The third Space Shuttle flight on Columbia carried instrumentation to measure thermal plasma density and temperature. Two separate investigations, the Plasma Diagnostics Package (PDP) and the Vehicle Charging and Potential Experiment (VCAP), carried a Langmuir Probe, and the VCAP also included a Spherical Retarding Potential Analyzer (SRPA). Only those measurements made while the PDP is in the payload bay are discussed here since the VCAP instrumentation remains in the payload bay at all times and the two measurements are compared. The wake behind a large structure (in this case the Space Shuttle Orbiter) flying through the ionospheric plasma is discussed. Much theoretical work was done regarding plasma wakes. The instrumentation on this mission gives the first data taken with a large vehicle in the ionospheric laboratory. First, the PDP Langmuir Probe and its data set will be presented, then the VCAP Langmuir Probe and SRPA with associated data. The agreement between the two data sets is discussed and then followed by some other PDP data which infers an even lower wake density

Murphy, G. B.

Pickett, J. S.

Raitt, W. S.

Shawhan, S. D.

English

NASA Technical Reports Server

: ELECTRON AND ION. DENSITY. DEPLETI:ONS MBASURliD. IN_THE..,_T$..=._...._O.RBI.T_RWAKe* ...........
G. B. Nurphy and J. S_. Pickett
University o£ Iowa
Iowa City, Iowa 52242 ........
W. S. Raltt
Utah State University
Losan, Utah 84322
S. O. Shawhan
NASA Headquarters
WaShington,_D,_C. 20546
The third Space Shuttle flight on Columbia carried instrumentation to measure
thermal plasma density and temperature. Two separate investigations, the Plasma
Diagnostics Package (PDP) and the Vehicle Char_ing and Potential Experiment (VCAP),
carried a Lan_muir Probe, and the VCAP also inclUded a Spherical Retarding Poten-
tial Analyzer (SRPA). The Langmuir Probe on the PDP made measurements while the
PDP was attached to the pallet in the Orbiter bay and while the PDP was articulated
by the. RMS.. Only those measurements made while the PDP is in the payload bay are
discussed here since the VCAP instrumentation remains in the payload bay at all
'_ times an_.,the two measurements are compared.
Figure I illustrates the location of the PDP andVCAP instrumentation on the
science payload pallet.
The principle thrust of this paper is co discuss the wake b_hind a large
structure (in this case the Space Shuttle Orbiter) flying through the ionospheric
plasma. Much theoretical work has been done regarding plasma wakes (ref. t) and to
a certain extent laboratory plasmas have provided an experimental and measurement
basis set for this theory. The instrumentation on this mission gives the first da-
ta_£aken with a large vehicle in the ionospheric laboratory.
First, the PDP Langmuir Probe and its data set will be presented, then the
VCAP Langmuir Probe and SRPA with associated data. A discussion of agreement be-
tween the two data sets is then followed by some other PDP data which infers an
even lowec.-wake.density.
Lastly, conclusions, caveats and a description of future work which will fur-
ther advance the measurement techniques and data set are put forth.
PDP T_GHUIR PROBE RESULTS
The PDP Langmuir Probe is a 6 cm diameter gold-plated sphere which is operated
in two modes, the AN/N mode and the swept mode. The swept mode which is of concern
'_ *This work i_ sunported by NASA/Lewis Research Grant No. NAG3-449
33
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https://ntrs.nasa.gov/search.jsp?R=19850014163 2020-03-22T18:42:11+00:00Z
: VCAP
i_ /SRPA!_!_./ VCAP
; STARBO
i_i_ PORT
_-_
-?_.
_ PDP- _'_" y
FORWARD.......
_;_ FiSure 1. - Science pa!let__confisuration on STS--3 showing location of instru-
_ mentat _on
_,_.. _
here is a 120 step voltage sweep which lasts 1.2 seconds and is executed 5 times! per minute. The nominal density range of the probe Is approximately 5 x 102 to
5 x I06/cm3 , the precise sensitivity depending on temperature. Operating in this
mode, the Langmu_Lr Probe has a current voltage characterlstic whose slope is pro-
portional, to I/T e and which has a "knee" in the curve proportional to Me.
There a_e two l£mltations to the PDP Langmuir Probe measurements. The first
_:_ occurs when the plasma is too dense to really see the entire knee of the curve re-
i_ suiting i_ instrument saturation and an underestimate of density. The second oc-
i_ curs when the plasma temperature is too high and density too low to get a reliable
:_ slope resulting in only an upper bound on density and lower bound on temperature,
i Figure 2 illustrates the electron density and temperature for one orbit as a ,_
functiOn of vehicle attitude. (_he data is repeated for a second orbit to provide
_ clar.ity for the graph and illustrate a periodicity which is real). The vehicle
, attitude Is described by _ i and _2 which are illustrated at the top of the figure.
!=i Maximum wake occurs when the vehicle flies tail first with the plas_ ramming into
i._ the Orbiter belly (e.g. G_ 83:20:48). At this point iu time, the _=hicle is
i-i_" flying a nose-to-sun attitude with a 2 times orbit roll. (See figu_'_ 9.in the pa-
_-_':'_ per "Suprathermal Plasma Observed on the STS-3 Hisslon by the Plas,a Diagnostics
! Package, by Paterson et al. (ref. 2) in this issue for a description of this
_ attltude.) This results in a once per o_;bit ram/wake cycle which is evident in
! figure 2 by the e- density and neutral-denslty (pressure) measurements ........................
Several important observations summarize figure 2:
1. Although density is near ambient while the paylaod bay is neither pointing
directly into the velocity vector or into the wake, there is evidence that
the density may be 2 to I0 times ambleut when the bay points close to the
velocity vector. The probe saturates making reliable measurement above
2 x I0_ difficult. The region cross hatched in flguze 2 is where this
higher density regime is encountered.
[
._ 34
!i,
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..... .................................................. 00000001-TSD03
2, Density decreases rapldly as the Or_blter rolls into wake cond____!.o-,
3. The B,tnimum rellable, measurement o_ density with the PDP probe is
app_roxlmately 5 x l_/cm 3 , At least another order of magnitude decrease
is required to pull the. sweep totally of£scale which is subsequently
observed to happen. The sweep ¢emalnS offscale for approxlmately
25 minutes centered around 83:20:48.
4. During all non-wake conditions, the temperature remains relatively
constant at about 1000° (+30%),
5, Temperature rises rapidly as density decreases,
6, The highest reliable temperatur.es occur at 6000°K, However, the trend_
continues suggesting temperatures in excess of 7000°K in.._.the deep wake.
It is also worthwhile to note that in near ram condition the neutral density
(pressure) was almost two orders of magntt-de above ambient ionospheric conditions
and feJ, l below 10 -7 torr (the instrument se ativity__!tmtt) during wake conditions,
THE___AP.LANGHUIR PROBEAND SRPA
Data on the characteristics of the ambient thermal plasma are extracted from
tim probes using a technique similar to that described by Raitt e_ al._ (r_f. 3).
This AC technique employe_ for..the probes enables direct measurement of the second
derivative of the SRPA current-voltage characteristic and the_first derivative of
the LP current-voltage characteristic.
The SRPA signal is obtained by adding two stnusotdal AC signals (at 8.5 kHz
and 10.7 kHz) to a sawtooth sweep voltage. The probe current is passed through a
narrow band amplifier that selects the difference freqency of 2.2 kHz, which is a
measure of the non-ltneartty of the probe current-voltage characteristic, and re-
sults in a signal proportional to the second derivative of the current-voltage
characteristic. Two ac current xanges are available: one from -76 dB to -24 dB ....
and the other from -40 dB to 0 dB relative to 10 -7 amp rma. Each successive sweep
of the probe alternates between the two ranges. Since the sweep period is
17 seconds the complete dynamic range is covered each 34 seconds.
The LP has only one AC signal (at 3.2 kHz) added to the sweep voltage. The
amplitude of the alternating component ot the probe current derived by using a nar-
row band amplifier tuned to 3.2 kHz enables the first derivative of the current
voltage characteristic to be measured directly. A single dynamic current range,
from -80 dB to +10 dB relative to 10-6 amp rms, is used for all sweeps. The range
of the sawtooth voltage is from -2 V to +3 V, the period and phase of the sweep be-
ing synchronized to the SRPA sweep.
Figure 3 illustrates data taken under similar conditions as that taken by the
PDP, although at a different time. In this case the vehicle attitude is differ-
ent, but the same angles are used to charac_erize the direction of the velocity
vector. _te addition of the dark bar on this figure serves to show when day and
night occur during the orbit.
35
00000001-TSD04
', L i
I
I
: ii_:,__¸ 36
_ .... .................. oooo -
!The results of the Langmuir Probe (dotted line) and SRPA (solid line) general-
i ly confirm, results of the PDP LangmuLr P_obe, VCAP Lan. "u[r Probe temperatur_._ are
not _lotted, but.the following result_ are notable:i
!i ....
1. Close to ambient (1000°K) ionspheric temperatures are measured during
non-wake coadition.
i
!-_, 2. As the Orbiter rolls into wake, a turbulence at all frequencies adds noise
i__ to the 3.2 kHz LP first derivative, but measurements indicate an increase .................
__ in temperature to beyond__4flfl0_K.
i-
i= t VCAP LP densities indicate the following: _i
!. 1 .... An upper botmd of electron density when the pa)_aod bay faces close to
the vel3city vector iS lO7/era 3.
i,-'- 2, Dens,ity during wake conditions drops to below the instrument sensitivityi_. "......................
i_: of 10_ elc_.
:-'_:_ The SRPA measurements are difficult to interpret since the peak in the second
derivative as a function of sweep voltage for the dominant ionospheric 0+ ion is
_i often contaminated by locally produced H2O+ and NO+. Nhen the .0+_peak is clearly
!! observable, s_veral observa_ons prevail:
_ I. Densities consistent with ambient ionospheric 0+ are observed for most
! conditions which shall be referred to as non-wake.2. _ 2 orders of mag_letlon occurs in the-near wake ....................................
_-_' ADDITIONAL EVIDENCE FOR LARGE DEPLETION
-_ Additional evidence for a many order of magnitude depletion in the electron ._
i _ density in the near wake is provided by what amounts to a sounder experiment. Re- ,... ii
_ !_, call that the VCAF SRPA is excited with a signal at 8.5 and I0..7 kllz. The PDP con-
_" rains a 16 channel (+-15Z bandwidth) spectrum analyzer capable of detecting electro-?
i:_, static or electromagnetic waves over a frequency range from 30 Hz to 178 kHz. The
i_ instrument has a saturation of approximately I V/m electric fleld-amplitude and a
usable dynamic range of about 95 dB.
_ During most of the orbit, the Spectrum Analyzer output is- dominated by broad-
i_ band orbiter generated electrostatic noise, (ref. 4) thruster firings or other
i-_ events Figure 4 illustrates that as the wake boundary is approached, the electro-! ,
i-: static noise disappears in all channels simultaneously and as the payload bay is
immersed.deeper in the orbiter's wake a signal in the 10 kHz channel grows to a
i- point of dominance in the spectrum. This in fact is the VCAP SRPA signal. AS the
: density drops so that the plasma frequency nears or drops below 10.7 kHz, this sig-
: hal can propagate to the PDP sensor. Detailed calculations and modeling are being
_ done taking field strengths and sensor separation into account, but preliminary
i_ work suggests that although the PDP Langmulr Probe infers densities, < 50/cm 3,
i' the density probably drops at least another order of magnitude to < 5/cm3. This
would be approximately six orders of magnitude of plasma depletion in the near wake
from that measured under ram condition.
37
00000001-TSD06
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¥isure 3. - Summary of VCAP Langmuir probe (dotted line) and SBPA (solid line)
results as function of vehicle attikude ....
38
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WAKE
1982 DOY83 HOUR.20
Fisure 4. - VL_ electric field spectr_m-showLns_in¢_easin8 intensity of received
SRL)A sisnal.
SUt_ARY
,1: (
_.,_ Although measurements are still in a primitive state, several conclusions can
be drawn from the STS-3 PDP and VCAE data.
_.,_:; l. Ram condttlons seem to result in a higher than expected electron density.
_i 2. Densl_y depletions of at least 4 orders of magnitude in the wake plasma
_'-_i_,' are observed and there :Ls evidence to suggest this depletion may be as,
"'_.':t-.! high as six orders of magnitude.
_. 3. Effective temperature measured by the thermal plasma probes indicate an
_:._, increase in electron temperature In the wake to ) 6000°K.
i;_i'.; 4. The thermal ions are excluded rapidly as the orbiter bay rolls into wake
_!_[ and only those locally produced H2O+ and NO+ are measurable.
_i_ 5. Both LP_s and the S_PA1ndfcate a degree of plasma density or velocity
_. turbulence which peaks in the-tre.usttion region between ram and wake. __
,,__,_, Several concerns about these measurements are that: first, the VCAP probes'
outputs are often contaminated by the turbulence which causes bias in the data;
_i": second, the ability of the PDP LP to measure density and temperature reliably be-
.... yond s certain limited range is questionable; and thlrd_ whether the sounder exper-
_ Iment setup between the VCAP SRPA and PDP Spectrum Analyzer is "calibratable" _s
_,,_ still an open question.
_i
00000001-TSD08
i
! The first concern is being worked and there Is confidence that corrections, for
i the turbulence can be computed. Recall that the PDR LP has a AN/N mode which-can
i_, provide upp_er bounds on the turbul_nce-_tCh£n a g_ven frequency band.
_- The eecond concern,, which applte_ to a lesser degree to the VCAPLP, is harder
___ to solve_ A_ the density decreases-and temperature increases, the size of-the
_ pr_e in relation to a debye length and thermal electron gyroradius changes drasti-
i tally, This means that approximations used to derive temperature and density are
no longer valid and new formulations must be used. A long-term research effort is
-_ underway to better understand the behavior Of swept probes in these extreme reg-
,2- imes. (See re£. 5 for a description of the probe theory). Meanwhile, effort has
i_ been made to Include data in this report derived from regimes where apprnximations
;_ hold. Thus, the densities and temperatures are probably good to a factor-of two.
_ It is encouraging to note that when comparisons are made to measurements made
_:. by the DE satellite, which f_lew through the same altitude and latitude regime
i-,_' within the same day, general agreement is found. The DE data show daystde condl-
_12_: ttons of Ne ffi .9 -1.1 x 106/cm 3 and Te ffi 1500 ° - 2000 ° while the PDP and VCAP data
_:i'_=i;_ taken dayside out of wake and also out of maximum ram condition indicate Ne = 2 to
:__',,, 10 x 106/cm3 andT e _--4000 ° (+30_.).
_o! The third concern Is currently being worked and if the "sounder" is calibrat-
e.} able, _ should p_ro_vtde valuable Input for theory.
_ The fact that elevated temperatures are observed in the near wake of a space-
craft is not without _recedence. Samir et al. (ref. 6) found evidence for elevated
i-". election temperatures In the wake of Explorer 31, a much smaller vehicle than_the
_ Shuttle Orbiter.
iN-
_=" Additional measurements by the PDP and VCAP instruments will be made on
,=_" Spacelab-2 where detaLted experiments have been designed to study the structure of
i=i_ the wake out-to approximately one kilometer from the vehicle.
i.
! REFERENCES_
_ 1. Santtr, U.; Wright, I. H., Jr.; and Stone, N. E.: The Expansion of a PlaSma
into a Vacuum--Basic Phenomena and Processes and Applications to. Space
_._i Plasma Physics. Submitted to Reviews of Geophysics and Space Physics
_-_ Space Science Laboratory, M_FC, Preprtnt 083-102.
2. Paterson, W.4 Frank, L. A.; Owens, H.; Pickett, J. S.; Murphy, G. g.; and
Shawhan, S. D°: Suprathermal Plasma ObServed on the STS-3 Mission by the
; Plasma Diagnostics Package. Submitted to Proceedings of the Spacecraft
; Environmental Interactions Conference, October 4-6, 1983.
i
i '
!
.... 40
_i_ .: _,_,. - -.;,,
00000001-TSD09
3. Raitt, N_ J.; DorLtng, g.. B.; Sheather, P..H.; and Blades, J." Ionospheric
Measurements from the-_ Sa£eIlite. Planet Space S_f., 23, 1085-!101,
1_75,-
4. Shawh_ln, S. D.| Murphy, G. B.; and P_ckett, J. S.: Plasma Diagnostics Package
X_._Ltttal Assessment. of the Shuttle Orbiter Plasma. Euvi.ron_nu. Accepted
for pttbl_c,_$.on, December 1983, J. Spacecraft and Rockets.
5. Robtr, stein, J. and Laframboise, J. G.: Theory of a Spherical Probe in a
Collisionless _lagnetoplasma. Phys. F_.uids25 (7),_Jt,ly1982, 1174-1182.
6. Samir, U. and Nrenn, G. L. : Experimental Evidence of an Electron Temperature
Enhancement in the Nake of-an loncspheric Satellite. Planet. Spa__Sc£_,
.TL972,Vol. 20, 889-904.
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O0000001-TSDIO


Electron and ion density depletions measured in the STS-3 orbiter wake

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