The differential energy spectra of electrons between 1 and 100 eV were measured by an electron spectrometer flown on an early shuttle. This energy range was scanned in 64 incremental steps with a resolution of 7%. The most striking feature that was observed throughout these spectra was a relatively flat distribution of the higher energy electrons out to 100 eV. This is in contrast to normal ambient spectra which consistently show a rapid decline in quantitative flux beyond 50 to 55 eV. The lower energy (1 to 2 eV) end of these spectra showed steep thermal trails comparable to normal ambient spectral structure. In general, daytime fluxes were significantly higher than those obtained during nighttime measurements. Quantitative flux excursions which may possibly be associated with thruster firing were frequently observed. Spectral structure suggestive of the N2 vibrational excitation energy loss mechanism was also seen in the data from some measurement periods. Examples of these spectra are shown and possible correlations are discussed

Delorey, D.

Hills, R.

Mcmahon, W.

Salter, R.

English

NASA Technical Reports Server

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D. Dolo_'oyBoston. College
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i"i The differential energy spectra of electrons between 1 and 100 eV were measured
by an electron spectrometer flown on.an early shuttle. This energy range-was scan-
ned in 64 incremental steps with a resolution of 7T. The most striking feature
:_ that was observed throughout these spectra was a relatively flat distribution of
'_ the higher energy electrons out to 100 eV. This is in contrast to normal ambient
'_ spectra v_hich consistently show a rapid decline in quantitative flux beyond 50=55
_ eV. The lower energy (1-2 eV) end of these spectra showed steep thermal trails
_;. comparable to normal ambient spectral structure. In general, daytime fluxes were
significantly higher than those obtained during nighttime measurements, quanti-
_. tati.ve flux excursions which may possibly be associated with thruster firing were
;' frequently observed.. Spectral structure suggestive of the N2 vibrational excitation
_ energy loss mechanismwas also seen in the data from somemeasurementperiods.
Examples of these Spectra are shownand possible correlations are discussed,
_:: INTRODUCTION
Ii\-
!:. The purposeof this experimentwas to examinethe role of low energyelectrons
,_. in the interactionbetweenlarge vehiclesand the space environment,and to assess.............
the extentof contaminationpresentedby these electrons. For this purpose,an
,_, electronspectrometerwas flown on an early shuttle. This instrumentwas a 1270
I!_ cylindricalelectrostaticdeflectionanalyzeressentiallyidenticalto thatdes-
_:._ cribed in referenceI. Differentialenergy spectrabetweenI and 100 eV were
_.i obtainedby applyingthe analyzervoltagein 64 incrementalsteps, The analyzer
_r,_" resolutionwas 7% and the steppingincrementswere set at 7% to match this and thus
':.:, avoid,gaps in the spectraldata, Dwell time per stepwas set at 1.0 sec, thereby
,: requiring64 secs to scan a full spectrum;this was found to be an undesirablylong
':,: time period,as will be later discussed. The instrumentwas mountednear the rear
;' of the shuttlebay with a lookdirectionout over the rightwing, along the -Y axis
i._ and tilted120 upward from it.
f
! Before showing the shuttledata it ,_i11be useful to first sllow,by way of
:.:_ contrast,wha':the ambientor relativelyuncontaminatedspectralook like. The
examplesshown in figure1 are spectraof ambientdaytimephotoelectronswhich were
i. obtained on a rocketflightusing an instrumentidenticalto that used on the shut-
_, tle. Tilehighestaltitudeshown,however,is about80 km below shuttlealtitude.
• Theseare typicalof many hundredsof spectraobtainedduring severalrocketflights
!
https://ntrs.nasa.gov/search.jsp?R=19850014165 2020-03-22T18:42:06+00:00Z
¸and _how-fourchiefcllaractoristicS:There is first a steep thernlaltail at very low
energies(<2oV),only th_ lowerp_rts of _hich are s_en in these partic_rlarspectra.
Second is the valley-like structure hetwe_n 2-5 eV--cau_odby energy loss duo to
resonant vibrational excitation of N2 by electron impact -- the so-call_onltroqen
bite-out.. [,_ should he noted, for later reference, that this feature diminishe_in
the ambient spectra as altitt_dn-increases toward 200 km and disappears ahow thi_,
well below sI_ttlealtitude. Third, in the 20-30 oV rm.lionf6r daytimespectra
only, there are _everal closely ._pacedp_aks due to photoioni;,ation of N2 and 0
hy the intensesolar lleII line at 304A; thi._structurei_ shown here as combined
by the limitedanalyzerresolutionto form on_ broadenedpeak. And fourth,there
is a rapidquantitativedecreasein photo_lectronsabove about 55 eV due to a
pronounceddecreasein the solar euv flux at higher'energies(wavelengthsless
than about 170A).
MEASUREMENTS
The shuttlespectrashoweddeparturesfromthe ambientcharacteristicsnoted
above, someof which were expectedand some of which were not. Figure2 shows a
typicalspectrumwhich is actuallythe averageof all Spectraobtainedduringthis
particulardata acquisitionperiod. In commonwith ambientspectra,a thermaltail j
below 2 eV is seen,as was expected. There is no apparentstructurein the 2-5 or
20_30 eV regionswhich was not unexpected. What was quite unexpected,however,is
the continuedrelativelyflatdistributionof electronshaving energiesgreater
than 50 eV, out to the highestenergymeasured (100eV). This was the most striking
characteristicof thesemeasurements;it was presentin every spectrumtaken during
the flight. The sourceof these higherenergyelectronsis, as yet, unexplained.
But it clearlyindicatesthe need for one experimentalchange for the next flight:
' that is to extendthe energy range coverageout to 500 or I000 eV to see where, or
: if, the expectedshouldercan be found. Figure3 shows three individualspectra
obtainedin this samemeasurementperiodduringwhich the vehiclewent fromdaylight
to darkness. As Shown,the quantitativeelectronfluxdecreasessignificantlyat
• night,clearlysuggestingthat locallygeneratedsolar photoelectronscontributein
largemeasureto the daytimeelectronenvironmentof the shuttle. Two of these
spectraalso show considerablescatteror data point excursions. In an attemptto
correlatethis scatterwith other vel_iclevents,periodsof thrusterfiringare
shown,althoughthis cannothe considereda direct sourceof electronsin the energy
rangeshown here. It shouldbe noted that thrusterfiringsmentionedhereinrefer
to all cases to the verniers. There was very littlefiringof the primarythrusters
durit_gan) of these data acquisitionperiods,and what littlethere was appearedto
have no effecton the data. Thrusteroperationinvolvesa hypergolicreaction
betweenmonomethylhydrazine(MMH)and nitrogentetroxide (N204)and while this
generateswhat might be calleda hot plasma,it cannotdirectlycontributeelectrons
havingenergiesmuch greaterthan about2 eV. Some subsequentacceleratingmech-
anismwould clearlybe requiredif thrusteractivityis to be associatedwith these
flux excursions.
Thrusterfiringsoccur for a minimumof 80 msec, althoughin many cases one or
more were activefor considerablylongerperiodsof time, But this pointsup
anotherway in which the experimentshould be changedfor a subsequentflight:TIie
presentscantimingsequence (lsec/step,64sec/scan)is simplytoo longto observe,
over the betterpart of a spectrum,any short term phenomenonwhich may have oc-
curred. Veryshortterm phenomenoncould thereforeappear as the excursionof a
7_
O0000001-TSF14
!single data point, or a few cOnSeCgtive points, which may account for the apparent
scattereffect. Forthe presentdata to be asSociated-withthrusterfiring,
however, requires that some sort of delayed-reaction or response tithe be inferred
-a seemingly plausible concept. But-the apparent response time involved ksnot
manifested in any constant manner. The topmost spectrum of figure 3, for example,
shows a-sharp drop in flux levels approximately 2 seconds after thruster firing
ceased while the mixldlespectrumshowsa delay of ]3-14seconds beforea similar
responseis observed. Additionali11ustrationof this is seen in figure4 which
shows three individualspectrataken during a common nighttimeperiod. The loweSt
spectrumshown (C)was completelyfree of thrusterfiringduring its measurement
and for a periodof 15 minutesprior to that. The topmostspectrum (A)was taken
followinga periodof moderatethrusteractivityanalin one during which there was
t nearlycontinuousfiringof all six vernierthrusters. The middle spectrum(B) was
taken immediatelyfollowingthis but representsa largelythruster-freemeasure-
ment; firingoccurredduring the very early (low energy)segmentof this spectrum
and then ceasedfor all data pointstaken above 3.4 eV. This spectrumseems to
indicatea tendencyto returnto the normallower nighttimeflux levels,but has
not reachedthis point some 50 secondsafter thrusterfiringceased. These ap,
parent variationsin responsetime,therefore,tend to obscurethe possible
associationof thrusterfiringswith data perturbations.
iJl An enhancedview of the steel)thermaltail below 2 eV is seen in the spectrum
;=_ of figure 5. This suggeststhat the generallynegative (1-3volts)vehiclepot-
;_ entialhad decreasedor gone positiveat this titne,allowingmore of the very low
;_ energyelectronsto _nter the analyzer,thu. allowingmore of this tail to be
_? observed. This is supportedby independentmeasurementsfrom a comp_anionexperi-
._ ment (ref.2), the data fromwhich indicatesa vehiclepotentialof about +0.8
_ volts at the t:imethis spectrumwas taken._
That th-usterfiringsmay contributeto higherflux levelsis consistentwith
the data shown next in figure 6. This relatesto a 35 secondorbitalmaneuvering
_i_ system (OMS)burnwhich occurredduringthis period. The OMS employsthe same
hypergolicreeactionas describedfor the thrustersexceptthat each of the two
engines in this system generates6000 pounds of thrustas comparedto only 24
:=: poundsfor each of the vernierthrusters. This figureshows two typicalindividual
spectra,the lowerof which was taken prior to the OMS burn, and the upper taken
duringand immediatelyafter the burn. Only a littleover two minutesseparated
these individualspectrain time and yet a differenceof an order of magnitudeor
i more is seen in the flux levels.
:-: A substantialnumberof spectrawere obLainedwhile the shuttlewas in a
__ bottomto-the-sunattitudeand while these were almostexclusivelydaytimedata,
their flux levelswere aiqongthe lowestof the flight. Typically,these spectra
_ were indistinguishablefrom nighttimemeasurements. It seems likelythat the term
"bottom-to-the-sun"can explainthese low fluxes:There was almost certainlylarge
numbersof solar photoelectronsbeing generatedat the sunlit bottomsurfacesof
the wings and fuselageduringthese periods. The subsequenttrajectoriesof these
electronswould largelybe governed,exclusiveof collisionprocesses,by the geo-
magneticfield lines. And no matterwhat the orientationof these lines,it seem
unlikelythat many of these electronscouldmigrateinto the shuttlebay area and
enter the acceptancecone of th_ analyzer.
/3
00000001-TSG01
IThere was little or no thruster activity during all measi_rementsof this mode and
therewas _ery Little variation between individual Spectra obtained during these
_-_ periods; this is agala consistent _ith the possible correl-ationdata perturbations
and thruster fi.ring, There was a passage from day to night in the I/)minute bottom-
to-the-sun run illustrated in figuce 7 wh$ch did not _ppea$ to haw any effect on
spectra except at very low energies,....This is a_ averaged spectrum but i.tvery
closely represents all those obtained, both day and night, except for the I-3 eV
region where an enhanced view.of the thermal tail again appears. Themaxim_n data--
point for this tail was a factor of 2 or _norel_igherthan shown in t_is averaged
spectrum, and-was fairly constant in the daytime spectra. Following eclipse,
this dropped immediately by a factor of 8 to 10 and remained quite constant in a-It
of the night spectca. This again suggests a diminished vehicle-potential and,
again, this was supported by the independent measurements of reference 2. It also
indicates the influence of daytime solar photoelectrons on the low energy (1-3
eV) region of these spectra but not elsewhere which, i#,turn, indicates the
influence of collis.)n processes since most of these el_ctrons must have come from
the sunlit side. Note alsn that the data points in the extreme low energy region
of this spectrum are seen to be reduced substantially below the m_ximum point, an
_.: effect observed in all the spectra of this run. This was due, almost certainly,
to geomagnetic shadowing. The orientatin of the geomagnetic field lines throughouti-
_ i the run was toward the aft quarter and downward with respect to the vehicle, or to
:_, the right and downward with respect to the analyzer look direction. This means
_ that some electrons spiraling about field lines in trajectories that would nor- :!
mally bring them within the acceptance cone of the analyzer could, in some cases, i
_ be intercepted _y vehicle or payload component_ and thus be lost to collection.
. The lower energy electrons are most vulnerable to suc_shadowing effects because
_ : of their shorter Larmor radii. In the case of _he high termal tail seen earlier
: in which no such effect was observed, the field lines were oriented to the left,
or forward, and above, where no vehicle or payload components existed to obstruct
_ the electron paths. •
_gure8 shows a spectrum typical of all those obtained during two of these
bottom-to-the-sun periods. These were daytime measurements during which there
were no thruster firings and virtually no difference between individual spectra. ,
. Note the valley-like structure in the 2-4 eV region. This feature was present
in all spectra of these two consecutive runs and is strongly suggestive of the
N2 vibrational excitation energy loss mechanism (the nitrogen bite-out) mention-,
ed earlier as a characteristic of ambient spectra below 200 km, This would not
.- be expected to appear in ambient spectra at the shuttle altitude (300 km). But
if sufficient N2 were present, the mechanism that produces this structural
feature should prevail. Residual N2 from the MMH/N204 thruster reaction might
possibly have provided this. The question of why thls feature was seen only in
these data, but seen consistently throughout them, has no immediate answer.
SUMMARY/CONCLUSIONS
Undoubtedly, the most remarkable feature of these measurements is the rel-
atively flat spectral distribution of higher energy electrcns whicllwas consist-
ently observed but remains essentially unexplained. And while the data asso-
ciating quantitative electron flux excursions with thruster and OMS firings can
be considered pursuasive, it cannot be considered conclusive based on these data
alone. The data do seem conclusive in indicating that locally generated solar
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.... , photoelectrons contribu.tesubstantially to the daytime electron environment of
_." the shuttle. Other ohserved departures from the general,characteristics of these
spectra atso lend themselves to explanations as, for.. exampLe, geomagnetic shadowingor the N2 bite-out mechanism, And the-occasional enhanced observati.ons of the
,._ steep thermal tail is _robably explained by departures in vehical potential- levels,
i Many questions remain, however, A reflight of this experiment, modified as eartier
' de_ibed, should.provide very useful add_nal-data and, possibly, some answers,_-
i REFERENCES
ii, 1, McMahon, W.J, and Heroux, L., Rocket Measurement o_ Thermospheric Ph.otoelectron
i Ener_ Spectra J. Geophys. Res., 83, 1390 (1978)., 2. Smiddy, M; Sul_ivan, W.P. Girouard_-'D.;and Anderson, P.B., Shuttle Electrical
Environment, P.roceedin.qsof Spacecraft Environmental Interactions Conference,
_. Paper I-4, NASALewis Research _nter, (1983).
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Measured electron contribution to Shuttle plasma environment: Abbreviated update

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