A POTENTIAL CONTROL METHOD FOR THERMAL PLASMA MEASUREMENTSON THE
DE-1 SPACECRAFT
R. C. Olsen & D. L. Gallagher
University of Alabama, Physics Department
Huntsville, AL 35899
C. R. Chappell & J. L. Green
Space Science Laboratory
Solar Terrestrial Physics Div. NASA/Marshall Space
Flight Center, Huntsville, AL 35812
S. D. Shawhan
University of Iowa
Department of Physics and Astronomy
Iowa City, IA 52242
ABSTRACT
Thermal plasma measurements from the Dynamics Explorer
1 spacecraft using an aperture bias in the outer plasmasphere
and over the polar cap show the existence of cold plasma (T
< 1 eV) which is otherwise hidden from the particle detector
by spacecraft potentials between +1 and +3 V. In eclipse, the
photoelectron current drops and the spacecraft potential
drops to near zero volts. At density levels between 10 and
100 cm s, the cold plasma becomes visible in eclipse, along
with a warmer tail to the distribution. Operations of the
RIMS instrument with the external aperture plane biased up
to -8 V with respect to the spacecraft show the existence of
low energy field-aligned flows over the polar cap.
Electrostatic models of the spacecraft indicate that if the
spacecraft potential is more than a few volts positive, there
can be a saddle point, or an electrostatic barrier inhibiting the
effectiveness of the aperture bias. Comparison of the aperture
bias experiments with the electrostatic models show that the
aperture bias is successful in drawing in the field-aligned
flows, with kinetic energy greater than the predicted barrier
height, but may not be sufficient to measure the isotropic
background.
1. INTRODUCTION
History
Plasma measurements in the magnetosphere should provide a
determination of the plasma density, temperature, mass
composition, and pitch angle distribution in order to
completely describe the environment. Complementary
particle and wave measurements have shown that at times
there is good agreement between the two. Gurnett and Frank
[1974] compared 88-eV to 38-keV particle measurements in
the plasma sheet with the total electron density as determined
from the lower cutoff frequency of continuum radiation as
measured the plasma wave instrument on Imp 6. GEOS-1
measurements in the plasmasphere showed good agreement
when the spacecraft potential was low (0 to +2 V, density
greater than 5 cm 3) [Decreau et al., 1978]. Outside the
plasmasphere, at higher spacecraft potentials (+2 to +5 V),
agreement was poorer, particularly with the ion detectors. It
was speculated that some part of the pitch angle distribution
was not being observed, that an unsampled high energy
population might provide the 'difference, or that the cold
plasma was excluded from the spacecraft ion detectors by the
positive spacecraft potential. One clue that this last answer
was the correct one came from data taken in eclipse by the
SCATHA satellite. Particle data taken with an electrostatic
analyzer showed that a cold plasma population (T 'L 0.5 eV)
with a density between 10 and 100 cm -3 appeared suddenly
when the satellite was eclipsed, but was hidden in sunlight
[Olsen, 1982). These measurements were not complemented
by wave data, however, and the analyzers used for these
measurements had a sharp drop in sensitivity below 2 eV that
left open the possibility that a more sensitive instrument
might succeed in measuring the cold plasma distribution in
sunlight as well as eclipse.
The purpose of this paper is to show further measurements of
ordinarily 'hidden' ion populations, and to demonstrate that
without some form of potential control, it is possible to miss
the major component (by density) of the ion plasma
distribution. Data from the Dynamics Explorer 1 (DE-1)
satellite will be shown from the outer plasmasphere and over
the polar cap, giving examples of potential variations in
eclipse, and the effectiveness of an external aperture bias in
allowing the cold plasma component to be measured. An
electrostatic model of the aperture bias process is presented,
and compared to data taken in the plasmasphere and polar
cap.
1.2 Spacecraft and Instruments
The DE-1 satellite was launched into a polar orbit on August
3, 1981, with a perigee of 675 km altitude, and an apogee of
4.65 RE. The 7.5 hour orbit began with apogee over the north
pole, with apogee moving towards the equator (in the
midnight sector) in the spring of 1982. The space-craft spins
in a reverse cartwheel fashion, at 10 rpm with its spin axis
perpendicular to the orbit plane.
The Retarding Ion Mass Spectrometer (RIMS), is composed
of 3 sensor assemblies. The radial sensor head views
perpendicular to the spin axis and provides pitch angle
distributions, while sensors on the ends of the spacecraft
view parallel to the spin axis to complete the phase space
coverage. The radial sensor head has an angular resolution of
t10° in the spin plane and ±55° perpendicular to the spin
plane. The end heads, or Z heads, have roughly conical
apertures with 55° half angles. The instrument covers the 132 AMU mass range with two channels. The low channel
covers the 1-8 AMU range, the high channel 4-32 AMU. In
this paper, we will show only H+ and He+ data using the low
(1 AMU) and high mass (4 AMU) channels in parallel. The 2
channels have different sensitivities and flux comparisons
between channels should not be done from the raw data. A
retarding potential analyzer (RPA) precedes the mass
analysis, with a voltage sweep from 0-50 V. In addition to
the RPA, there is an external aperture plane which is
normally grounded to the spacecraft body, but can be biased
with respect to the spacecraft to values of -2, -4, and -8V.
This was provided in order to overcome the expected
positive spacecraft potential [Chappell et al., 1981].
The plasma wave instrument (PWI) experiment provides
wave information from 1.8 Hz up to 409 kHz, using a low
frequency correlator (LFC) from 1.8 - 100 Hz, and step
frequency receivers (SFR) from 104 Hz to 409 kHz. The
LFC covers the 1.8-100 Hz frequency range in 8 roughly
logarithmic steps, and the SFR covers the 105 Hz to 409 kHz
range in 128 steps. In the normal operational mode, a
complete frequency sweep takes 32 s--just over 5 spins of the
spacecraft. The data shown in this paper come from the long
electric antenna (200 m tip- to-tip) perpendicular to the spin
axis (Ex) [Shawhan et al., 1981]. Magnetic field data are
obtained from the DE-1 Goddard Space Flight Center
magnetometer [Farthing et al., 1981]. These data provide the
gyrofrequency and pitch angle information needed to
interpret the wave frequency spectra and particle angular
distributions.
2. OBSERVATIONS
2.1 Eclipse and Sunlight in the Plasmasphere
Observations from GEOS and SCATHA [Decreau et al.,
1978; Olsen, 1982] show that a typical floating potential for
a satellite in the outer plasmasphere is between +2 and +10V.
In eclipse, the spacecraft potential would drop to a negative
potential of the order of the electron thermal energy, if it
were not for secondary emission. With 100-1000 eV
electrons present, i.e., at the inner edge of the plasma sheet, it
is possible to have a floating potential in eclipse of up to +5V
[Olsen, 1982]. As the cold electron flux or density increases
with respect to the warm electron flux, the potential should
drop.
Data from the DE-1 taken on March 7, 1982, illustrate the
change in spacecraft potential which occurs during an eclipse
when the ambient plasma density is between 60 and 70 cm -3
and constant through the eclipse exit period. The plasma
density is inferred from the plasma wave data using a
spectral feature identified as the upper hybrid resonance
[Green et al. 1977]. The Retarding Potential Analyzer (RPA)
curves from the -Z detector (90° pitch angle) in sunlight and
eclipse are shown in Figures la and lb. The spacecraft is at L
= 4.5, near the magnetic equator, at local midnight. Figure la
shows the hydrogen curves in sunlight and eclipse; 1b, the
hydrogen and helium curves in eclipse. Note the large
difference for the observed count rate between the eclipse
and sun observations.
Figure la. RPA curves for hydrogen in sunlight and eclipse,
day 66.
Figure lb. RPA curves for hydrogen and helium in eclipse,
day 66.
There is a cold hydrogen population visible between 0 and 1
eV in both la and lb, which can be modeled with a
temperature of 0.3 - 0.4 eV a density of 30 to 80 cm-3, and a
spacecraft potential of +0.3 to +0.4 V. The "warm" tail of the
distribution, from 1.0 to 3.0 eV, can be modeled with a 0.7 to
0.8 eV, 2 to 4 cm-3 distribution. This fit to the warm tail
matches the sunlight data if the spacecraft potential is
increased to 1.7 V. The cold component is lost in sunlight
data and cannot be inferred from the tail of that component
because of the warmer component. (NOTE: All RPA
analysis uses the thin-sheath model developed by Comfort et
al. [1982].)
Earlier in this same orbit, we have an example of a plasma
population which can be measured in sunlight as well as
eclipse. During eclipse entry, at L = 3.5, the plasma density
was 400 cm-3, and the cold (0.25 eV temperature) component
is clearly visible in sunlight and eclipse. Spacecraft potentials
in sunlight and eclipse are nominally within a few tenths of a
volt of zero. In this environment, there is also a warmer tail
above 1 eV, with a temperature of about 0.5 eV, contributing
about 10% of the density. Therefore, it appears that if the
plasma density exceeds a few hundred particles per cubic
centimeter, on DE-1 all the cold plasma can be measured.
distribution. In the presence of two-temperature distributions
such as those shown here, even an 'ideal' instrument could
not infer the cold plasma distribution because of the
obscuring effects of the warm tail.
2.2 Aperture Bias
Figure 2a. Distribution function for hydrogen in eclipse,
day 67.
Control of the spacecraft potential is clearly necessary to
make a complete determination of the plasma properties. The
RIMS experiment on DE-1 was, therefore, provided with an
external aperture plane. The diameter of the aperture plane
was 20 cm, substantially smaller than the spacecraft
dimensions of 1 meter in height and 1.4 m in diameter. This
aperture plane could be biased to 0 V, -2 V, -4 V, and -8 V
with respect to the spacecraft. The retarding voltages are
referenced to the aperture plane, so that to the RIMS detector
the effect of the bias should be the same as a change in
spacecraft potential, aside from the effects of local fields.
The first example of aperture bias data is from the
plasmasphere, in order to illustrate the effect of the bias in an
isotropic plasma, with a spacecraft potential less than 0.5 V.
Next, measurements of field-aligned ions on the night-side
illustrate the results of aperture bias at space-craft potentials
of a few volts. Finally, field-aligned ion measurements from
the dayside, at spacecraft potentials of 5 to 8 V show
evidence of a barrier effect, and variations in the spacecraft
potential caused by the bias.
2.2.1. Observations in the Plasmasphere
The first orbit using aperture bias data was on October 14,
1981, illustrated in Figure 3, a projection of the orbit plane in
solar-magnetic coordinates. Data from this orbit are shown in
Figures 4a and 4b, a plot of the RPA curves taken deep in the
plasmasphere, where the spacecraft potential is near 0 V. The
cold plasma simply shifts in energy as the aperture plane is
biased, with no new plasma population coming into view.
Figure 2b. Distribution function for helium in eclipse, day
67.
A second example of the thermal plasma data obtained in
eclipse comes from the following day, March 8, 1982. The
wave data show that the density is 75 cm-3, near the end of
the eclipse period. The RPA curves have been converted to
distribution functions in Figure 2 using a simple differencing
technique. At this time, the technique provides a clear
illustration of the two temperature components of the cold
plasma, and plasma parameters similar to those given by the
thin sheath RPA analysis. Figure 2a shows the hydrogen
data, Figure 2b shows the helium data. Below 1 eV, both
hydrogen and helium show a 0.2 to 0.25 eV population, with
a 0.5 eV population at higher energies. For this same orbit
just after the eclipse, the RPA curves in sunlight (not shown)
give counts at or below the 1 per accumulation level.
The data from day 66 and 67 shown in Figure 2 illustrate two
important elements of the potential control problem. First, for
typical detectors, if the spacecraft potential exceeds the
plasma temperature by more than a factor of 2 or 3, it is
difficult to measure even the tail of the cold plasma
Figure 3. Orbit plot for day 287 of 1981.
Figure 4a. RPA curves for hydrogen during an aperture bias
sequence in the plasnasphere, day 287. Plasma
parameters: density, 2000 cm-3, Temperature 0.36
eV, ram velocity, 6 km/s (Mach 0.7); satellite
potential +0.5 V.
Figure 5. Energy-time and spin angle-time spectrogram for
hydrogen, day 287.
Figure 6. Frequency-time spectrogram for the long electric
antenna, day 287. The dashed white line extending
from 30 kHz on the left to 80 kHz on the right is the
electron gyrofrequency.
Figure 4b. RPA curves for helium during an aperture bias
sequence, day 287. Plasma parameters: density 720
cm-3, temperature 0.36 eV, ram velocity 6 km/s
(Mach 1.45), satellite potential +0.2 V.
The data shown here were taken over a four-minute period,
looking in the ram direction. The spacecraft velocity is 6
km/s, giving a flow energy to the hydrogen which is less than
the thermal energy. A spacecraft potential of a few tenths of
a volt positive is assumed in the fits. The gap between the fit
and the -8 V data suggests that there is a small shift (0.1 to
0.2 V) in the spacecraft potential caused by the shift in
aperture potential. This could be caused by an increase in
escaping photocurrent from the biased conductors.
Otherwise, the data are clearly explainable by shifting the
'spacecraft potential' of the RPA, and it appears that there are
no major sheath or barrier effects under these conditions, i.e.
plasma density near 1000 cm 3, spacecraft potential near 0 V.
2.2.2. Observations in the Polar Cap
Continuing on the same day, we next present data taken at a
higher spacecraft potential, between +3 and +5 V. In the
nightside polar cap, the aperture bias operations revealed a
high velocity (10 - 20 km/s) field-aligned hydrogen flow, in
spite of a spacecraft potential of +3 to +5 V. Figure 5 shows
the hydrogen data from the radial detector in a spin-energy
time spectrogram. The top panel shows the RPA sweeps for
the data taken along the magnetic field line, the bottom curve
the angular distribution of the data taken at energies between
0.0 and 1.0 eV. The aperture bias is sequenced in a regular
pattern throughout this period, 0 V, -2 V, 0 V, -4 V, 0 V, -8
V, and so on. The highest fluxes are seen at -8 V, as
expected. From 2015 to 2050 UT, the top panel shows green
at energies less than 1 eV, showing 5 to 10 counts per
accumulation in the field-aligned direction even at 0 V bias.
In addition to the field-aligned flow, a cold rammed plasma
appears at -8 V bias as seen in the lower panel. The detector
view direction is plotted on the vertical axis of the lower
panel, with views in the direction of the spacecraft motion
(RAM direction) at 0°. The relationship to the magnetic field
directions is indicated by the solid and dashed white lines
running horizontally across the plot. The solid line is for
180° pitch angle (flow from the north geographic pole),
while the dashed line is for 0° pitch angle.
The plasma wave instrument provides a different perspective
on this time period. Wave data can often be used to obtain a
plasma density, using features associated with the plasma
frequency. In this case, the wave data suggest that this is an
unusual day, and that there may be an isotropic background
which is difficult to measure, even with the aperture bias.
The ten counts per accumulation seen in the ram direction
may be the tail of that distribution. The plasma wave data are
shown in Figure 6, a frequency-time spectrogram for data
taken with the Ex antenna. The plotted quantity is the
uncalibrated, logarithmically compressed, antenna voltages
on a 0-96 color coded scale. Figure 6 shows a feature visible
near 100 kHz which appears to be the upper hybrid
resonance though the upper hybrid resonance has not been
reported in the polar cap. This gives a plasma density near
100 cm -3 which is higher than normally reported for the
polar cap. This density is similar to that during the eclipse
observations of hidden ions shown earlier. By comparison, it
is reasonable to suspect that if the ambient plasma is cold, it
would be hidden at 0 V bias. The question, then is, do the
aperture bias operations reveal a cold plasma? RPA curves
taken along the ram direction at -8 V bias (not shown) give
extremely low counts (10 or less per accumulation). If there
are no barrier potential effects, the wave and particle
measurements cannot be brought into agreement. It is
therefore necessary to examine the potential distribution
around the satellite during a period of positive of spacecraft
potential with the aperture bias in operation.
Figure 7. NASCAP model equipotential contours.
2.2.3 Models
Interpretation of the plasma data requires a model for the
fields around the satellite during aperture bias measurements.
Two models have been developed for this purpose, and have
been compared to the data. These comparisons are not
complete, but are in qualitative agreement with the models.
The first model is the NASA Charging Analyzer Program
(NASCAP), a three-dimensional numerical code designed to
work on a 16 x 16 x 33 grid, solving Laplace's equation for a
given potential distribution and a specified geometry. This
code is normally used to model spacecraft charging
dynamics, solving for the currents to and from the spacecraft,
and iteratively adjusting the spacecraft potentials and
currents. In this case, only the potential solving portion of the
code was utilized. The code was successfully used to match
the differential charging effects observed on ATS 6 [Olsen et
al., 1981]. A short cylinder was used to simulate the satellite,
and a three cell by three cell surface element was used to
simulate the aperture plane. The code resolution is essentially
one cell.
Figure 8. NASCAP model potential curves.
The results for two sets of potentials are shown in Figure 7,
which displays isopotential contours in a cut perpendicular to
the spacecraft axis through the center of the detector. A
saddle point is formed in front of the aperture, with a
magnitude below one volt. The potential distribution along a
nominal particle trajectory radially away from (or towards)
the detector is shown in Figure 8. The potential maximum 4
grid units (0.4 m) away from the detector is a barrier to low
energy ions.
A complete set of spacecraft and aperture potentials was run
with this model, with results shown in Figure 9, a plot of the
barrier height versus spacecraft potential for the different
aperture biases. The NASCAP predictions are shown in
combination with the results from an analytic model to be
described next.
The large size of the NASCAP code makes it awkward to
use, and a faster, more adaptable code is needed to study the
effects of aperture bias and angular effects. To accomplish
this, the spacecraft was modeled as a sphere, with the
aperture modeled as a cap of 5°-20° half-angle. Setting the
sphere at spacecraft potential, and biasing the cap to the
aperture potential, it was possible to expand the spherical
potential distribution using Legendre polynomials, in a
multipole fit. The potential distribution away from the sphere
can then be accurately determined with this spherical
expansion, and these terms are easily evaluated.
2.2.4. Comparison of Data with Models
The last step in the process is to compare these models with
particle data taken in the aperture bias mode. The fieldaligned flows illustrated in Figure 5 are reasonably stable
over a 12-minute period, and the spacecraft potential is
apparently +3 to +5 V, making these data useful for a
comparison with the model.
The field-aligned flow observed from 20:35 to 20:40 is
illustrated in Figure 11, using RPA curves with 0 V, -2 V, -4
V, and -8 V aperture bias. The direction of the flow is nearly
perpendicular to the spacecraft velocity vector, so it is
apparent that the flow velocity is high, greater than 10 km/s
at least. This gives the field-aligned ions a kinetic energy of
greater than 1 eV, comparable to the barrier height. If the
bulk of the plasma has an energy greater than the barrier
height, the RPA curves would show little evidence of a
barrier effect. Modeling without the barrier (not shown) gave
similar fits and plasma parameters. These data are therefore
not conclusive on the question of the effect of the aperture
bias.
Figure 9. Barrier height versus spacecraft potential. The
circled values are from NASCAP, the lines from
the multipole fit.
Figure 11. RPA curves for field-aligned hydrogen, day 287.
Plasma parameters: density, 4 cm 3; temperature, 0.2
eV; flow velocity, 20 km/s (Mach 3.25); satellite
potential, 5 V; bias -2, -4, -8 V; barrier height 3.6,
3.2, and 2.7 V.
Figure 10. Multipole model equipotential contours.
The results from this model are illustrated in Figure 10, a plot
of the potential distribution for a cap of 10° half-angle. Again
a barrier results. The barrier height obtained in this way is
also plotted in the smooth curves in Figure 9. The two sets of
plots agree in the monotonic rise in barrier height with
spacecraft potential, and the drop with increasing aperture
bias. The magnitudes are in rough agreement. Closer
agreement could be obtained with a different solid angle for
the cap, but it is felt that 10° accurately matches the ratio of
areas on the actual spacecraft. The difference seen in this plot
is probably largely due to the difference between cylindrical
and spherical geometry.
The background plasma measurements do show evidence of
a barrier effect because of the low flux, but the interpretation
is still not clear. The question of whether there is a cold 100
cm -3 background, as suggested the wave data, remains
unanswered because of the unusual nature of wave
measurements. Electrostatic analyzer data for electrons may
help determine the spacecraft potential, enabling us to
resolve this question.
A slightly different set of ambient plasma conditions are
found on the dayside orbit, where the flow energy is lower,
and apparently the ambient plasma density is lower. Figure
12 shows an energy time spectrogram for 3 minutes of
aperture bias data. This time period was shown by Chappell
et al. [1982, Plate 2d] and is the first high altitude
observation of the long predicted polar wind.
Figure 12. Energy-time spectrogram for hydrogen and
helium, day 287.
plasmasphere and beyond can limit the measurement of some
of the most important elements of the ambient plasma. Given
the existence of two-temperature distributions, even the most
ideal particle detectors cannot measure the lowest energy
portion of the ion population when the spacecraft potential is
greater than two volts. Active methods of potential control
are clearly important for the complete determination of the
ambient plasma characteristics. Aperture bias techniques are
one of the simplest methods of accomplishing this goal, and
have shown some success. This method may be limited by
the existence of barrier effects, and a possible increase in
spacecraft potential caused by the biasing. This latter effect
would not be as severe on a satellite with more conducting
surface area, and it remains clear that a conducting surface is
imperative for any scientific satellite. Active potential control
by means of devices such as plasma emitters offer the
possibility of setting the entire spacecraft to near zero volts
potential, alleviating the problem of non-zero potentials and
barrier effects [Olsen, 1981].
4. ACKNOWLEDGEMENTS
Figure 13. RPA curves for field-aligned hydrogen, day 287.
Plasma parameters: density, 7.5 cm-3; temperature,
0.6 eV; flow velocity, 10 km/s (Mach 0.94);
satellite potential, 6.0 V; aperture bias, -4 and -8 V:
harrier height, 3.0 and 3.0 V.
Figure 13 shows two RPA curves from this period. It is
difficult to fit these curves simultaneously, varying only the
aperture bias. It appears that as the aperture bias is increased
from 0 to -8 V, the spacecraft potential rises from +3 or +4 V
to +6 V. The -8 V bias data can be fitted with a spacecraft
potential of +4 V and no barrier using a 3 eV, 6 cm 3, 5 km/s
flow (Mach 0.2), or as shown in Figure 13, by raising the
spacecraft potential to 6 V, the density to 7.5 cm 3, and a
barrier of 3 V. It is difficult to interpret the data
unambiguously. Even without barrier effects, there is a
substantial range of plasma parameters which fit the data [see
Sojka et al., 1983]. Electron data from the electrostatic
analyzer will be examined to see if the spacecraft potential
can be determined unambiguously, and if a shift in spacecraft
potential can be observed. These last data show the most
evidence of a barrier effect, but are complicated by an
apparent change in the spacecraft potential. After comparing
the two sets of field-aligned ion measurements to the model,
the applicability of the model to the data is still not clear.
3. CONCLUSIONS
Analysis of the eclipse and aperture bias data sets show that
the normal positive spacecraft potentials found in the outer
The authors would like to express their gratitude to the Data
Systems Technology Program for allowing us access to the
central computer facilities through the Space-plasma
Computer Analysis Network (SCAN). Special thanks go to
M. Sugiura for use of his magnetic field data in the
determination of the particle pitch angles. The research at
The University of Alabama in Huntsville was supported by
NASA contract NAS8-33982 and NSF grant ATM8-300426,
and at the University of Iowa through contract NAS5-25690.
The authors are indebted to the engineering and science staff
of the University of Texas at Dallas and to the RIMS team at
Marshall Space Flight Center. We are grateful to the
programming staff of the Intergraph and Boeing
Corporations for assistance with the data reduction software.
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