A POTENTIAL CONTROL METHOD FOR THERMAL PLASMA MEASUREMENTSON THE DE-1 SPACECRAFT

Transcription

1 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 C. R. Chappell & J. L. Green Space Science Laboratory Solar Terrestrial Physics Div. NASA/Marshall Space Flight Center, Huntsville, AL S. D. Shawhan University of Iowa Department of Physics and Astronomy Iowa City, IA 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 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

2 plane. The end heads, or Z heads, have roughly conical apertures with 55 half angles. The instrument covers the 1-32 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 Hz, and step frequency receivers (SFR) from 104 Hz to 409 khz. The LFC covers the 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 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 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.

3 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 Observations in the Plasmasphere Figure 2b. Distribution function for helium in eclipse, day 67. 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. A second example of the thermal plasma data obtained in eclipse comes from the following day, March 8, 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.

4 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 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

5 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 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.

6 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 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.

7 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. 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 NAS and NSF grant ATM , and at the University of Iowa through contract NAS 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. 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

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