solar i[lunainationfor the case
of n_ atmosphere (dashed lln_)and with an atmos-
phere (solidllhe)
errors are of the same ot'der indicating that the atmospheric model is consistent
with the accuracy inl_er_ntin the problem.
4. CONCLUSIONS
The voltage data in Fiffare 3 can be fit as a _nction of time between roughly
-2000 and -10,000 volts by:
Day 59: V _ -103 60. 016(T+530) (4)
Day 66: V _ -10 3 e0.05(T+1563} (5)
whet'e V -=vbltag_. (volts)
; ' T = time (seconds}.
: 199
:=% i
• t , = i r I '
• :. n.o ._. ....._-- , ...._o,_. , o_.....oo "- ..... 00000003-TSAll
Assuming 6 : 23.5 km and Xo : 145 km in the atmospheric model, the ,_olar
illumination vvr_u_ voltage for Day_ 50 and tiff are plotted in Figure 8. The error
bar_ are for the results arrived at by ansuming no atmosphere (upper) or by
assuming 6 , _0 km (lower).
As the percentage of ,qolar illumination in directly related to the Fhotoeleetron
flux (Figure 7), Figure It ran be interpreted as illustrating the effect of varying the
photoelectron flux on the satellite potential. Timing errors in the ephemeris and
the atmospheric model may alter the quantitative results of Figure 8 within roughly
the error bars shown, but will not alter the qualitative result tnhel, ent in the
figure .. namely that the voltage is proportional to the exponential of the photoelec- _..
tron flux. This resultisof realvalue as itcan be u._edas a testofthe predictions
of various theoreticalmodels. In particular,itcan be used to check the modeling
of photoelectronvarlatiolls.As itisalmost impossibleto distinguishsecondary
and back-scatteredelectronsfrom photoelectrons,the resultshave broader impli-
cation_since theycan be appliedto testingany model which includesthe etfect._of
variableelectronemission. This latterobservationisparticularly"mpurtantwhen
one recallsthatstandard sheathmodels imply a logarithmic,not exponential,
relationbetween electronemission and voltage.
I00 9 ' I
__ 8o Y
_ _o
,,
2o I i
I I
I . I
0 l, -I
I( IOt 102 I0a 104
SATELLITE[SOI"ENTIAL(volil}
Figure 8. Composite Plotof the Solar Illumination(or
PllotoeleetronFlux)as a Functionof SatellitePotential
for Day 59 and Day 66
2O0
- I _ i ' I LI ' l'......................._L................_ ...._ ....................._ ............................i..................! .........................L _,_
00000003-TSA12
"_ ,_t I t s u J
Acknowledgments
I would like to thank Drs. DoFora,_t and Whipple for prc_viding th_ ATe-(; data
and taking the time to explain data format,_. U,_eful dl._cu,_,_lon,_ were h¢,Id with
Dr,_t. _urke (INJtIN V data), Hlntt.roggor (AE-C data), Hubin, ltothwoll0 arid Plkp,
References _'
7 I. DeForest, S.E. (1972) Spacecraft charging at synchronous orbit, J.Geophys.
_" Res. 77:651-659.
i 2. DeForest, S.E., and Mcllwain, C.E. (1971) Plasma clouds in the magneto-
': sphere, J. G_ophys. Res. 76:3587-3611.
3. Hinteregger, H.E., Bedo, D, E., and Manson, J.E. (1973) The EUV spectro-
i photometer on Atmosphere Explorer, Had. Set. 8(No. 4):349-359.
201
=°_ .
±
00000003-TSA] 3


Analysis of penumbral eclipse data

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