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Plasma effects of active ion beam injections in the ionosphere at rocket altitudes
7, 1992
0273-1177192 $15.00
Adv. Space Res. Vol.12 No.12, pp.(12)l—(12) Printed in Great Britain. All rights reserved.
Copyright © 1992 COSPAR
PLASMA EFFECTS OF ACTIVE ION BEAM INJECTIONS IN THE IONOSPHERE AT ROCKET ALTITUDES R. L. Arnoldy,* L. J. Cahill Jr,** P. M. Kintner,*** T. E. Mooref and C. J. Pollock*** Institutefor the Study of the Earth, Oceans and Space, University of New Hampshire, Durham, NH 03824, U.S.A. * * School of Physics andAstronomy, University of Minnesota, Minneapolis, MN 55455, U.S.A. *** Department of Electrical Engineering, Cornell University, Ithaca, NY 14853, U.S.A. t NASA/Marshall Space Flight Center, Huntsville, AL 35812, U.S.A. ABSTRACT *
Data from ARCS rocket ion beam injection experimentswill be primarily discussed in this paper. There are three results from this series of active experiments that are ofparticular interest in space plasma physics. These are the transverse acceleration of ambient ions in the large beam volume, the scattering of beam ions near the release payload, and the possible acceleration ofelectrons very close to the plasma generator which produce intense high frequency waves. The ability of 100 ma ion beam injections into the upper E and F regions of the ionosphere to produce these phenomena appear to be related solely to the process by which the plasma release payload and the ion beam are neutralized. Since the electrons in the plasma release do not convect with the plasma ions, the neutralization of both the payload and beam must be accomplishedby large field-aligned currents (milliamperes/square meter) which are very unstable to wave growth of various modes. Future work will concentrate on the wave production and wave-particle interactions that produce the plasma/energetic particle effects discussed in this paper and which have direct application to natural phenomena in the upper ionosphere and magnetosphere. INTRODUCTION This report will briefly review those plasma effects created by the injection of ion beams into the ionosphere that may provide a laboratory for investigating the physics of natural phenomena in the ionosphere. A very limited number of ion beam experiments have been performed and we will primarily concentrate on the results from the series of flights called ARCS (argon release for controlled studies). The ion guns used aboard the ARCS flights are based upon a design developed by the Hughes Corporation in connection with the early ECHO rocket series. These guns are basically scaled downion thrusters having a heated filament within a trapping field produced by an electromagnet, and a positively biased cylindrical conductor concentric with the electromagnet windings toward which electrons from the filament are accelerated. Argon gas flowing through the gun is ionized by the acceleratedelectron population in regions of positive potential relative to the exterior plasma. The argon ions are acceleratedout of the gun having an energy close to the anode potential, whereas superthermal electrons are extracted from the gun by any space-charge potentials created to achieve charge neutrality. The direction and location of the argon beam relative to the wave and particle sensors and the local geomagnetic field varied for the four ARCS flights. For ARCS 1 the diagnostic sensors were on the same payload as the plasma gun (which fired at angles between 0°and 90°to the magnetic field). For the ARCS 2 and 3 flights the plasma release payload was separated both along and across B from the diagnostic payload and the plasma releases were also centered along and transverse to B. The ARCS 4 flight, which was recently launched (February, 1990), injected the ion beam transverse to B at two different energies and the plasma release payload and diagnostic payload were very closely aligned on the same flux tube. NEUTRALIZATION EFFECTS
A significant result obtained from the Porcupine experiments, where xenon plasma was injected transverse to B, was that the polarization field resulting from magnetization ofthe injected electrons was only ten percent of the value needed to convect these electrons along with the energetic beam ions /1,2/. Payload neutralization can then be achieved only if the superthermal gun electrons can leave the payload and its vicinity along the magnetic field. For beam injections parallel to the magnetic field there is no problem in payload neutralization but during the ARCS 3 flight transverse beam injections resulted in payload charging up to 100 volts /3/. This result can be seen in Figure 1 which summarizes the ions measured aboard the separated diagnostic payload for the entire flight. The anode potential for both the parallel and transverse to B injections was 200 volts. The operation periods for the parallel and transverse injections are given below the third panel. The transverse injections resulted in ions at the diagnostic payload with energies in the 60-119 eV window whereas the parallel injections were seen in the energy window which included the anode potential. Kaufmann et al. /4/ have suggested that the inability of the gun electrons to leave the plasma release payload along the lines of magnetic force to effect payload neutralization may be an artifact of the particular gun design, i.e. strong magnetic field leakage from the ARCS 3 gun which when combined with the Earth’s field would bring many of the electrons back to the payload. The transverse beam injections of the ARCS 4 flight did not resultin any payload charging of significance as can be seen in the summary ion data plotted in Figure 2. This figure gives energy spectrograms of the superthermal ign detectors aboard the plasma release payload (lower panel) and the diagnosticpayload (upper panel); The alternating bursts of ions are the beam ions at the two different anode potentials applied to the gun. For both panels the lower energy ions had maximum fluxes at energies between 80 and 100 eV and the higherenergies between 170 and 200 eV. Since the beam ions returning to the release payload are very close to the same energy at those detected aboard the separated diagnostic payload one can say that the release payload did not charge significantly as a result of the ion beam release. The dark lines in the spectrograms near a few eV energy are the ambient ions and will be discussed next. (12)1
(12)2
R. L. Amoldy et aL
NASA Flight # 29015 AR~ ION BEAM EXPERIMENT IUNH Copped Hemliphere)
Fig. 1. Summary of the ion detectorcount rates on the diagnostic payload in three energy windows for the ARCS 3 flight. The time and direction with respect to B of the ion beam releases are given beneath the third panel.
2
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Flight Time (sec) Fig. 2. Summary of the ion detect count rates in spectrogram format for the beam release payload, lower panel, and for a detector on the diagnosticpayload, upper panel. The gun alternately released 100 and 200 eV ions transverse to the magnetic field. However, the sequencing of the gun changed phase for some unknown reason at about 420 seconds flight time. The periods of response over a wide range of energy channels represent natural auroral ions with possible some auroral ultraviolet contamination at 580 seconds.
Active Ion Beam
Injections
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The importance of the neutralization of the ion beam release payload is that the requirement that it be accomplished by electron flow along field lines results in very large field-aligned currents (milliamperes/square meter) which are very unstable to wave growth /5/,particularly electrostatic waves near the lower hybrid frequency. Ion cyclotron waves and hydrogen Bernstein waves generally are observed in association with beam releases /6,7,8,9,10,11,12,13/14/15/. Some subset of these waves must be responsible for the transverse acceleration of ambient ions observed during ion beam releases /15/ and which occurs naturally in association with auroral phenomena.
AMBIENT IONS AND TRANSVERSELY ACCELERATED IONS As can be seen in the top panel of Figure 1, (ARCS 3 flight), a very curious reduction in the intensity of the measured ambient ion population occurred during the period of time when the diagnostic payload measured beam ions. A cylindrical volume of space 800 meters above the release payload and a radius perpendicular to B equal to the argon ion gyrodiameter (400 meters) was effected. This reduction occurred immediately after the first gun firing and did not
recover until beam ions were not longer measured aboard the diagnostic payload (no ion measurements were made aboard the release payload). Apparently space charge created by the intense charged particle fluxes of the plasma release precluded the measurement of the ambient thermals. Although the release payload did not neutralize, as discussed above, this charge could hardly effect the large volume of space in which the ambients were depleted. It is possible that the argon beam itself was not neutralized effectively by ionospheric electrons which could then create the large volume space charge. Another possible interpretation is that the detector itself developed a space charge since there were aluminum surfaces in the analysis volume of the detector that could have had an insulating surface of aluminum oxide and therefore become charged. Above a few eV energy the detector did appear to operate properly and measured a superthermal population (10-36 eV) confmed to pitch angles near 90°as seen in Figure 3. In this figure the high energy ions are apparently gun ions (why they have intensity maxima at 90°and 180°pitch angle will be discussed later).
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Fig. 3. Top panel. Three dimensional plot of directional intensity of ions measured on the diagnostic payload of the ARCS 3 ifight. Bottom panel. Integration of this flux over all pitch angles. In contrast to these results, the ion sensors aboard the ARCS 4 diagnostic payload measured the ambient population irrespective of whether the gun was firing or not, while the detector aboard the release payload measured the ambient ions only when the gun was not firing. The disruption of the ambient population in the immediate vicinity of the plasma release where a great deal of wave turbulence has been created is not unexpected. These results can be seen in Figure 2 where the continuous line in the top panel at the diagnostic payload corresponds to the everpresent ambients while the burst events in the bottom panel at low energy are the ambient ions between gun firings at the release payload. In addition, the ion detector aboardthe diagnostic (top panel) payload did not measure the discrete superthermal population as seen during the ARCS 3 flight discussed above. There is, however, some evidence in the ARCS 4 data that there were superthermal ions between ambient and gun energies. The ARCS 4 flight flew through a series of inverted V auroral events so natural ions were present around 350, 445 and 580 seconds flight time. The first few injections did have a considerable spread in the energy of the beam ions measured at the diagnostic payload but show no evidence of a well-defined separate population as seen by the ARCS 3 detectors. It is safe to say that the ion beam injected by the ARCS 4 gun was not as efficient in the transverse acceleration or heating of ions as thatfrom an identical gun flown aboard the ARCS 3 flight. The reason for this is not understood. One significant difference between the two flights is the reduction of the ambient density in the ARCS 3 beam volume as discussed above. The measurement of naturally occurring conics at rocket altitudes seems to also favor regions ofreduced ambient density /16/.
R. L. Arnoldy et aL
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SCATI’ERING OF BEAM IONS Pollock et al. /3/ have shown how the presence of ions at fairly large pitch angles at the ARCS 3 diagnostic payload, during injections of ions upward along B toward the diagnostic payload (as seen in Figure 3), could be explained as nominal ion trajectories to the position of the diagnostic payload if the beam is scattered right near the release payload to create an isotropic source of ions. The beam ions measured in ARCS 4 by the diagnosticpayload below the release payload also requires this scattering of the ion beam very close to the release payload. Figure 4 is an eight second ARCS 4 180
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Fig. 4. Pitch angle spectrogram of the response of the ion detector on the diagnostic payload for the third gun pulse of the ARCS 4 flight. The intense counts at low energies are due to the ambient ion population and the three spots at high energies are measurements of the ion beam.
average of ARCS 4 diagnostic payload ion data during the second gun firing plotted with pitch angle as the ordinate, energy as abscissa and average count rate as gray scale. The rammed thermal population at low energies appears over a range of pitch angles due to the eight second averaging since the pitch angle bin the thermal population will appear in depends on the azimuthal angle of the detector measured with respect to the ram direction. The gun ions are the three populations at the 17th energy step (100 eV), located at pitch angles of +80°and -80°(these values depend on the separation of the release and diagnostic payloads along B) and at 0°pitch angle. The anomalous population for an ion beam injected transverse to B is, of course, the 0°pitch angle ions. These ions would, however, be seen if the beam ions were severely scattered in pitch angle (but not significantly in energy) near the point ofinjection if the two payloads were nearly on the same line of force. A mapping of particles from an isotropic source at the release payload to the diagnostic payload is given in Figure 5 where the displacement ofthe two payloads across B is the ordinate and detected
ARCS 4
J: Pitch Angle (deg)
Fig. 5. A study of the trajectories of ions from an isotropic source at the release payload as a function of transverse to B separation of the diagnosticand release payloads and the pitch angle of detected ions by the diagnostic payload. This plot is for the beam injection at 269 seconds flight time when two payloads were separated along B by about 300 meters.
Active Ion Beam Injections
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particle pitch angle at the diagnostic payload is the abscissa. For near zero transverse displacements (both payloads nearly on the same line of force) the curve shows that ions should be seen at 0°pitch angle and above 75°pitch angle which is the case for the data given in Figure 4. Nominally, the two payloads were supposed to be magnetically connected since the plasma release payload was separated from the diagnostic payload after alignment along B by an attitude control system accurate to within a few degrees. However, an analysis ofbeam ions reaching the diagnostic payload using this technique that thepayload two payloads transverse to B at a rate of 0.1 n~1secand explains the missing injection event at shows the diagnostic at 660 separated seconds (see Figure 3). The important result here is that both the ARCS 3 and ARCS 4 missions require that the beam ions be severely scattered in pitch angle as they leave the release payload. Although the ARCS 2 payload did not have pitch angle-imaging ion detectors, thereis evidence that the ions detected away from the injection payload also required such scattering. The full cone beam width of the plasma gun, as measured in a vacuum tank, does not exceed 60°which means the ions still have to undergo another 60°of scattering to produce the results discussed above. Very large electric fields near the injection payload appear to be required. Electrostatic repulsion of the Argon ions from the beam is intense due to the very high perveance of the 100 ma, 200 volt ion gun. For a 100% unneutralized release the ions at the outer surface of the beam will be deflected topitch angles near 90°within 10cm of the release payload. Neutralizing gun electrons would become magnetized by the Earth’s field within this distance leaving the beam unneutralized by the gun beyond this point and therefore capable of injecting ions at all pitch angles via electrostatic repulsion. SUPERTHERMAL (HECTOVOL1) ELECTRONS NEAR THE RELEASE PAYLOAD
ARCS 1 and 4 were the only two flights in the ARCS series which had particle detectors aboard the ion beam release payload to make measurements inside what must be a very turbulent plasma regime surrounding the payload /17/. Moore et al. /18/ and Kaufmann et al. /12/report an anomalous electron distribution measured aboard the ARCS 1 release payload. Moore et al. /18/ characterize these electron distributions as a 2 knilsec streaming of low energy electrons down the field line and an isotropic heating ofelectrons near a few hundredeV energy. Kaufmann et al. /12/ note that all downgoing electrons were accelerated, whether or not they passed through or near the ion beam, and postulated, as Moore et al. /18/ had done earlier, that the downgoing electrons were acceleratedto a few hundred eV by a potential structure developed near the release payload as a result of the interruption of a naturally occurring aurora! current by wave fields in the turbulent volume surrounding the release payload. Although the ARCS 1 electron detectors did not image pitch angle, they did detect some azimuthal asymmetry in the downgoing electrons. The pitch angle-imaging electron detector aboard the ARCS 4 plasma release payload also detected superthermal electrons with maximum energy very near to the anode voltage of the plasma gun. The pitch angle distribution of these electrons is given in the data shown in Figure 6 where the top panel gives the energy sweep of the detector and the bottom panel I IllIllIllIll
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Flight Time (sec) Fig. 6. Top panel. Energy sweep of the electron detector on the ARCS 4 plasma release payload. Bottom panel. Pitch angle spectrogram (abscissa is time or energy as given in the top panel) of this detectorduring the second gun firing. The dark line in the middle ofthe spectrogram are counts that were not imaged due to saturation of the imaging electronics.
(12)6
R.L.ArnoldyetaL
displays the pitch angle distribution as function of time (or energy sweep). The intensity of the fluxes reaching Utis detector was so high that its imaging capability was saturated which means that not all counts are imaged but many wcrc placed in one of the bins (the dark streak at the center of the distribution). Those counts that were imaged, however, give a correct pitch angle distribution. Like the ARCS 1 results, the electrons have a population streaming down the field line but this population is not azimuthally isotropic in that pitch angles of 330°should see the same count rate as 30°which is not the case. The measured electron population is not fixed in space with the detector rotating through it but rather rotates with the detector and is therefore always seen in the same detector pitch angle bins. The only source for these electrons is the plasma gun located above and about 35°removed in azimuth from the detector head. The plasma gun should release only a few eV electrons and not a distribution extending to the gun energy and dependent on the gun energy. One has reason to be very skeptical of particle measurements made aboard a beam-emitting payload. However, when the release and diagnostic payloads were separated by less than 10 meters, three different detectors aboard the diagnostic payload (a conventional cylindrical electrostatic analyzer, a sweeping pitch angle-imaging detector, and a fixed energy pitch angle-imaging detector) all measured these electrons giving a very similar energy spectrum and pitch angle distribution. At this time we have to believe these electron measurements, but whether they are similar to those made aboard the ARCS 1 flight is questionable since the ARCS 1 electrons extended well above the energy of the 60 volt plasma gun flown. The most likely interpretation of the ion gun-related electrons, which have a maximum in their energy spectrum that tracks with the maximum ion energy or with the anode voltage applied to the ion gun, is that they are accelerated near the ion gun by a potential that is oscillating between 0 and negative anode voltage. We have argued above, when discussing the gun ions, that they present no evidence for a DC or even quasi-static charging ofthe ion release payload. This is true up to the temporal resolution of the detectors, which was 8 ms. Kaufmann et al. /4/ have presented evidence from the ARCS 1 flight that supertheimal gun electrons may have modified the the ambient population around that payload at the frequency of the AC filament power supply of the gun (17 KHz) which could have produced the Langmuir probe oscillations measured at nearly 1 KHz frequency as an aliased measurement of the true 17 KHz plasma fluctuations. It is possible that the filament of the ARCS 4 gun also fluctuated several volts around ambient plasma potential at a frequency of 30 KHz, thereby creating bursts of superthermal electrons emitted by the gun, which when emitted neutralized the gun (and perhaps the payload), but when attracted to the cathode and kept within the gun momentarily, resulted in an unneutralized plasma (payload?) that accelerated electrons as observed. This mechanism might also explain the energy distribution of ions detected from the gun on both the release payload and the diagnostic payload as a result of energy loss in moving away from a partially unneutralized source. An Argon ion retarded in energy in leaving the gun due a negative potential would not necessarily regain that energy in returning to a detector on the release payload since the payload would most likely be at a different floating potential when the ion is detected than when the ion left the payload due to the large difference in the ion gyrofrequency and the frequency of any payload potential fluctuation. Such an argument cannot be made for electrons accelerated away from the release payload and being detected by a sensor electrically attached to the same payload because of the short travel time of electrons. Moreover, a radial electric field surrounding an unneutralized release payload would just E x B drift ambientelectrons around the payload and not impart significant energy to the electrons. Since gun-related electrons were detected on the release payload and on the diagnostic payload, when within several meters of the gun payload, at gun anode energies and lower, it appears that the acceleration of the electrons must have occurred within the plasma generator itself due to space charge electric fields created when the superthermal electrons were prevented from leaving the gun as a result of a small oscillating positive potential on the filament as discussed above. The electrons apparently accelerated by the ion gun have a phase space distribution which is quite unstable, therefore these electrons are clearly the source of intense waves measured during ion beam injections between the electron plasma frequency and the upper hybrid frequency as well as below the electron gyrofrequency. To produce the waves the electrons must resonate with a wave mode whose phase velocity matches the electron speed. The index ofrefraction for 2 occurs only between the electron plasma frequency and a typical cold plasma at the rocket altitude shows that large n the upper hybrid frequency and below the electron cyclotron frequency where the waves were observed. These wave observations will be more thoroughly discussed in subsequentreports. SUMMARY There are three results ofthe ion beam experiments described in this report that are of particular interest in space plasma physics and have application to natural phenomena occurring in the upper ionosphere and magnetosphere. These are the transverse acceleration of ambientions in the large beam volume, the scattering of beam ions near the release payload, and the acceleration of electrons very close to the plasma generator which in turn are unstable to the growth of intense high frequency waves. All of these processes undoubtedly involve strong wave-particle interactions. The ability of 100 ma ion beam injections to produce these phenomena appear to be related solely to the process by which the plasma release payload and the ion beam are neutralized. Since the electrons in the plasma release do not convect with the plasma ions, the neutralization of both the payload and beam must be accomplished by large field-aligned currents (milliamperes/meter2) which are very unstable to wave growth of various modes. Future work will concentrate on the wave production and wave-particle interactions that produce the plasma/energetic particle effects discussed in this paper. It is planned to directly study the heating, acceleration and scattering ofparticles with the injection of waves from rocket payloads into the upper ionosphere. ACKNOWLEDGEMENTS Work at the University of New Hampshire was supported under NASA Grant NAG 6-12, at the University of Minnesota under NASA Grant NAG6-l 1, at Cornell University under NASA Grant NAG5-601 and at Marshall Space Flight Centerunder NASA RTOP 432-36-55.
Active Ion Beam Injections
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REFERENCES 1.
G. Haerendel, and R.Z. Sagdeev, Adv. Space Res., 1, 29, 1981.
2.
B. Hausler, R.A. Treuman, O.H. Bauer, G. Haerendel, R. Bush, C.W. Carlson, B. Theile, M.C. Kelley, V.S. Dokukin, and Yu Ya Ruzhin, J. Geophys. Res., 91, 287, 1986.
3.
C.J. Pollock, R.L. Arnoldy, R.E. Erlandson, and L.J. Cahill, Jr., J. Geophys. Res., 93, 11,473, 1988.
4.
R.L. Kaufmann, D.N. Walker, J.C. Holmes, C.J. Pollock, R.L. Arnoldy, L.J. Cahill, Jr., and P.M. Kintner, J. Geophys. Res., 94, 453, 1989.
5.
J.M. Kindel, and C.F. Kennel, J. Geophys. Res., 76, 3055, 1971.
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D.G. Cartwright, and P.J. Kellogg, J. Geophys. Res., 79, 1439, 1974.
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T.W. Jones, and P.J. Kellogg, J. Geophys. Res., 78, 2166, 1973.
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P.M. Kintner, and M.C. Kelley, J. Geophys. Res., 88, 357, 1983.
9.
P.M Kintner, J. LaBelle, M.C. Kelley, L.J. Cahill, Jr., T.E. Moore, and R.L. Arnoldy, Geophys. Res. Len.,
11, 19, 1984. 10. M. Malingre, and R. Pottelette, Geophys. Res. Len., 12, 275, 1985. 11. P.M. Kintner, J. LaBelle, W.A. Scales, R.E. Erlandson, and L.J. Cahill, Jr., Ion Acceleration in the Magnetosphere and Ionsphere, Geophys. Monogr. Ser., 18, 206, AGU, Washington, D.C., 1986. 12. R.L. Kaufmann, R.L. Arnoldy, T.E. Moore, P.M. Kintner, L.J. Cahill, Jr., and D.N. Walker, J. Geophys.Res., 90, 9595, 1985. 13. D.N. Walker, J. Geophys. Res., 91, 3305, 1986. 14.
R.E. Erlandson, L.J. Cahill, Jr., C.J. Pollock, R.L. Arnoldy, W.A. Scales, and P.M. Kintner, J. Geophys. Res., 92, 4601, 1987.
15. R.E. Erlandson, L.J. Cahill, Jr., R.L. Kaugmann, C.J. Pollock, and R.L. Arnoldy, J. Geophys. Res., 94, 2645, 1989. 16. AS.W. Yau, F.A. Whalen, and P.M. Kintner, Ion Acceleration in the Magnetosphere and Ionosphere, Geophys. Monogr. Ser., 18, 206, AGU, Washington, D.C., 1986. 17. R.L. Arnoldy, C.J. Pollock, L.J. Cahill, Jr., R.E. Erlandson, and P.M. Kintner, Adv. Space Res., 10, (7)107, 1990. 18.
T.E. Moore, R.L. Arnoldy, R.L. Kaufmann, L.J. Cahill, Jr., P.M. Kintner, and D.N. Walker, J. Geophys. Res., 87, 7569, 1982.
0273-1177192 $15.00
Adv. Space Res. Vol.12 No.12, pp.(12)l—(12) Printed in Great Britain. All rights reserved.
Copyright © 1992 COSPAR
PLASMA EFFECTS OF ACTIVE ION BEAM INJECTIONS IN THE IONOSPHERE AT ROCKET ALTITUDES R. L. Arnoldy,* L. J. Cahill Jr,** P. M. Kintner,*** T. E. Mooref and C. J. Pollock*** Institutefor the Study of the Earth, Oceans and Space, University of New Hampshire, Durham, NH 03824, U.S.A. * * School of Physics andAstronomy, University of Minnesota, Minneapolis, MN 55455, U.S.A. *** Department of Electrical Engineering, Cornell University, Ithaca, NY 14853, U.S.A. t NASA/Marshall Space Flight Center, Huntsville, AL 35812, U.S.A. ABSTRACT *
Data from ARCS rocket ion beam injection experimentswill be primarily discussed in this paper. There are three results from this series of active experiments that are ofparticular interest in space plasma physics. These are the transverse acceleration of ambient ions in the large beam volume, the scattering of beam ions near the release payload, and the possible acceleration ofelectrons very close to the plasma generator which produce intense high frequency waves. The ability of 100 ma ion beam injections into the upper E and F regions of the ionosphere to produce these phenomena appear to be related solely to the process by which the plasma release payload and the ion beam are neutralized. Since the electrons in the plasma release do not convect with the plasma ions, the neutralization of both the payload and beam must be accomplishedby large field-aligned currents (milliamperes/square meter) which are very unstable to wave growth of various modes. Future work will concentrate on the wave production and wave-particle interactions that produce the plasma/energetic particle effects discussed in this paper and which have direct application to natural phenomena in the upper ionosphere and magnetosphere. INTRODUCTION This report will briefly review those plasma effects created by the injection of ion beams into the ionosphere that may provide a laboratory for investigating the physics of natural phenomena in the ionosphere. A very limited number of ion beam experiments have been performed and we will primarily concentrate on the results from the series of flights called ARCS (argon release for controlled studies). The ion guns used aboard the ARCS flights are based upon a design developed by the Hughes Corporation in connection with the early ECHO rocket series. These guns are basically scaled downion thrusters having a heated filament within a trapping field produced by an electromagnet, and a positively biased cylindrical conductor concentric with the electromagnet windings toward which electrons from the filament are accelerated. Argon gas flowing through the gun is ionized by the acceleratedelectron population in regions of positive potential relative to the exterior plasma. The argon ions are acceleratedout of the gun having an energy close to the anode potential, whereas superthermal electrons are extracted from the gun by any space-charge potentials created to achieve charge neutrality. The direction and location of the argon beam relative to the wave and particle sensors and the local geomagnetic field varied for the four ARCS flights. For ARCS 1 the diagnostic sensors were on the same payload as the plasma gun (which fired at angles between 0°and 90°to the magnetic field). For the ARCS 2 and 3 flights the plasma release payload was separated both along and across B from the diagnostic payload and the plasma releases were also centered along and transverse to B. The ARCS 4 flight, which was recently launched (February, 1990), injected the ion beam transverse to B at two different energies and the plasma release payload and diagnostic payload were very closely aligned on the same flux tube. NEUTRALIZATION EFFECTS
A significant result obtained from the Porcupine experiments, where xenon plasma was injected transverse to B, was that the polarization field resulting from magnetization ofthe injected electrons was only ten percent of the value needed to convect these electrons along with the energetic beam ions /1,2/. Payload neutralization can then be achieved only if the superthermal gun electrons can leave the payload and its vicinity along the magnetic field. For beam injections parallel to the magnetic field there is no problem in payload neutralization but during the ARCS 3 flight transverse beam injections resulted in payload charging up to 100 volts /3/. This result can be seen in Figure 1 which summarizes the ions measured aboard the separated diagnostic payload for the entire flight. The anode potential for both the parallel and transverse to B injections was 200 volts. The operation periods for the parallel and transverse injections are given below the third panel. The transverse injections resulted in ions at the diagnostic payload with energies in the 60-119 eV window whereas the parallel injections were seen in the energy window which included the anode potential. Kaufmann et al. /4/ have suggested that the inability of the gun electrons to leave the plasma release payload along the lines of magnetic force to effect payload neutralization may be an artifact of the particular gun design, i.e. strong magnetic field leakage from the ARCS 3 gun which when combined with the Earth’s field would bring many of the electrons back to the payload. The transverse beam injections of the ARCS 4 flight did not resultin any payload charging of significance as can be seen in the summary ion data plotted in Figure 2. This figure gives energy spectrograms of the superthermal ign detectors aboard the plasma release payload (lower panel) and the diagnosticpayload (upper panel); The alternating bursts of ions are the beam ions at the two different anode potentials applied to the gun. For both panels the lower energy ions had maximum fluxes at energies between 80 and 100 eV and the higherenergies between 170 and 200 eV. Since the beam ions returning to the release payload are very close to the same energy at those detected aboard the separated diagnostic payload one can say that the release payload did not charge significantly as a result of the ion beam release. The dark lines in the spectrograms near a few eV energy are the ambient ions and will be discussed next. (12)1
(12)2
R. L. Amoldy et aL
NASA Flight # 29015 AR~ ION BEAM EXPERIMENT IUNH Copped Hemliphere)
Fig. 1. Summary of the ion detectorcount rates on the diagnostic payload in three energy windows for the ARCS 3 flight. The time and direction with respect to B of the ion beam releases are given beneath the third panel.
2
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650
750
Flight Time (sec) Fig. 2. Summary of the ion detect count rates in spectrogram format for the beam release payload, lower panel, and for a detector on the diagnosticpayload, upper panel. The gun alternately released 100 and 200 eV ions transverse to the magnetic field. However, the sequencing of the gun changed phase for some unknown reason at about 420 seconds flight time. The periods of response over a wide range of energy channels represent natural auroral ions with possible some auroral ultraviolet contamination at 580 seconds.
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The importance of the neutralization of the ion beam release payload is that the requirement that it be accomplished by electron flow along field lines results in very large field-aligned currents (milliamperes/square meter) which are very unstable to wave growth /5/,particularly electrostatic waves near the lower hybrid frequency. Ion cyclotron waves and hydrogen Bernstein waves generally are observed in association with beam releases /6,7,8,9,10,11,12,13/14/15/. Some subset of these waves must be responsible for the transverse acceleration of ambient ions observed during ion beam releases /15/ and which occurs naturally in association with auroral phenomena.
AMBIENT IONS AND TRANSVERSELY ACCELERATED IONS As can be seen in the top panel of Figure 1, (ARCS 3 flight), a very curious reduction in the intensity of the measured ambient ion population occurred during the period of time when the diagnostic payload measured beam ions. A cylindrical volume of space 800 meters above the release payload and a radius perpendicular to B equal to the argon ion gyrodiameter (400 meters) was effected. This reduction occurred immediately after the first gun firing and did not
recover until beam ions were not longer measured aboard the diagnostic payload (no ion measurements were made aboard the release payload). Apparently space charge created by the intense charged particle fluxes of the plasma release precluded the measurement of the ambient thermals. Although the release payload did not neutralize, as discussed above, this charge could hardly effect the large volume of space in which the ambients were depleted. It is possible that the argon beam itself was not neutralized effectively by ionospheric electrons which could then create the large volume space charge. Another possible interpretation is that the detector itself developed a space charge since there were aluminum surfaces in the analysis volume of the detector that could have had an insulating surface of aluminum oxide and therefore become charged. Above a few eV energy the detector did appear to operate properly and measured a superthermal population (10-36 eV) confmed to pitch angles near 90°as seen in Figure 3. In this figure the high energy ions are apparently gun ions (why they have intensity maxima at 90°and 180°pitch angle will be discussed later).
Beam Event 2...
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Fig. 3. Top panel. Three dimensional plot of directional intensity of ions measured on the diagnostic payload of the ARCS 3 ifight. Bottom panel. Integration of this flux over all pitch angles. In contrast to these results, the ion sensors aboard the ARCS 4 diagnostic payload measured the ambient population irrespective of whether the gun was firing or not, while the detector aboard the release payload measured the ambient ions only when the gun was not firing. The disruption of the ambient population in the immediate vicinity of the plasma release where a great deal of wave turbulence has been created is not unexpected. These results can be seen in Figure 2 where the continuous line in the top panel at the diagnostic payload corresponds to the everpresent ambients while the burst events in the bottom panel at low energy are the ambient ions between gun firings at the release payload. In addition, the ion detector aboardthe diagnostic (top panel) payload did not measure the discrete superthermal population as seen during the ARCS 3 flight discussed above. There is, however, some evidence in the ARCS 4 data that there were superthermal ions between ambient and gun energies. The ARCS 4 flight flew through a series of inverted V auroral events so natural ions were present around 350, 445 and 580 seconds flight time. The first few injections did have a considerable spread in the energy of the beam ions measured at the diagnostic payload but show no evidence of a well-defined separate population as seen by the ARCS 3 detectors. It is safe to say that the ion beam injected by the ARCS 4 gun was not as efficient in the transverse acceleration or heating of ions as thatfrom an identical gun flown aboard the ARCS 3 flight. The reason for this is not understood. One significant difference between the two flights is the reduction of the ambient density in the ARCS 3 beam volume as discussed above. The measurement of naturally occurring conics at rocket altitudes seems to also favor regions ofreduced ambient density /16/.
R. L. Arnoldy et aL
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SCATI’ERING OF BEAM IONS Pollock et al. /3/ have shown how the presence of ions at fairly large pitch angles at the ARCS 3 diagnostic payload, during injections of ions upward along B toward the diagnostic payload (as seen in Figure 3), could be explained as nominal ion trajectories to the position of the diagnostic payload if the beam is scattered right near the release payload to create an isotropic source of ions. The beam ions measured in ARCS 4 by the diagnosticpayload below the release payload also requires this scattering of the ion beam very close to the release payload. Figure 4 is an eight second ARCS 4 180
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Fig. 4. Pitch angle spectrogram of the response of the ion detector on the diagnostic payload for the third gun pulse of the ARCS 4 flight. The intense counts at low energies are due to the ambient ion population and the three spots at high energies are measurements of the ion beam.
average of ARCS 4 diagnostic payload ion data during the second gun firing plotted with pitch angle as the ordinate, energy as abscissa and average count rate as gray scale. The rammed thermal population at low energies appears over a range of pitch angles due to the eight second averaging since the pitch angle bin the thermal population will appear in depends on the azimuthal angle of the detector measured with respect to the ram direction. The gun ions are the three populations at the 17th energy step (100 eV), located at pitch angles of +80°and -80°(these values depend on the separation of the release and diagnostic payloads along B) and at 0°pitch angle. The anomalous population for an ion beam injected transverse to B is, of course, the 0°pitch angle ions. These ions would, however, be seen if the beam ions were severely scattered in pitch angle (but not significantly in energy) near the point ofinjection if the two payloads were nearly on the same line of force. A mapping of particles from an isotropic source at the release payload to the diagnostic payload is given in Figure 5 where the displacement ofthe two payloads across B is the ordinate and detected
ARCS 4
J: Pitch Angle (deg)
Fig. 5. A study of the trajectories of ions from an isotropic source at the release payload as a function of transverse to B separation of the diagnosticand release payloads and the pitch angle of detected ions by the diagnostic payload. This plot is for the beam injection at 269 seconds flight time when two payloads were separated along B by about 300 meters.
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particle pitch angle at the diagnostic payload is the abscissa. For near zero transverse displacements (both payloads nearly on the same line of force) the curve shows that ions should be seen at 0°pitch angle and above 75°pitch angle which is the case for the data given in Figure 4. Nominally, the two payloads were supposed to be magnetically connected since the plasma release payload was separated from the diagnostic payload after alignment along B by an attitude control system accurate to within a few degrees. However, an analysis ofbeam ions reaching the diagnostic payload using this technique that thepayload two payloads transverse to B at a rate of 0.1 n~1secand explains the missing injection event at shows the diagnostic at 660 separated seconds (see Figure 3). The important result here is that both the ARCS 3 and ARCS 4 missions require that the beam ions be severely scattered in pitch angle as they leave the release payload. Although the ARCS 2 payload did not have pitch angle-imaging ion detectors, thereis evidence that the ions detected away from the injection payload also required such scattering. The full cone beam width of the plasma gun, as measured in a vacuum tank, does not exceed 60°which means the ions still have to undergo another 60°of scattering to produce the results discussed above. Very large electric fields near the injection payload appear to be required. Electrostatic repulsion of the Argon ions from the beam is intense due to the very high perveance of the 100 ma, 200 volt ion gun. For a 100% unneutralized release the ions at the outer surface of the beam will be deflected topitch angles near 90°within 10cm of the release payload. Neutralizing gun electrons would become magnetized by the Earth’s field within this distance leaving the beam unneutralized by the gun beyond this point and therefore capable of injecting ions at all pitch angles via electrostatic repulsion. SUPERTHERMAL (HECTOVOL1) ELECTRONS NEAR THE RELEASE PAYLOAD
ARCS 1 and 4 were the only two flights in the ARCS series which had particle detectors aboard the ion beam release payload to make measurements inside what must be a very turbulent plasma regime surrounding the payload /17/. Moore et al. /18/ and Kaufmann et al. /12/report an anomalous electron distribution measured aboard the ARCS 1 release payload. Moore et al. /18/ characterize these electron distributions as a 2 knilsec streaming of low energy electrons down the field line and an isotropic heating ofelectrons near a few hundredeV energy. Kaufmann et al. /12/ note that all downgoing electrons were accelerated, whether or not they passed through or near the ion beam, and postulated, as Moore et al. /18/ had done earlier, that the downgoing electrons were acceleratedto a few hundred eV by a potential structure developed near the release payload as a result of the interruption of a naturally occurring aurora! current by wave fields in the turbulent volume surrounding the release payload. Although the ARCS 1 electron detectors did not image pitch angle, they did detect some azimuthal asymmetry in the downgoing electrons. The pitch angle-imaging electron detector aboard the ARCS 4 plasma release payload also detected superthermal electrons with maximum energy very near to the anode voltage of the plasma gun. The pitch angle distribution of these electrons is given in the data shown in Figure 6 where the top panel gives the energy sweep of the detector and the bottom panel I IllIllIllIll
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Flight Time (sec) Fig. 6. Top panel. Energy sweep of the electron detector on the ARCS 4 plasma release payload. Bottom panel. Pitch angle spectrogram (abscissa is time or energy as given in the top panel) of this detectorduring the second gun firing. The dark line in the middle ofthe spectrogram are counts that were not imaged due to saturation of the imaging electronics.
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R.L.ArnoldyetaL
displays the pitch angle distribution as function of time (or energy sweep). The intensity of the fluxes reaching Utis detector was so high that its imaging capability was saturated which means that not all counts are imaged but many wcrc placed in one of the bins (the dark streak at the center of the distribution). Those counts that were imaged, however, give a correct pitch angle distribution. Like the ARCS 1 results, the electrons have a population streaming down the field line but this population is not azimuthally isotropic in that pitch angles of 330°should see the same count rate as 30°which is not the case. The measured electron population is not fixed in space with the detector rotating through it but rather rotates with the detector and is therefore always seen in the same detector pitch angle bins. The only source for these electrons is the plasma gun located above and about 35°removed in azimuth from the detector head. The plasma gun should release only a few eV electrons and not a distribution extending to the gun energy and dependent on the gun energy. One has reason to be very skeptical of particle measurements made aboard a beam-emitting payload. However, when the release and diagnostic payloads were separated by less than 10 meters, three different detectors aboard the diagnostic payload (a conventional cylindrical electrostatic analyzer, a sweeping pitch angle-imaging detector, and a fixed energy pitch angle-imaging detector) all measured these electrons giving a very similar energy spectrum and pitch angle distribution. At this time we have to believe these electron measurements, but whether they are similar to those made aboard the ARCS 1 flight is questionable since the ARCS 1 electrons extended well above the energy of the 60 volt plasma gun flown. The most likely interpretation of the ion gun-related electrons, which have a maximum in their energy spectrum that tracks with the maximum ion energy or with the anode voltage applied to the ion gun, is that they are accelerated near the ion gun by a potential that is oscillating between 0 and negative anode voltage. We have argued above, when discussing the gun ions, that they present no evidence for a DC or even quasi-static charging ofthe ion release payload. This is true up to the temporal resolution of the detectors, which was 8 ms. Kaufmann et al. /4/ have presented evidence from the ARCS 1 flight that supertheimal gun electrons may have modified the the ambient population around that payload at the frequency of the AC filament power supply of the gun (17 KHz) which could have produced the Langmuir probe oscillations measured at nearly 1 KHz frequency as an aliased measurement of the true 17 KHz plasma fluctuations. It is possible that the filament of the ARCS 4 gun also fluctuated several volts around ambient plasma potential at a frequency of 30 KHz, thereby creating bursts of superthermal electrons emitted by the gun, which when emitted neutralized the gun (and perhaps the payload), but when attracted to the cathode and kept within the gun momentarily, resulted in an unneutralized plasma (payload?) that accelerated electrons as observed. This mechanism might also explain the energy distribution of ions detected from the gun on both the release payload and the diagnostic payload as a result of energy loss in moving away from a partially unneutralized source. An Argon ion retarded in energy in leaving the gun due a negative potential would not necessarily regain that energy in returning to a detector on the release payload since the payload would most likely be at a different floating potential when the ion is detected than when the ion left the payload due to the large difference in the ion gyrofrequency and the frequency of any payload potential fluctuation. Such an argument cannot be made for electrons accelerated away from the release payload and being detected by a sensor electrically attached to the same payload because of the short travel time of electrons. Moreover, a radial electric field surrounding an unneutralized release payload would just E x B drift ambientelectrons around the payload and not impart significant energy to the electrons. Since gun-related electrons were detected on the release payload and on the diagnostic payload, when within several meters of the gun payload, at gun anode energies and lower, it appears that the acceleration of the electrons must have occurred within the plasma generator itself due to space charge electric fields created when the superthermal electrons were prevented from leaving the gun as a result of a small oscillating positive potential on the filament as discussed above. The electrons apparently accelerated by the ion gun have a phase space distribution which is quite unstable, therefore these electrons are clearly the source of intense waves measured during ion beam injections between the electron plasma frequency and the upper hybrid frequency as well as below the electron gyrofrequency. To produce the waves the electrons must resonate with a wave mode whose phase velocity matches the electron speed. The index ofrefraction for 2 occurs only between the electron plasma frequency and a typical cold plasma at the rocket altitude shows that large n the upper hybrid frequency and below the electron cyclotron frequency where the waves were observed. These wave observations will be more thoroughly discussed in subsequentreports. SUMMARY There are three results ofthe ion beam experiments described in this report that are of particular interest in space plasma physics and have application to natural phenomena occurring in the upper ionosphere and magnetosphere. These are the transverse acceleration of ambientions in the large beam volume, the scattering of beam ions near the release payload, and the acceleration of electrons very close to the plasma generator which in turn are unstable to the growth of intense high frequency waves. All of these processes undoubtedly involve strong wave-particle interactions. The ability of 100 ma ion beam injections to produce these phenomena appear to be related solely to the process by which the plasma release payload and the ion beam are neutralized. Since the electrons in the plasma release do not convect with the plasma ions, the neutralization of both the payload and beam must be accomplished by large field-aligned currents (milliamperes/meter2) which are very unstable to wave growth of various modes. Future work will concentrate on the wave production and wave-particle interactions that produce the plasma/energetic particle effects discussed in this paper. It is planned to directly study the heating, acceleration and scattering ofparticles with the injection of waves from rocket payloads into the upper ionosphere. ACKNOWLEDGEMENTS Work at the University of New Hampshire was supported under NASA Grant NAG 6-12, at the University of Minnesota under NASA Grant NAG6-l 1, at Cornell University under NASA Grant NAG5-601 and at Marshall Space Flight Centerunder NASA RTOP 432-36-55.
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11, 19, 1984. 10. M. Malingre, and R. Pottelette, Geophys. Res. Len., 12, 275, 1985. 11. P.M. Kintner, J. LaBelle, W.A. Scales, R.E. Erlandson, and L.J. Cahill, Jr., Ion Acceleration in the Magnetosphere and Ionsphere, Geophys. Monogr. Ser., 18, 206, AGU, Washington, D.C., 1986. 12. R.L. Kaufmann, R.L. Arnoldy, T.E. Moore, P.M. Kintner, L.J. Cahill, Jr., and D.N. Walker, J. Geophys.Res., 90, 9595, 1985. 13. D.N. Walker, J. Geophys. Res., 91, 3305, 1986. 14.
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