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Electron beams and their interactions with the ionosphere — A review of the E| |B series
12)lS—(l2)28, 1992 Adv. Space Res. Britain. Vol.12 No.12, pp.( reserved. Printed in Great All rights
0273-1177/92 $15.00 Copyright © 1992 COSPAR
ELECTRON BEAMS AND THEIR INTERACTIONS WITH THE IONOSPHERE A REVIEW OF THE El lB SERIES
-
P. J. Kellogg School of Physics and Astronomy, University of Minnesota, Minneapolis, MN 55455, U.S.A.
ABSTRACT
A review is presented of results from a series of experiments which began with an attempt to reflect electrons from the potential change which is thought to generate the aurora. In order to understand the experiment, this series then expanded to study beam-plasma interactions in the laboratory as well as in the ionosphere. Results from the electron reflection experiment, as well as in situ and ground-based observations of the waves generated by beam-plasma interactions will be reviewed. Measurements bearing on modifications of the ambient plasma by the beam-emitting rocket will be presented. INTRODUCTION The El lB series of rocket flights was initiated by William Bernstein, then of the National Ocean and Atmospheric Administration, with the primary purpose of investigating the electric field structure of the auroral generation region by detecting reflected electrons generated by an on-board electron accelerator. From the beginning it was realized that beam plasma interactions as well as the interactions of the charged, beam-emitting payload with the surrounding plasma would play a large role in the experiment, and so it developed into a complex set of measurements to investigate all of these effects. At a certain point in the series, the experiments were therefore renamed ‘SCEX” for Several Compatible Experiments to reflect these goals. Rockets which may be considered to have belonged to the series are as follows: ExDeriment NVB-02 El lB NVB-O6 SCEX I SCEX II SCEX III
Date 5 9 3 27 31 1
Mar. Apr. Dec. Jan. Jan. Feb.
References and Comments 1977 1978 1979 1982 1987 1990
- - -- - - -
guns failed to open /1/ partly successful, some echoes /2/,/3/,/4/,/5/ successful, no echoes /5/,/6/,/7/,/8/,/9/ echoes, extremely successful /1O/,/ll/ partial gun failure precluded echoes /12/ some deployments failed, data analysis in progress
In addition, the rocket program has been supported by research in laboratory plasma chambers /13/ to /29/ and theory /30/,/31/. A more broadly based review of the many experiments has been given by Szuszczewicz /32/ and so this review will concentrate on the El IB-SCEX series. As stated above, the rocket experiment program was conceived as an attempt to reflect electrons from the electric field that is thought to generate the aurora. This electric field is thought to be situated at a distance of about 1 earth radius. The rocket emits pulses of electrons successively at a number of different energies and from the round trip time and the existence or non-existence of echoes one can establish the total potential drop and the location of the reflecting layer. Electrons which are reflected from such a great height will suffer significant drifts, mostly due to electric fields perpendicular to the magnetic field, B, and so will not return to the electron-emitting payload. To detect the returning electrons a number of Throw-Away-Detectors (TADs) are released from the main rocket and go out to cover the surrounding area. In the earlier flights these TAD5 were instrumented with electron detectors by Klaus Wilhelm, Max Planck Institute, Lindau, and in SCEX III the instruments were provided by Roy Torbert, University of New Hampshire. Although the configurations have varied slightly from flight to flight most experiments have had a configuration approximately like that shown in Fig. 1, which represents the nominal configuration of SCEX III, the latest in the series. On reaching a reasonable altitude a forward payload, which mainly carries plasma diagnostics and wave antennas, is ejected, and 4 TAD’s are ejected to the sides, so that the rocket payload divides into six parts. In the (12)15
(12)16
P. J. Kellogg
B (I3~) GEOGRAPHIC VERTICAL
2 rn/sec Forward Paylo tin”,
10
AT 209 ASIMUTH
58*
/ ~TAD 2
~JJ
gmi~rProb.s
w— Alt Payload
TAD
TETHER
~AD
Fig. 1.
A typical configuration for rocket experiments of the
EIIB
and SCEX series.
latest configuration, SCEX II and SCEX III, one of the TADs was fitted with a plasma wave receiver and the other three with particle detectors for echoing electrons. In SCEX I there were two wave receivers and two particle detectors. The two Canadian experiments (NVB-O2 and NVB-O6) did not carry the throw-away-detectors. ECHOES Originally it was thought that successful detection of returning electrons would be very difficult and would depend on a very fortunate alignment of detector position and pitch angle. The laboratory results showed that beam was greatly spread in energy and pitch angle by beam-plasma interactions, which makes detection of returned electrons much more probable. In El lB a few echoes were detected /4/. However, in SCEX I, the experiment was extremely successful and more than 200 echoes were detected by the two TAD5 carrying electron instrumentation /10/, /11/. These experiments, however, raised further problems which we will discuss below, but first we discuss the results from the point of view of success. On each of the two TAD’s of SCEX I there were four analyzers set to the three different gun energies which were used (two detectors were set to 4 key). The gun energies were 2, 4, and 8 key, at currents of nominally 1, 10, and 100 mA. In Fig. 2, taken from Ref. 11, we show an overview of the observations. The lower part of the figure shows the rocket trajectory together with the periods when auroral precipitation was detected. The upper part of the figure shows the delay time of the detected echoes as a function of flight time. It will be seen that there is considerable scatter in the delay times but that in general the delay times, of .2 to .5 sec, correspond to reflection of a height of 3000 to 5000 kilometers. The reflections detected by TAD 2 were confined to the first period of auroral precipitation, and cease when the precipitation ceases. No reflections were detected in the two shorter periods of precipitation which follow the main one. This can always be understood by supposing that the TADs were in the wrong place. In order to determine the expected return position of the electrons we need a way of measuring the electric field drift. Unfortunately the electric field experiment itself was a failure on SCEX I owing to an error which resulted in boom deployment before despin and consequent breaking of the electric field antennas. However, a thermal ion detector provided by Brian Whalen of the National Research Council of Canada measured the velocity of the ionospheric plasma with respect to the payload. A representation of these data is shown in Fig. 3. It will be seen that for a period corresponding to TAD 2 separation of 150-200 m the electric field drift would not have returned reflected electrons to the vicinity of TAD 2. This roughly accounts for the gap in echoes at TAD 2 between separations of 150-175 m. At separation — 300 m the return position moves slightly but not strongly away from TAD 2. This might account for the end of detected echoes.
Election Beams
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Fig. 2. Observations of delayed Echoes from SCEX I. In the lower panel is shown the rocket trajectory together with times during which auroral electrons were observed. In the upper panel, on the same time scale, are shown the clear cases of returning electrons. The different symbols show which TAD detected the returns and the energy (2 or 4 key) of the emitted gun pulse. No delayed echoes were detected for 8 keV pulses. NORTH 330’
340’
350’
0’
10’
20’
30’
320’
40’ 400 ‘~WAKE”
IE
+
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310’ TAD2 300’’
+ +
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+
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100
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WEST 270’
4
80’ 90°EAST
260°
100°
250°
110°
240°
230°220° 200° 180’ 160’ SOUTH
140° 130°
120°
Fig. 3. The expected return position of reflected electrons as a function of distance from the main payload based on the observation of the drifts of ions of the ambient plasma. .~
12,12-C
(12)18
P. J. Kellogg
However, for 15 sec after release, the TAD experiments also detected what Wilhelm labeled “prompt responses”, i.e., electron echoes with time delays between 0 and 40 msec. These are shown for both TADs in Fig. 4. More recently, R.J. Nemzek and J.R. Winckler /33/ in Electron Echo VII have found a continuous distribution of responses, which must therefore be considered to be prompt responses, with delay times up to 180 msec. At an average delay of 20 msec as observed in SCEX I and at 4 kV an electron could only have returned from a point not more than 400 km away and it is not possible that reflection layers exist in that region, or else there would be no reflections from higher up. There are therefore three possibilities for the cause of the reflected electrons. A) The electrons are reflected by a layer of strong electric field; B) the electrons are reflected by the fields of strong beam plasma interactions; and C) the electrons are not reflected but are precipitated from the trapped radiation by the fields of strong beam plasma interactions. Because of the prompt responses, not all of the echoes can be due to the first process. The entire sequence of reflected electrons is consistent with interpretation (B), that the electrons are reflected by strong beam-plasma interactions, with the gap between prompt responses and delayed echoes being due to unfavorable electric field drift. Nevertheless there are still some conceptual puzzles. If the beam-plasma interactions are strong enough to reflect electrons after 20 msec then it would seem that they are strong enough to thermalize the beam and therefore the beam will no longer be able to generate the instabilities which, farther up the field line, cause the delayed echoes. A possible explanation might be that the fastest particles go ahead in a temporal time-of-flight mechanism and the bump on tail structure of the beam may be reconstituted in this way. Or as Papadopoulos has suggested (private communication) a different beam interaction takes place at the height where the electron-cyclotron frequency becomes higher than the plasma frequency, of the order of thousands of km. But also a very reasonable explanation is that the prompt responses are due to (B) electrons reflected by beam-plasma interactions, and the I
Sequence I
Sequence 2
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20
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—
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l.A.
I
10
Delay times for prompt responses detected by the TADs in SCEX I.
delayed echoes are due to (A) reflection by electric field ‘ayers at about 1 earth radius. The scatter in the transit times would then be due to two processes- -first the beam-plasma interactions
which the beam must traverse,
and second irregularities
in the reflecting
layer
itself which is now believers to consist of a large number of small potential drops. Clearly there are a number of features which are not yet understood and which would be clarified by further data, even without additional instrumentation. connection with the observed fields of the next section, we need to consider what fields are necessary to bring a beam electron back. Kellogg et al. /20/ found in plasma chamber work that the observed beam electron velocity spread was consistent with the measured plasma frequency field strength provided that the most extreme velocity changes were for electrons which were exactly resonant, i.e., which caught the perfect wave and rode it to the target.
In
Electmn Beams and their Interactions
(12)19
For the prompt responses and the delayed echoes this cannot be the right concept for the whole reflection process, as we cannot expect the perfect wave to turn around and bring the electron back. Rather the electrons must diffuse in velocity by interacting with many waves. The net change in velocity is then ~v—f~
öv
(1)
where ~v is the total velocity change, n is the number of interactions, and 6v is the velocity change in one interaction. The following order of magnitude estimate assumes that these interactions are with waves near the plasma frequency. If a single interaction with a plasma frequency wave has a duration of l/f~sec and gives
(i—)
6v—~E
(2)
The number of interactions in a typical delayed echo time of TD velocity change of
—
.4 sec gives
a maximum
~v-J~(~E)
(3) 3 MHz, TD
9cm/sec, corresponding to a 4 keV beam and f For ~v a lower 3.8 lOlimit of gives p E—
.4
sec,
.6V/m
this
(4)
For a prompt response in 20 msec, the field must be about five times larger. BEAN PLASMA INTERACTIONS AND WAVES above it seems that the prompt responses are most likely due to reflection of electrons by strong beam-plasma interactions. A considerable effort has been devoted then to measuring directly the waves which are the result of beam-plasma interactions and the cause of the prompt responses. In the discussion
The full wave spectrum from near D.C. to several MHz has been measured on different flights at several different positions around the main payload, at distances up to 1 km. A typical spectrum is shown in Fig. 5. It will be seen that the largest signals, in terms of electric field strength, are present at low frequencies, yet the waves responsible for the reflections must be resonant with the beam electrons and it is usually believed that they are due to the standard two-stream instability at the plasma frequency. Our observations of the field strengths at the plasma frequency, however, show that it is quite weak. We note that in the supporting experiments carried out in Chamber A of Johnson Space Flight Center
~
1~55
“3’’~”I~ ~“T’
SCEX 3 Forward Payload
SCEX 3 Forward Payload
flight time 168.926 sec.
-
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.
.
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flight time 162.268 sec.
-
-
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.01
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10
,~
Spectra of transverse electric field for a high current (left) and a medium current At this time the forward payload was 130 meters from the gun along B and roughly 10 meters from the beam. gun pulse.
P. J. Kellogg
(12)20
in the late 1970’s, electric fields were detected in the plasma frequency range which were capable of explaining the observed effects on the electron beam, namely fields of 30 V/rn, even though the distance available to these fields in that case was only 20 meters. These electric field observations have been discussed in Kellogg et al. /20/ and the effects of such fields on the electron spectrum have been discussed by Jost et al. /17/. It is known from the work of Jost et al. /18/, also in Chamber A at JSC and from theory /34/ that the plasma frequency electric fields are strongly confined to the region of the beam itself, so it is immediately suggested that the reason we have never seen sufficiently strong electric fields is because none of the payloads have ever passed through the beams. In three flights therefore (SCEX II, SCEX III, and Electron Echo VII), attempts have been made to send one payload parallel to the magnetic field and through the electron beam. These attempts have so far not been successful, as determined by trying to detect the beam electrons with particle detectors. A principal problem has been that too much time elapses between payload ejection and gun turn-on, necessitated in order that the guns outgas sufficiently. Combined with the errors in ejection direction this means that the payload does not quite pass through the electron beam. Recently a number of plasma simulations on supercomputers have been made which are becoming more and more realistic. It is reasonable to turn to such simulations to investigate the behavior of the electric field outside the beam to get some idea of how close the payloads have come and what electric field might be expected inside the beam. In Fig. 6 are shown typical electric field spectra from a recent simulation /35/. Similar and more detailed results have been obtained in calculations especially aimed at simulating the experiment Echo 7 /36/ /37/. Note that the low frequency electric field dominates, even within the beam. In most flights the beam generates waves near the plasma and upper hybrid frequencies. Low current beams generate these most clearly, as is seen in Fig. 7 for the 1 mA pulse at 139.4 sec. At higher currents the spectrum is nearly featureless and fills the whole observed frequency range, even in the stop band for electromagnetic radiation above the electron cyclotron frequency, as is seen in Fig. 5. In most flights we have also seen harmonics of the plasma frequency. At first it was considered possible that these harmonics were generated in the electronics, but all recent experiments have shown clearly that the harmonics are part of the signal on the antenna. This has been done by periodically attenuating the signal and noting that the harmonics are present when the signal is well within the linear range of the electronics. The harmonics might, of course, be generated in the sheath around the antenna. However, we now present evidence that these harmonics are generated in the beam, in that the number of harmonics and their amplitude as a function of harmonic number appears more compatible with what is expected from bunching of the beam than with the kind of distortion to be expected in a sheath. FIeld History
Power Spectrum
Power Spectrum
Field History
fL~1:ebW!3 ::~J I F ~ t/L~Lm1~:4L/~1~øL\~~2 fr~’~J~1 .2L~1~10EF~’°1~ ocr’
ForwUrd of thearnc.rnatf
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Acrosa the MaQneuic FIeld
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0
40
Wp0t80
120
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0
40
80
120
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Fig. 6. Some simulation results from Winglee and Pritchett. The lefthand panel shows the transverse electric field at 3 points up the beam from the rocket. A is the cell length in the simulation and should be taken to be between 10 cms and 1 meter for comparison with the experiments. The righthand panel shows the variation of electric field across the beam at a distance of x — A. Electric fields are scaled to a beam energy per 2 A. Note the preponderance of the low frequency electric fields even inside the beam.
Electron Beams and their Interactions
(12)21
Although there was a failure of the gun system in SCEX II in that an arc developed across the batteries for the gun high voltage power supply, nevertheless for a short period the guns functioned in a way which resulted in some useful data. It appears that the arc allowed a voltage drop of about ten volts across the battery terminals and that this voltage was sufficient to drive the gun power supply to generate a basin of a few hundred volts and a few or a few tens of milliamperes. In Fig. 7 and 8 we show observations from that flight which contain a large number of harmonics of the plasma frequency. At the time of the measurements, the forward payload was 150 m up the field and about 12 m away from the beam. The figure shows nine harmonics of the plasma frequency. The amplitude as a function of harmonic number for the gun pulse at 141.913 sec is plotted in Fig. 8. These waves were from a gun pulse of 1.4 mA at 680 eV energy. The power falls as harmonic number to the fourth power.
•1
. ~
20-
~: GUNS I
c’
~
1~I~TII ~:I. ~ uII_i~uII._I~II..
Guns v 139.
140
142.
14S.
FLIGHT TIME (eec)
Fig. 7. A period of wave observations for the short time when the SCEX II gun operated. The spectrum at 141.913 shows nine harmonics of the plasma frequency and is interpreted to indicate full bunching of the beam. Several pulses including the spectra at 139.224 and 139.787 show a clear upturn at the highest frequencies.
Scex U January 31, 1987 OsmPalec at 141.913 eec, 680 volts, 1.4 mA
_______
____
____ __ ___
-
—
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~~ii~iii
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Fig. 8. The amplitudes of the harmonics of the plasma frequency for the gun pulse at 141.913 sec of Fig. 7.
(12)22
P. J. Kellogg
The standard explanation of such harmonic generation was given by O’Neill et al. /38/. The harmonic generation is supposed to arise from complete bunching of the beam which then becomes a series of thin disks propagating along the field. The Fourier analysis of the field of these thin disks is clearly rich in harmonics. O’Neill et al. calculated the spectrum of the electric field of such thin disks for an infinitely wide beam and found that the amplitude of the harmonics fell like harmonic number squared. We would not expect more than qualitative agreement with their results because the observations are made outside the beam and because we are probably observing electromagnetic radiation resulting from the conversion of electrostatic waves. Plasma waves which are resonant with the beam would have a wave length of a few meters and at frequencies that are significantly above the plasma frequency such waves are attenuated also in a few meters. We can use this observation to estimate a lower limit on the field strengths which would be required of plasma frequency waves. At the time of these observations the forward payload was d 150 m above the main payload.
w
The wavelength for resonant waves is given by — kvb, where vb is the beam velocity. To bunch the beam, electrons must be moved by one Ralf wavelength in the travel time to the forward payload, d/vb, hence 5
_A_~2_iat2
1
E
d
This estimate of E is 3 E—
~.5V/m
(6)
in rough agreement with the earlier estimate of what is required. Thus, the indirect evidence indicates that fields of the required magnitude exist inside the beam. The largest plasma frequency fields which we have measured directly in earlier flights (SCEX III data are not yet fully analyzed) are of the order of a few mV/rn, hence 100 times smaller.
ROCKET ENVIRONMENT DURING BEAM FIRING The design of experiments for investigating the environment of the rocket during beam firing depends on a certain picture of this environment. In Fig. 9 we show what we think the environment must look like. Our picture is similar that of Managadze et al. /39/. When the rocket emits an electron beam it must become positive at a very fast rate. If the rocket were in vacuum we can estimate its capacitance to be about 200 pF and so for a 100 mAmpere beam the rocket potential would go positive at a rate of .5 kV/msec. In plasma the dielectric constant reduces this potential considerably since the dielegtric constant at frequencies below the ion cyclotron frequency may be of the order of 10 . On a time scale in which the ions cannot move the dielectric constant is smaller by the electron-ion mass ratio, with further complications in the electron cyclotron and plasma frequency regions. We expect that the response of the surrounding plasma will be complex and time-dependent on scales from the ion-cyclotron frequency to the plasma frequency. We therefore expect a picture like the following. As the rocket becomes positive, the first response is for the electrons to move toward the payload, a response which is carred outward by a Langmuir wave, or by a resonance-cone (longitudinal) whistler. This response should then travel outward at a speed of the order of the electron thermal speed, which appears to be instantaneous on our time resolution (about 1 rnsec). If the rocket continues to go positive, it repels away all of the positive ions of the ionosphere and will therefore be surrounded by a region which contains only electrons. There will presumably be an ion-enhanced region but these ions ought quickly to be neutralized by electrons pulled along field lines outside the ion void. For field lines that intersect the payload the electrons will be pulled into the payload, but together with secondaries made by the action of the beam on the neutral atmosphere. For field lines which pass close to the payload, electrons will be pulled toward the maximum positive potential and will therefore have gained considerable energy. In the ion void then electrons will be oscillating back and forth along B, passing through the maximum of the positive potential in the equatorial plane. At the equatorial plane they will have considerable energy, some of which will be perpendicular energy so that electrons which pass close to the payload may have sufficiently large Larmor radii to intersect the payload. The zeroth order effective area is then larger than the physical area /40/. The rocket is moving through the plasma at a speed which is generally of the order of .3 to 1 kin/sec. Therefore at large distances from the rocket the plasma will appear to be streaming in response to an electric field. The electrons of the plasma will appear to
Electron Beams and their Interactions
(12)23
Electrons Depleted
Enhanced Ions
:\:IOfl~
:\~:vo1d)
Low Frequency Instabilities
Fig. 9.
A conceptual picture of the environment of a beam-emitting rocket.
2. The velocity streamlines in such a drift by with velocityin given x B)/B situation are a sketched Fig. by 10 V for— (E ambient electrons. Electrons which are going to miss the payload by a large distance will go around it. Within a certain separatrix all electrons are trapped and will drift around the payload. From this picture it appears that no electrons can be collected by the main payload from its equatorial plane. All electron collection is then along field lines, which either strike the payload or become part of the electron cloud. There may be some evaporation from this cloud which contributes hot electrons which travel back up the field lines. Otherwise electrons will remain in the cloud until they diffuse into the payload. If there is no evaporation, then the size of the cloud is determined by Linson’s limit /41/ 2 (7)
where r and r current~ and rR.
are the cloud and effective rocket radii, and I and I are the beam electron saturation current which would be col~ected~y a rocket of size
In early work by Winckler /42/ it was found that the rocket neutralization was maintained reasonably well at rather low potentials. This coupled with our own experience with floating potential probe booms, which we still do not understand, lead us to believe that the rocket would be charged to at most 1MO~2OOvolts and this belief dictated the design of the early experiments. However, this wa~in spite of Managadze et al.s’ /39/ results showing that the rocket could charge to tne beam energy and to some extent we attributed Managadze’s results to the very clean payload end very high altitude which he reached. In 1985 however, the MAIMIC rocket experiment /43/ showed clearly that in a tenuous plasma the payload could charge to the full beam energy and beyond. We had ourselves demonstrated this in plasma chamber work /21/ and in retrospect it was an error not to consider large payload charging more seriously. The electron cloud in the ion void consists of fairly energetic electrons since these have fallen from large distances into the potential of the rocket, that is to say electrons having an energy of the order of tens to hundreds of eV. These electrons then are capable of ionizing the neutral background. The resulting ions will be hurled outward by the electric field of the rocket and a measure of their energy spectrum at large distance would allow a measure of the rocket potential.
(12)24
P. J. Kellogg
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40
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—40
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—30
Illilill
—20
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—10
11111111111
0 METERS
10
20
30
Fig. 10. Trajectories of low energy electrons under E x B forces in the frame of the equatorial plane of a rocket. The rocket has been assumed ~o be charged to 400 volts and the field is assumed to fall off around the rocket like l/r
A simple experiment was designed to be mounted on the TAD5 to measure the spectrum of these ions. The TAD5 have uncontrolled orientation and it was desired to have as large a solid angle for the detectors as possible even at some sacrifice of energy resolution. However, the energy resolution sacrificed in a planar detector of even one sterradian was considered to be too great and so rather simple hemispherical retarding potential analyzers were used. These were four grid analyzers with the outermost grid grounded to the rocket body. The next grid was an electron repeller. The third grid was an ion energy analyzer and the fourth grid suppressed secondary emission and shielded the collector from capacitive coupling to the swept grid. In SCEX II the RPA’s were designed to measure ion energies up to 200 eV. In SCEX III this was increased to 400 eV. Some results were shown in Fig. 11 taken from Kellogg and Monson /12/. In these the effects of capacitive coupling between the ion analyzer grid and the collector have been removed by subtracting a background of data taken just before launch. However, there is still a large apparent ion current at the beginning of each sweep. This is attributed to energetic auroral electrons which can penetrate the electron repeller grid but which are attracted to and captured by the ion analyzer grid when it is very positive. The fact that they then do not arrive at the collector appears as a positive, i.e. ion resembling, current. The true ion currents are clearly seen in coincidence with the gun pulses. In this flight, SCEX II, an arc developed across the gun battery. It is likely that outgassing products from this arc maintained a higher than neutral density in the vicinity of the rocket and that the amount of neutralization current due to ionization is therefore larger than it would normally be. From these ion currents we /12/ determined that (1) a significant fraction of the payload neutralization current was due to this ionization process and (2) that no maximum ion energy was observed so that the accelerator payload charged to more than 200 volts. It appears that this whole electron cloud region is quite unstable to low frequency electrostatic plasma waves which then replace collisions, as suggested long ago by Linson /41/, and allow electrons to diffuse into the main payload and neutralize it. They are probably also responsible for heating the plasma cloud. The wave experiments give information on the low frequency instabilities and on the expanding first response surface. The best evidence on the first response surfaces has been obtained from our participation in the experiment Echo VII of J.R. Winckler /44/. The measurements were made by a forward payload which was ejected at an angle of about 5 degrees to the beam at a speed of 4.2 rn/sec. Since the experiment lasted about 400 sec the forward payload traveled about 1.6 km up the beam. The antennas of the forwar -vioad consisted of two stainless bands wrapped
Electmn Scams and their Intemctions
1A04(NO!TH) .
CPA
(12)25
i
..
~
20
TAD2(WEST)
teAl
—
CURCENTI?~ GUN i
te~ VOLTS
~
~
tood~ ..-.._.,.~___________ 0 (4!
Fig. 11. Energetic ions which have been a charged rocket and expelled outward by occur periodically are a spurious effect seen in conjunction with the gun pattern
-
s:...~sJ+J ~
~
~
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42 FLIGHT TINE (sect
143
created by ionization processes in the vicinity of an electric field. The sharp decaying pulses which due to auroral electrons. The ion currents can be which is shown below.
around a fiberglass section of the nosecoise and separated by .8 meters. Since the nosecone was spin stabilized with its axis roughly along the magnetic field, this antenna measures the component of electric field parallel to the magnetic field. In Fig. 12 we show the first response of this antenna to gun pulses for some early times and again for some later times. The sign of the first part of the pulse changed with distance, the change occurring at a distance of approximately 800 meters. The change in sign of the first pulse could be due to the fact that this first pulse is carried out by a dispersive wave, or it could be due to a change in the relative importance of the payload charging and the charge density of the beam itself. Clearly, however, the region of low frequency disturbance extends at least 800 in along B.
cuc’moe
ELECTRON WOO 0 NOOL COME a lC~ PIT’S 01161.2 30 AT 82 082 SEC 402 METERS It,e 6)38
ECHO 7 NOSE COFIE AHGL2 110 It?? METERS IRON CI’S
31 cnn PITCII A? 383.387 SEC
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I______
11110 8582 El.1C1509 ECHO 7 NOSE CORE 30 KnY PITCh ASCIi III A? 83,882 SEC 406 PYlON
METORS
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008
88
~ Kay
8822 CONS
PITCH 411012 ItO
A? 382.880 SEC
See NETENS
FROM GUN
__
TIME WOlF
Fig. 12. The first response of the electric field measured in the nose experiment on Echo VII which traveled up the beam at an angle of about S degrees to the magnetic field. The fields measured are roughly parallel to the magnetic field and a positive electric field is downward toward the electron accelerator.
(12)26
P. J. Kellogg
We also have measured, but less accurately, the variation of the low frequency waves with distance perpendicular to B from the payload. The best evidence which we have analyzed comes from SCEX I. The spectrum is obtained from data which were returned as samples of a broadband logarithmized signal. On the ground the data were exponentiated to reconstitute the original waveform and then Fourier analyzed. The length of the gun pulses, 50 msec, sets an effective lower limit to the frequency resolution of 20 Hz, so the ion cyclotron frequency is not resolved. In Fig. 13 we show the behavior of the total power of these low frequency waves in the band from 30 Hz to 1 kllz as TAD 1 moves away from the main payload. When the TAD is close to the main payload there is a fair amount of saturation and the points should be regarded only as lower limits. It will be seen that there is no sharp outer boundary to the low frequency waves but that their amplitude decreases in a fairly regular way and extends to about 100 meters. We believe that these low frequency waves are responsible for the diffusion of the electrons of the electron cloud to the main payload as suggested long ago by Linson /41/. The waves which do the diffusing seem not to be the lower hybrid waves. In the waveforms shown in Fig. 12 only one of the waveforms shows strong lower hybrid waves, the one at 1280 meters, and in this case the lower hybrid waves were already present before the gun was turned on. Further, of the two spectra shown in Fig. 5, the low current spectrum shows a strong lower hybrid peak (7 kHz), but the high current spectrum shows the lower hybrid peak submerged in the low frequencies. The waves are irregular but the major part of their spectrum falls well below the lower hybrid range into the ion-cyclotron frequency range. A number of workers /45/ /46/ have observed that the plasma around a beam-emitting rocket is heated out to distances of the order of 100 meters. It is likely also that these low frequency waves are responsible for part of this heating. As suggested above, some of the heating may also be due to evaporation of electrons from the cloud. GROUND OBSERVATIONS Following the successful detection of HF radiation from the electron beam in Electron Echo IV /47/, we set up ground stations for the observation of radiation from the natural aurora and from the artificial electron beam in each of the flights in which we have participated. In this way we have detected from HF radiation from the natural aurora /48, 49/. A total of about 70 such events have been measured. Some of the radiation detected in Echo IV was exactly (to .1%) at the second harmonic of the electron cyclotron frequency and this same identification implies height limits on the generation region for emissions from the natural aurora. In another flight in the Electron Echo series /7/, radiation was detected on the ground which must have been at the plasma frequency. This is shown in Fig. 14 which shows the signal on a receiver which is sweeping from 2.9 MHz to 3.4 MHz, together with a representation below of the gun program. This radiation was identified as being at the plasma frequency by comparison with the local electron density as measured by the Chatanika radar. However, we have never succeeded in measuring radiation from the beam during any of the SCEX flights. This is perhaps because we are always seeking to send the SCEX experiments over a stable auroral arc and in the vicinity of such an arc the ionospheric critical frequency is higher than the 3 MHz second harmonic of the electron cylotron frequency. While it might .1
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across the
Electron Beams and their Interactions
ELECTRON ECHO z
(12)27
NOVEMBER
I
Ii, 1979
~ 240
250
260
270
FLIGHT TIME I5ECI
Fig. 14. Ground observations of waves at the plasma frequency from Echo V. is represented below.
The gun program
appear desirable to try to measure the third harmonic at about 4.5 MHz, this frequency falls into a busy communications band and the interference makes measurements more difficult. Still measurements have been made several times at that frequency and have never been successful. ACKNOWLEDGEMENTS First this whole program was begun by William Bernstein, recently retired from Rice University, and we are grateful to him for the initiative and guidance which he has provided. The program has involved the contributions of a large number of experimenters, who have designed, built and tested their instruments for the various measurements of the program. These include Brian Whalen, Malcolm Duncan and Andrew Yau of the National Research Council of Canada, Klaus Wilhelm and Chris Becker of the Max Planck Institute, Lindau, Roy Torbert and Craig Kletzing of the University of New Hampshire, and Ed Szuszczewicz and Hugh Anderson of SAIC. The author is grateful to Klaus Wilhelm, to Hugh Anderson, and to Tom Hallinan for the guidance and support they have given him during the tense moments of a launch. University of Minnesota work on NVB-02 and NVB-06 was supported by NOAA grants 0378-B0l-6O and NA-79RACOO51, SCEX I through III were supported by NASA through grant NSG 5373. REFERENCES
1. 2.
3. 4. 5. 6. 7. 8. 9.
10. 11. 12. 13.
14. 15. 16. 17.
P.J. Kellogg, S.J. Monson and BA. Whalen, Geophys. Res. Lett. 5, 47-50 (1978). W. Bernstein, P.J. Kellogg, S.J. Monson, R.H. Holzworth and B.A. Whalen, Recent observations of beam plasma interactions in the ionosphere and a comparison with laboratory studies of the beam plasma discharge, in Artificial Particle Beams in Space Plasma Studies, NATO Advanced Study Institute Series B, Vol. 79, Plenum Press, New York, 1982. R.H. Holzworth and H.C. Koons, J. Geoohys. Rca. 86, 853-857 (1981). K. Wilhelm, W. Bernstein and B.A. Whalen, Geophys. Res. Lett. 7, 117 (1980). P.J. Kellogg, S.J. Monson, W. Bernstein and B.A. Whalen, J. Geophys. Res. 91, 1206512077 (1986) G.R.J. Duprat, B.A. Whalen, A.C. McNamara, W. Bernstein, J. Ceotihys. Res. 88, 3095-3108 (1983). P.J. Kellogg and S.J. Monson, Adv. Space Res. 1, 61-68 (1981). R.T. Goerke, P.J. Kellogg and S.J. Monson, J. Geophys. Res. 95, 4277-4283 (1990). P.J. Kellogg and S.J. Monson, A second kind of BPD - rocket and laboratory results, in Active Experiments In Space. Proceedings of an International Symposium held in Alpbach, Austria 1983, ESA SP-195, p. 157-159. K. Wilhelm, W. Bernstein, P.J. Kellogg and B.A. Whalen, Geophys. Res. Lett. 11, 11761179 (1984). K. Wilhelm, W. Bernstein, P.J. Kellogg and B.A. Whalen, J. Geophys. Rca. 90, 491-504 (1985). P.J. l~ellogg and S.J. Monson, J. Geomag. Geoelectr. 40, 1257-1267 (1988). W. Bernstein, H. Leinbach, P.J. Kellogg, S.J. Monson, T.J. Hallinan, O.K. Garriott, A. Konradi, J. McCoy, P. Daly, B. Baker and H.R. Anderson, Geophys. Rca. Lett. 5, 127-130 (1978). W. Bernstein, H. Leinbach, P.J. Kellogg, S.J. Monson and T.J. Hallinan, J. Geochys. ~ 84, 7271-7278 (1979). W. Bernstein and P.J. Kellogg, Adv. Space Rca. 1, 347-360 (1981). R.W. Boswell and P.J. Kellogg, Geophys. Res. Lett. 10, 565-568 (1983). R.J. Jost, H.R. Anderson and J.O. McGarrity, Geophys. Res. Lett. 7, 509 (1980).
(12)28
P. J. Kellogg
18.
R.J. Jost, HR. Anderson, W. Bernstein and P.J. Kellogg, Radial dependence of HF field strength in the BPD column, in Artificial Particle Beams in Soace Plasma Studies, Plenum Press, New York, 1982.
19.
T.J.
Hallinan,
H. Leinbach,
G. Mantjoukis
and W. Bernstein,
J.
Geoohvs.
Res.
89, 23335-
2347 (1984). 20.
21.
22. 23.
24. 25. 26.
27.
28.
29. 30.
31. 32. 33. 34. 35. 36. 37. 38. 39. 40.
P.J. Kellogg, S.J. Monson, R.H. Holzworth and R.J. Jost, IEEE Trans. on Plasma Science, Vol. PS-l4, 891-901 (1986). P.J. Kellogg, H.R. Anderson, W. Bernstein, T.J. Hallinan, R.W. Holzworth, R.J. Jost, H. Leinbach and E.P. Szuszczewicz, Laboratory simulation of injection of particle beams in the ionosphere, in Artificial Particle Beams in Space Plasma Studies, NATA Advanced Study Inst. Series B, Vol. 79, Plenum Press, New York, 1982. X. Llobet, W. Bernstein and A. Konradi, J. Geophys. Res, 90, 5187-5196 (1985). T.J. Hallinan, H. Leinbach and W. Bernstein, Visible signatures of the multipatep transition to a beam-plasma-discharge, in Artificial Particle Beams in Space Plasma Studies, NATA Advanced Study Inst. Series B, Vol. 79, Plenum Press, New York 1982. P.J. Kellogg and R.W. Roswell, Phys. Fluids 29, 1669-1674 (1986). W.E. Sharp, Geophys. Rca. Lett. 9, 869-872 (1982). E.P. Szuszczewicz, and C.S. Lin, Time-dependent plasma behavior triggered by a pulsed electron gun under conditions of beam-plasma-discharge, in Artificial Particle Beams in Space Plasma Studies, NATA Advanced Study Inst. Series B, Vol. 79, Plenum Press, New York, 1982. E.P.
Szuszczewicz,
K. Papadopoulos,
W. Bernstein,
C.S.
Lin and D.N. Walker,
J.
Geophys.
~ 1565 (1982). D.N. Walker, E.P. Szuszczewicz and C.S. Lin, Ignition of the beam plasma discharge and its dependence on electron density, in Artificial Particle Beams in Space Plasma Studies, NATA Advanced Study Inst. Series B, Vol. 79, Plenum Press, New York, 1982. D.N. Walker and E.P. Szuszczewicz, J. Geophys. Res. 90, 1691-1697 (1985). K. Papadopoulos, Theory of beam plasma discharge, in Artificial Particle Beams in Space Plasma Studies, NATA Advanced Study Inst. Series B, Vol. 79, Plenum Press, New York, 1982. H.L. Rowland, C.L. Chang and K. Papadopoulos, J. Geophys. Res, 86, 9215-9218 (1981). E.P. Szuszczewicz, J. Atmos. Terr. Phys. 47, 1189 (1985). R.J. Nemzek and J.R. Winckler, ‘Prompt’ electron pulses generated by the Echo 7 electron beam, ~ Trans. Am. Geophys. Un. 70, 439 (1989. A.W. Trivelpiece and R.W. Could, J. Aool. Phys. 30, 1784-1793 (1959). R.M. Winglee and P.L. Pritchett, J, Ceophys. Res, 93, 5823-5828 (1988). R.M. Winglee and P.J. Kellogg, J. Geophys. Res. 95, 6167-6190 (1990). R.M. Winglee, J. Ceophys. Res, 95, 6167-6190 (1990). T.M. O’Neil, J.H. Winfrey and J.H. Malmberg, Phys. Fl. 14, 1204-1212 (1971). CC. Managadze, V.M. Balebanov, A.A. Burchudladze, T.I. Cagua, N.A. Leonov, S.B. Lyakhov, A.A. Martinson, A.D. Mayorov, W.K. Riedler, M.F. Friedrich, K.M. Torkar, A.N. Laliashvili, Z. Klos and Z. Zbysznski, Planet. Space Sd. 36, 399-410 (1988). L.W. Parker and B.L. Murphy, J. Ceophys. Res. 72, 1631-1636 (1967).
41.
L.M. Linson,
42. 43.
45. 46.
R.W. McEntire, R.A. Hendrickson and J.R. Winckler, J. Ceoohys. Res. 79, 2343 (1974). RN. Maehlum, J. Troim, N.C. Maynard, W.F. Denig, M. Friedrich and K.M. Torkar, Geophys. Res. Lett. 15, 725-728 (1988). JR. Winckler, P.R. Malcolm, R.L. Arnoldy, W.J. Burke, K.N. Erickson, J. Ernstmeyer, R.C. Franz, T.J. Hallinan, P.J. Kellogg, S.J. Monson, K. A. Lynch, C. Murphy and R.J. Nemzek, Echo 7, an electron beam experiment in the magnetosphere, ~ Trans. Am. Geophys. Union 70, 657-668 (1989). D.G. Cartwright, P.J. Kellogg and S.J. Monson, J. Ceophys. Res. 83, 16-24 (1978). R.L. Arnoldy and J.R. Winckler, J. Geophys. Res. 86, 575-584 (1981).
47.
S.J.
48. 49.
P.J. Kellogg and S.J. Monson, Ceophys. Res. Lett. 6, 297-300 (1979). P.J. Kellogg and S.J. Monson, Radio Science 19, 551-555 (1984).
44.
J.
Geophys. Rca.
Monson and P.J.
Kellogg,
74,
J,
2368
(1969).
Ceophys.
Res.
83, 121-131
(1978).
0273-1177/92 $15.00 Copyright © 1992 COSPAR
ELECTRON BEAMS AND THEIR INTERACTIONS WITH THE IONOSPHERE A REVIEW OF THE El lB SERIES
-
P. J. Kellogg School of Physics and Astronomy, University of Minnesota, Minneapolis, MN 55455, U.S.A.
ABSTRACT
A review is presented of results from a series of experiments which began with an attempt to reflect electrons from the potential change which is thought to generate the aurora. In order to understand the experiment, this series then expanded to study beam-plasma interactions in the laboratory as well as in the ionosphere. Results from the electron reflection experiment, as well as in situ and ground-based observations of the waves generated by beam-plasma interactions will be reviewed. Measurements bearing on modifications of the ambient plasma by the beam-emitting rocket will be presented. INTRODUCTION The El lB series of rocket flights was initiated by William Bernstein, then of the National Ocean and Atmospheric Administration, with the primary purpose of investigating the electric field structure of the auroral generation region by detecting reflected electrons generated by an on-board electron accelerator. From the beginning it was realized that beam plasma interactions as well as the interactions of the charged, beam-emitting payload with the surrounding plasma would play a large role in the experiment, and so it developed into a complex set of measurements to investigate all of these effects. At a certain point in the series, the experiments were therefore renamed ‘SCEX” for Several Compatible Experiments to reflect these goals. Rockets which may be considered to have belonged to the series are as follows: ExDeriment NVB-02 El lB NVB-O6 SCEX I SCEX II SCEX III
Date 5 9 3 27 31 1
Mar. Apr. Dec. Jan. Jan. Feb.
References and Comments 1977 1978 1979 1982 1987 1990
- - -- - - -
guns failed to open /1/ partly successful, some echoes /2/,/3/,/4/,/5/ successful, no echoes /5/,/6/,/7/,/8/,/9/ echoes, extremely successful /1O/,/ll/ partial gun failure precluded echoes /12/ some deployments failed, data analysis in progress
In addition, the rocket program has been supported by research in laboratory plasma chambers /13/ to /29/ and theory /30/,/31/. A more broadly based review of the many experiments has been given by Szuszczewicz /32/ and so this review will concentrate on the El IB-SCEX series. As stated above, the rocket experiment program was conceived as an attempt to reflect electrons from the electric field that is thought to generate the aurora. This electric field is thought to be situated at a distance of about 1 earth radius. The rocket emits pulses of electrons successively at a number of different energies and from the round trip time and the existence or non-existence of echoes one can establish the total potential drop and the location of the reflecting layer. Electrons which are reflected from such a great height will suffer significant drifts, mostly due to electric fields perpendicular to the magnetic field, B, and so will not return to the electron-emitting payload. To detect the returning electrons a number of Throw-Away-Detectors (TADs) are released from the main rocket and go out to cover the surrounding area. In the earlier flights these TAD5 were instrumented with electron detectors by Klaus Wilhelm, Max Planck Institute, Lindau, and in SCEX III the instruments were provided by Roy Torbert, University of New Hampshire. Although the configurations have varied slightly from flight to flight most experiments have had a configuration approximately like that shown in Fig. 1, which represents the nominal configuration of SCEX III, the latest in the series. On reaching a reasonable altitude a forward payload, which mainly carries plasma diagnostics and wave antennas, is ejected, and 4 TAD’s are ejected to the sides, so that the rocket payload divides into six parts. In the (12)15
(12)16
P. J. Kellogg
B (I3~) GEOGRAPHIC VERTICAL
2 rn/sec Forward Paylo tin”,
10
AT 209 ASIMUTH
58*
/ ~TAD 2
~JJ
gmi~rProb.s
w— Alt Payload
TAD
TETHER
~AD
Fig. 1.
A typical configuration for rocket experiments of the
EIIB
and SCEX series.
latest configuration, SCEX II and SCEX III, one of the TADs was fitted with a plasma wave receiver and the other three with particle detectors for echoing electrons. In SCEX I there were two wave receivers and two particle detectors. The two Canadian experiments (NVB-O2 and NVB-O6) did not carry the throw-away-detectors. ECHOES Originally it was thought that successful detection of returning electrons would be very difficult and would depend on a very fortunate alignment of detector position and pitch angle. The laboratory results showed that beam was greatly spread in energy and pitch angle by beam-plasma interactions, which makes detection of returned electrons much more probable. In El lB a few echoes were detected /4/. However, in SCEX I, the experiment was extremely successful and more than 200 echoes were detected by the two TAD5 carrying electron instrumentation /10/, /11/. These experiments, however, raised further problems which we will discuss below, but first we discuss the results from the point of view of success. On each of the two TAD’s of SCEX I there were four analyzers set to the three different gun energies which were used (two detectors were set to 4 key). The gun energies were 2, 4, and 8 key, at currents of nominally 1, 10, and 100 mA. In Fig. 2, taken from Ref. 11, we show an overview of the observations. The lower part of the figure shows the rocket trajectory together with the periods when auroral precipitation was detected. The upper part of the figure shows the delay time of the detected echoes as a function of flight time. It will be seen that there is considerable scatter in the delay times but that in general the delay times, of .2 to .5 sec, correspond to reflection of a height of 3000 to 5000 kilometers. The reflections detected by TAD 2 were confined to the first period of auroral precipitation, and cease when the precipitation ceases. No reflections were detected in the two shorter periods of precipitation which follow the main one. This can always be understood by supposing that the TADs were in the wrong place. In order to determine the expected return position of the electrons we need a way of measuring the electric field drift. Unfortunately the electric field experiment itself was a failure on SCEX I owing to an error which resulted in boom deployment before despin and consequent breaking of the electric field antennas. However, a thermal ion detector provided by Brian Whalen of the National Research Council of Canada measured the velocity of the ionospheric plasma with respect to the payload. A representation of these data is shown in Fig. 3. It will be seen that for a period corresponding to TAD 2 separation of 150-200 m the electric field drift would not have returned reflected electrons to the vicinity of TAD 2. This roughly accounts for the gap in echoes at TAD 2 between separations of 150-175 m. At separation — 300 m the return position moves slightly but not strongly away from TAD 2. This might account for the end of detected echoes.
Election Beams
_________________________________________
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(12)17
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0
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0
~
Churchill Azimuth 139°
5,
0
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300
Fig. 2. Observations of delayed Echoes from SCEX I. In the lower panel is shown the rocket trajectory together with times during which auroral electrons were observed. In the upper panel, on the same time scale, are shown the clear cases of returning electrons. The different symbols show which TAD detected the returns and the energy (2 or 4 key) of the emitted gun pulse. No delayed echoes were detected for 8 keV pulses. NORTH 330’
340’
350’
0’
10’
20’
30’
320’
40’ 400 ‘~WAKE”
IE
+
I
310’ TAD2 300’’
+ +
+
290’ Episodes of delayed 280° responses
+
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50° 60’
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+
100
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WEST 270’
4
80’ 90°EAST
260°
100°
250°
110°
240°
230°220° 200° 180’ 160’ SOUTH
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120°
Fig. 3. The expected return position of reflected electrons as a function of distance from the main payload based on the observation of the drifts of ions of the ambient plasma. .~
12,12-C
(12)18
P. J. Kellogg
However, for 15 sec after release, the TAD experiments also detected what Wilhelm labeled “prompt responses”, i.e., electron echoes with time delays between 0 and 40 msec. These are shown for both TADs in Fig. 4. More recently, R.J. Nemzek and J.R. Winckler /33/ in Electron Echo VII have found a continuous distribution of responses, which must therefore be considered to be prompt responses, with delay times up to 180 msec. At an average delay of 20 msec as observed in SCEX I and at 4 kV an electron could only have returned from a point not more than 400 km away and it is not possible that reflection layers exist in that region, or else there would be no reflections from higher up. There are therefore three possibilities for the cause of the reflected electrons. A) The electrons are reflected by a layer of strong electric field; B) the electrons are reflected by the fields of strong beam plasma interactions; and C) the electrons are not reflected but are precipitated from the trapped radiation by the fields of strong beam plasma interactions. Because of the prompt responses, not all of the echoes can be due to the first process. The entire sequence of reflected electrons is consistent with interpretation (B), that the electrons are reflected by strong beam-plasma interactions, with the gap between prompt responses and delayed echoes being due to unfavorable electric field drift. Nevertheless there are still some conceptual puzzles. If the beam-plasma interactions are strong enough to reflect electrons after 20 msec then it would seem that they are strong enough to thermalize the beam and therefore the beam will no longer be able to generate the instabilities which, farther up the field line, cause the delayed echoes. A possible explanation might be that the fastest particles go ahead in a temporal time-of-flight mechanism and the bump on tail structure of the beam may be reconstituted in this way. Or as Papadopoulos has suggested (private communication) a different beam interaction takes place at the height where the electron-cyclotron frequency becomes higher than the plasma frequency, of the order of thousands of km. But also a very reasonable explanation is that the prompt responses are due to (B) electrons reflected by beam-plasma interactions, and the I
Sequence I
Sequence 2
27.01.5 sctx tEL 2 27 JAN 982
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Delay times for prompt responses detected by the TADs in SCEX I.
delayed echoes are due to (A) reflection by electric field ‘ayers at about 1 earth radius. The scatter in the transit times would then be due to two processes- -first the beam-plasma interactions
which the beam must traverse,
and second irregularities
in the reflecting
layer
itself which is now believers to consist of a large number of small potential drops. Clearly there are a number of features which are not yet understood and which would be clarified by further data, even without additional instrumentation. connection with the observed fields of the next section, we need to consider what fields are necessary to bring a beam electron back. Kellogg et al. /20/ found in plasma chamber work that the observed beam electron velocity spread was consistent with the measured plasma frequency field strength provided that the most extreme velocity changes were for electrons which were exactly resonant, i.e., which caught the perfect wave and rode it to the target.
In
Electmn Beams and their Interactions
(12)19
For the prompt responses and the delayed echoes this cannot be the right concept for the whole reflection process, as we cannot expect the perfect wave to turn around and bring the electron back. Rather the electrons must diffuse in velocity by interacting with many waves. The net change in velocity is then ~v—f~
öv
(1)
where ~v is the total velocity change, n is the number of interactions, and 6v is the velocity change in one interaction. The following order of magnitude estimate assumes that these interactions are with waves near the plasma frequency. If a single interaction with a plasma frequency wave has a duration of l/f~sec and gives
(i—)
6v—~E
(2)
The number of interactions in a typical delayed echo time of TD velocity change of
—
.4 sec gives
a maximum
~v-J~(~E)
(3) 3 MHz, TD
9cm/sec, corresponding to a 4 keV beam and f For ~v a lower 3.8 lOlimit of gives p E—
.4
sec,
.6V/m
this
(4)
For a prompt response in 20 msec, the field must be about five times larger. BEAN PLASMA INTERACTIONS AND WAVES above it seems that the prompt responses are most likely due to reflection of electrons by strong beam-plasma interactions. A considerable effort has been devoted then to measuring directly the waves which are the result of beam-plasma interactions and the cause of the prompt responses. In the discussion
The full wave spectrum from near D.C. to several MHz has been measured on different flights at several different positions around the main payload, at distances up to 1 km. A typical spectrum is shown in Fig. 5. It will be seen that the largest signals, in terms of electric field strength, are present at low frequencies, yet the waves responsible for the reflections must be resonant with the beam electrons and it is usually believed that they are due to the standard two-stream instability at the plasma frequency. Our observations of the field strengths at the plasma frequency, however, show that it is quite weak. We note that in the supporting experiments carried out in Chamber A of Johnson Space Flight Center
~
1~55
“3’’~”I~ ~“T’
SCEX 3 Forward Payload
SCEX 3 Forward Payload
flight time 168.926 sec.
-
gun
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-
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.
.
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flight time 162.268 sec.
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10
,~
Spectra of transverse electric field for a high current (left) and a medium current At this time the forward payload was 130 meters from the gun along B and roughly 10 meters from the beam. gun pulse.
P. J. Kellogg
(12)20
in the late 1970’s, electric fields were detected in the plasma frequency range which were capable of explaining the observed effects on the electron beam, namely fields of 30 V/rn, even though the distance available to these fields in that case was only 20 meters. These electric field observations have been discussed in Kellogg et al. /20/ and the effects of such fields on the electron spectrum have been discussed by Jost et al. /17/. It is known from the work of Jost et al. /18/, also in Chamber A at JSC and from theory /34/ that the plasma frequency electric fields are strongly confined to the region of the beam itself, so it is immediately suggested that the reason we have never seen sufficiently strong electric fields is because none of the payloads have ever passed through the beams. In three flights therefore (SCEX II, SCEX III, and Electron Echo VII), attempts have been made to send one payload parallel to the magnetic field and through the electron beam. These attempts have so far not been successful, as determined by trying to detect the beam electrons with particle detectors. A principal problem has been that too much time elapses between payload ejection and gun turn-on, necessitated in order that the guns outgas sufficiently. Combined with the errors in ejection direction this means that the payload does not quite pass through the electron beam. Recently a number of plasma simulations on supercomputers have been made which are becoming more and more realistic. It is reasonable to turn to such simulations to investigate the behavior of the electric field outside the beam to get some idea of how close the payloads have come and what electric field might be expected inside the beam. In Fig. 6 are shown typical electric field spectra from a recent simulation /35/. Similar and more detailed results have been obtained in calculations especially aimed at simulating the experiment Echo 7 /36/ /37/. Note that the low frequency electric field dominates, even within the beam. In most flights the beam generates waves near the plasma and upper hybrid frequencies. Low current beams generate these most clearly, as is seen in Fig. 7 for the 1 mA pulse at 139.4 sec. At higher currents the spectrum is nearly featureless and fills the whole observed frequency range, even in the stop band for electromagnetic radiation above the electron cyclotron frequency, as is seen in Fig. 5. In most flights we have also seen harmonics of the plasma frequency. At first it was considered possible that these harmonics were generated in the electronics, but all recent experiments have shown clearly that the harmonics are part of the signal on the antenna. This has been done by periodically attenuating the signal and noting that the harmonics are present when the signal is well within the linear range of the electronics. The harmonics might, of course, be generated in the sheath around the antenna. However, we now present evidence that these harmonics are generated in the beam, in that the number of harmonics and their amplitude as a function of harmonic number appears more compatible with what is expected from bunching of the beam than with the kind of distortion to be expected in a sheath. FIeld History
Power Spectrum
Power Spectrum
Field History
fL~1:ebW!3 ::~J I F ~ t/L~Lm1~:4L/~1~øL\~~2 fr~’~J~1 .2L~1~10EF~’°1~ ocr’
ForwUrd of thearnc.rnatf
~
Acrosa the MaQneuic FIeld
~
0
40
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120
“UI
0)“h”nn
0
40
80
120
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Fig. 6. Some simulation results from Winglee and Pritchett. The lefthand panel shows the transverse electric field at 3 points up the beam from the rocket. A is the cell length in the simulation and should be taken to be between 10 cms and 1 meter for comparison with the experiments. The righthand panel shows the variation of electric field across the beam at a distance of x — A. Electric fields are scaled to a beam energy per 2 A. Note the preponderance of the low frequency electric fields even inside the beam.
Electron Beams and their Interactions
(12)21
Although there was a failure of the gun system in SCEX II in that an arc developed across the batteries for the gun high voltage power supply, nevertheless for a short period the guns functioned in a way which resulted in some useful data. It appears that the arc allowed a voltage drop of about ten volts across the battery terminals and that this voltage was sufficient to drive the gun power supply to generate a basin of a few hundred volts and a few or a few tens of milliamperes. In Fig. 7 and 8 we show observations from that flight which contain a large number of harmonics of the plasma frequency. At the time of the measurements, the forward payload was 150 m up the field and about 12 m away from the beam. The figure shows nine harmonics of the plasma frequency. The amplitude as a function of harmonic number for the gun pulse at 141.913 sec is plotted in Fig. 8. These waves were from a gun pulse of 1.4 mA at 680 eV energy. The power falls as harmonic number to the fourth power.
•1
. ~
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140
142.
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FLIGHT TIME (eec)
Fig. 7. A period of wave observations for the short time when the SCEX II gun operated. The spectrum at 141.913 shows nine harmonics of the plasma frequency and is interpreted to indicate full bunching of the beam. Several pulses including the spectra at 139.224 and 139.787 show a clear upturn at the highest frequencies.
Scex U January 31, 1987 OsmPalec at 141.913 eec, 680 volts, 1.4 mA
_______
____
____ __ ___
-
—
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~~ii~iii
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Fig. 8. The amplitudes of the harmonics of the plasma frequency for the gun pulse at 141.913 sec of Fig. 7.
(12)22
P. J. Kellogg
The standard explanation of such harmonic generation was given by O’Neill et al. /38/. The harmonic generation is supposed to arise from complete bunching of the beam which then becomes a series of thin disks propagating along the field. The Fourier analysis of the field of these thin disks is clearly rich in harmonics. O’Neill et al. calculated the spectrum of the electric field of such thin disks for an infinitely wide beam and found that the amplitude of the harmonics fell like harmonic number squared. We would not expect more than qualitative agreement with their results because the observations are made outside the beam and because we are probably observing electromagnetic radiation resulting from the conversion of electrostatic waves. Plasma waves which are resonant with the beam would have a wave length of a few meters and at frequencies that are significantly above the plasma frequency such waves are attenuated also in a few meters. We can use this observation to estimate a lower limit on the field strengths which would be required of plasma frequency waves. At the time of these observations the forward payload was d 150 m above the main payload.
w
The wavelength for resonant waves is given by — kvb, where vb is the beam velocity. To bunch the beam, electrons must be moved by one Ralf wavelength in the travel time to the forward payload, d/vb, hence 5
_A_~2_iat2
1
E
d
This estimate of E is 3 E—
~.5V/m
(6)
in rough agreement with the earlier estimate of what is required. Thus, the indirect evidence indicates that fields of the required magnitude exist inside the beam. The largest plasma frequency fields which we have measured directly in earlier flights (SCEX III data are not yet fully analyzed) are of the order of a few mV/rn, hence 100 times smaller.
ROCKET ENVIRONMENT DURING BEAM FIRING The design of experiments for investigating the environment of the rocket during beam firing depends on a certain picture of this environment. In Fig. 9 we show what we think the environment must look like. Our picture is similar that of Managadze et al. /39/. When the rocket emits an electron beam it must become positive at a very fast rate. If the rocket were in vacuum we can estimate its capacitance to be about 200 pF and so for a 100 mAmpere beam the rocket potential would go positive at a rate of .5 kV/msec. In plasma the dielectric constant reduces this potential considerably since the dielegtric constant at frequencies below the ion cyclotron frequency may be of the order of 10 . On a time scale in which the ions cannot move the dielectric constant is smaller by the electron-ion mass ratio, with further complications in the electron cyclotron and plasma frequency regions. We expect that the response of the surrounding plasma will be complex and time-dependent on scales from the ion-cyclotron frequency to the plasma frequency. We therefore expect a picture like the following. As the rocket becomes positive, the first response is for the electrons to move toward the payload, a response which is carred outward by a Langmuir wave, or by a resonance-cone (longitudinal) whistler. This response should then travel outward at a speed of the order of the electron thermal speed, which appears to be instantaneous on our time resolution (about 1 rnsec). If the rocket continues to go positive, it repels away all of the positive ions of the ionosphere and will therefore be surrounded by a region which contains only electrons. There will presumably be an ion-enhanced region but these ions ought quickly to be neutralized by electrons pulled along field lines outside the ion void. For field lines that intersect the payload the electrons will be pulled into the payload, but together with secondaries made by the action of the beam on the neutral atmosphere. For field lines which pass close to the payload, electrons will be pulled toward the maximum positive potential and will therefore have gained considerable energy. In the ion void then electrons will be oscillating back and forth along B, passing through the maximum of the positive potential in the equatorial plane. At the equatorial plane they will have considerable energy, some of which will be perpendicular energy so that electrons which pass close to the payload may have sufficiently large Larmor radii to intersect the payload. The zeroth order effective area is then larger than the physical area /40/. The rocket is moving through the plasma at a speed which is generally of the order of .3 to 1 kin/sec. Therefore at large distances from the rocket the plasma will appear to be streaming in response to an electric field. The electrons of the plasma will appear to
Electron Beams and their Interactions
(12)23
Electrons Depleted
Enhanced Ions
:\:IOfl~
:\~:vo1d)
Low Frequency Instabilities
Fig. 9.
A conceptual picture of the environment of a beam-emitting rocket.
2. The velocity streamlines in such a drift by with velocityin given x B)/B situation are a sketched Fig. by 10 V for— (E ambient electrons. Electrons which are going to miss the payload by a large distance will go around it. Within a certain separatrix all electrons are trapped and will drift around the payload. From this picture it appears that no electrons can be collected by the main payload from its equatorial plane. All electron collection is then along field lines, which either strike the payload or become part of the electron cloud. There may be some evaporation from this cloud which contributes hot electrons which travel back up the field lines. Otherwise electrons will remain in the cloud until they diffuse into the payload. If there is no evaporation, then the size of the cloud is determined by Linson’s limit /41/ 2 (7)
where r and r current~ and rR.
are the cloud and effective rocket radii, and I and I are the beam electron saturation current which would be col~ected~y a rocket of size
In early work by Winckler /42/ it was found that the rocket neutralization was maintained reasonably well at rather low potentials. This coupled with our own experience with floating potential probe booms, which we still do not understand, lead us to believe that the rocket would be charged to at most 1MO~2OOvolts and this belief dictated the design of the early experiments. However, this wa~in spite of Managadze et al.s’ /39/ results showing that the rocket could charge to tne beam energy and to some extent we attributed Managadze’s results to the very clean payload end very high altitude which he reached. In 1985 however, the MAIMIC rocket experiment /43/ showed clearly that in a tenuous plasma the payload could charge to the full beam energy and beyond. We had ourselves demonstrated this in plasma chamber work /21/ and in retrospect it was an error not to consider large payload charging more seriously. The electron cloud in the ion void consists of fairly energetic electrons since these have fallen from large distances into the potential of the rocket, that is to say electrons having an energy of the order of tens to hundreds of eV. These electrons then are capable of ionizing the neutral background. The resulting ions will be hurled outward by the electric field of the rocket and a measure of their energy spectrum at large distance would allow a measure of the rocket potential.
(12)24
P. J. Kellogg
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40
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Fig. 10. Trajectories of low energy electrons under E x B forces in the frame of the equatorial plane of a rocket. The rocket has been assumed ~o be charged to 400 volts and the field is assumed to fall off around the rocket like l/r
A simple experiment was designed to be mounted on the TAD5 to measure the spectrum of these ions. The TAD5 have uncontrolled orientation and it was desired to have as large a solid angle for the detectors as possible even at some sacrifice of energy resolution. However, the energy resolution sacrificed in a planar detector of even one sterradian was considered to be too great and so rather simple hemispherical retarding potential analyzers were used. These were four grid analyzers with the outermost grid grounded to the rocket body. The next grid was an electron repeller. The third grid was an ion energy analyzer and the fourth grid suppressed secondary emission and shielded the collector from capacitive coupling to the swept grid. In SCEX II the RPA’s were designed to measure ion energies up to 200 eV. In SCEX III this was increased to 400 eV. Some results were shown in Fig. 11 taken from Kellogg and Monson /12/. In these the effects of capacitive coupling between the ion analyzer grid and the collector have been removed by subtracting a background of data taken just before launch. However, there is still a large apparent ion current at the beginning of each sweep. This is attributed to energetic auroral electrons which can penetrate the electron repeller grid but which are attracted to and captured by the ion analyzer grid when it is very positive. The fact that they then do not arrive at the collector appears as a positive, i.e. ion resembling, current. The true ion currents are clearly seen in coincidence with the gun pulses. In this flight, SCEX II, an arc developed across the gun battery. It is likely that outgassing products from this arc maintained a higher than neutral density in the vicinity of the rocket and that the amount of neutralization current due to ionization is therefore larger than it would normally be. From these ion currents we /12/ determined that (1) a significant fraction of the payload neutralization current was due to this ionization process and (2) that no maximum ion energy was observed so that the accelerator payload charged to more than 200 volts. It appears that this whole electron cloud region is quite unstable to low frequency electrostatic plasma waves which then replace collisions, as suggested long ago by Linson /41/, and allow electrons to diffuse into the main payload and neutralize it. They are probably also responsible for heating the plasma cloud. The wave experiments give information on the low frequency instabilities and on the expanding first response surface. The best evidence on the first response surfaces has been obtained from our participation in the experiment Echo VII of J.R. Winckler /44/. The measurements were made by a forward payload which was ejected at an angle of about 5 degrees to the beam at a speed of 4.2 rn/sec. Since the experiment lasted about 400 sec the forward payload traveled about 1.6 km up the beam. The antennas of the forwar -vioad consisted of two stainless bands wrapped
Electmn Scams and their Intemctions
1A04(NO!TH) .
CPA
(12)25
i
..
~
20
TAD2(WEST)
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~
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Fig. 11. Energetic ions which have been a charged rocket and expelled outward by occur periodically are a spurious effect seen in conjunction with the gun pattern
-
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~
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42 FLIGHT TINE (sect
143
created by ionization processes in the vicinity of an electric field. The sharp decaying pulses which due to auroral electrons. The ion currents can be which is shown below.
around a fiberglass section of the nosecoise and separated by .8 meters. Since the nosecone was spin stabilized with its axis roughly along the magnetic field, this antenna measures the component of electric field parallel to the magnetic field. In Fig. 12 we show the first response of this antenna to gun pulses for some early times and again for some later times. The sign of the first part of the pulse changed with distance, the change occurring at a distance of approximately 800 meters. The change in sign of the first pulse could be due to the fact that this first pulse is carried out by a dispersive wave, or it could be due to a change in the relative importance of the payload charging and the charge density of the beam itself. Clearly, however, the region of low frequency disturbance extends at least 800 in along B.
cuc’moe
ELECTRON WOO 0 NOOL COME a lC~ PIT’S 01161.2 30 AT 82 082 SEC 402 METERS It,e 6)38
ECHO 7 NOSE COFIE AHGL2 110 It?? METERS IRON CI’S
31 cnn PITCII A? 383.387 SEC
~ti1
I______
11110 8582 El.1C1509 ECHO 7 NOSE CORE 30 KnY PITCh ASCIi III A? 83,882 SEC 406 PYlON
METORS
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~ Kay
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See NETENS
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__
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Fig. 12. The first response of the electric field measured in the nose experiment on Echo VII which traveled up the beam at an angle of about S degrees to the magnetic field. The fields measured are roughly parallel to the magnetic field and a positive electric field is downward toward the electron accelerator.
(12)26
P. J. Kellogg
We also have measured, but less accurately, the variation of the low frequency waves with distance perpendicular to B from the payload. The best evidence which we have analyzed comes from SCEX I. The spectrum is obtained from data which were returned as samples of a broadband logarithmized signal. On the ground the data were exponentiated to reconstitute the original waveform and then Fourier analyzed. The length of the gun pulses, 50 msec, sets an effective lower limit to the frequency resolution of 20 Hz, so the ion cyclotron frequency is not resolved. In Fig. 13 we show the behavior of the total power of these low frequency waves in the band from 30 Hz to 1 kllz as TAD 1 moves away from the main payload. When the TAD is close to the main payload there is a fair amount of saturation and the points should be regarded only as lower limits. It will be seen that there is no sharp outer boundary to the low frequency waves but that their amplitude decreases in a fairly regular way and extends to about 100 meters. We believe that these low frequency waves are responsible for the diffusion of the electrons of the electron cloud to the main payload as suggested long ago by Linson /41/. The waves which do the diffusing seem not to be the lower hybrid waves. In the waveforms shown in Fig. 12 only one of the waveforms shows strong lower hybrid waves, the one at 1280 meters, and in this case the lower hybrid waves were already present before the gun was turned on. Further, of the two spectra shown in Fig. 5, the low current spectrum shows a strong lower hybrid peak (7 kHz), but the high current spectrum shows the lower hybrid peak submerged in the low frequencies. The waves are irregular but the major part of their spectrum falls well below the lower hybrid range into the ion-cyclotron frequency range. A number of workers /45/ /46/ have observed that the plasma around a beam-emitting rocket is heated out to distances of the order of 100 meters. It is likely also that these low frequency waves are responsible for part of this heating. As suggested above, some of the heating may also be due to evaporation of electrons from the cloud. GROUND OBSERVATIONS Following the successful detection of HF radiation from the electron beam in Electron Echo IV /47/, we set up ground stations for the observation of radiation from the natural aurora and from the artificial electron beam in each of the flights in which we have participated. In this way we have detected from HF radiation from the natural aurora /48, 49/. A total of about 70 such events have been measured. Some of the radiation detected in Echo IV was exactly (to .1%) at the second harmonic of the electron cyclotron frequency and this same identification implies height limits on the generation region for emissions from the natural aurora. In another flight in the Electron Echo series /7/, radiation was detected on the ground which must have been at the plasma frequency. This is shown in Fig. 14 which shows the signal on a receiver which is sweeping from 2.9 MHz to 3.4 MHz, together with a representation below of the gun program. This radiation was identified as being at the plasma frequency by comparison with the local electron density as measured by the Chatanika radar. However, we have never succeeded in measuring radiation from the beam during any of the SCEX flights. This is perhaps because we are always seeking to send the SCEX experiments over a stable auroral arc and in the vicinity of such an arc the ionospheric critical frequency is higher than the 3 MHz second harmonic of the electron cylotron frequency. While it might .1
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S 1 I I. I I , 80 100 120 140 160 180 200 DISTANCE FROM ACCELERATOR, H. Fig. 13. The power in low frequency waves (30 Hz- 1 kHz) as a payload travels magnetic field. .0001
,
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across the
Electron Beams and their Interactions
ELECTRON ECHO z
(12)27
NOVEMBER
I
Ii, 1979
~ 240
250
260
270
FLIGHT TIME I5ECI
Fig. 14. Ground observations of waves at the plasma frequency from Echo V. is represented below.
The gun program
appear desirable to try to measure the third harmonic at about 4.5 MHz, this frequency falls into a busy communications band and the interference makes measurements more difficult. Still measurements have been made several times at that frequency and have never been successful. ACKNOWLEDGEMENTS First this whole program was begun by William Bernstein, recently retired from Rice University, and we are grateful to him for the initiative and guidance which he has provided. The program has involved the contributions of a large number of experimenters, who have designed, built and tested their instruments for the various measurements of the program. These include Brian Whalen, Malcolm Duncan and Andrew Yau of the National Research Council of Canada, Klaus Wilhelm and Chris Becker of the Max Planck Institute, Lindau, Roy Torbert and Craig Kletzing of the University of New Hampshire, and Ed Szuszczewicz and Hugh Anderson of SAIC. The author is grateful to Klaus Wilhelm, to Hugh Anderson, and to Tom Hallinan for the guidance and support they have given him during the tense moments of a launch. University of Minnesota work on NVB-02 and NVB-06 was supported by NOAA grants 0378-B0l-6O and NA-79RACOO51, SCEX I through III were supported by NASA through grant NSG 5373. REFERENCES
1. 2.
3. 4. 5. 6. 7. 8. 9.
10. 11. 12. 13.
14. 15. 16. 17.
P.J. Kellogg, S.J. Monson and BA. Whalen, Geophys. Res. Lett. 5, 47-50 (1978). W. Bernstein, P.J. Kellogg, S.J. Monson, R.H. Holzworth and B.A. Whalen, Recent observations of beam plasma interactions in the ionosphere and a comparison with laboratory studies of the beam plasma discharge, in Artificial Particle Beams in Space Plasma Studies, NATO Advanced Study Institute Series B, Vol. 79, Plenum Press, New York, 1982. R.H. Holzworth and H.C. Koons, J. Geoohys. Rca. 86, 853-857 (1981). K. Wilhelm, W. Bernstein and B.A. Whalen, Geophys. Res. Lett. 7, 117 (1980). P.J. Kellogg, S.J. Monson, W. Bernstein and B.A. Whalen, J. Geophys. Res. 91, 1206512077 (1986) G.R.J. Duprat, B.A. Whalen, A.C. McNamara, W. Bernstein, J. Ceotihys. Res. 88, 3095-3108 (1983). P.J. Kellogg and S.J. Monson, Adv. Space Res. 1, 61-68 (1981). R.T. Goerke, P.J. Kellogg and S.J. Monson, J. Geophys. Res. 95, 4277-4283 (1990). P.J. Kellogg and S.J. Monson, A second kind of BPD - rocket and laboratory results, in Active Experiments In Space. Proceedings of an International Symposium held in Alpbach, Austria 1983, ESA SP-195, p. 157-159. K. Wilhelm, W. Bernstein, P.J. Kellogg and B.A. Whalen, Geophys. Res. Lett. 11, 11761179 (1984). K. Wilhelm, W. Bernstein, P.J. Kellogg and B.A. Whalen, J. Geophys. Rca. 90, 491-504 (1985). P.J. l~ellogg and S.J. Monson, J. Geomag. Geoelectr. 40, 1257-1267 (1988). W. Bernstein, H. Leinbach, P.J. Kellogg, S.J. Monson, T.J. Hallinan, O.K. Garriott, A. Konradi, J. McCoy, P. Daly, B. Baker and H.R. Anderson, Geophys. Rca. Lett. 5, 127-130 (1978). W. Bernstein, H. Leinbach, P.J. Kellogg, S.J. Monson and T.J. Hallinan, J. Geochys. ~ 84, 7271-7278 (1979). W. Bernstein and P.J. Kellogg, Adv. Space Rca. 1, 347-360 (1981). R.W. Boswell and P.J. Kellogg, Geophys. Res. Lett. 10, 565-568 (1983). R.J. Jost, H.R. Anderson and J.O. McGarrity, Geophys. Res. Lett. 7, 509 (1980).
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P. J. Kellogg
18.
R.J. Jost, HR. Anderson, W. Bernstein and P.J. Kellogg, Radial dependence of HF field strength in the BPD column, in Artificial Particle Beams in Soace Plasma Studies, Plenum Press, New York, 1982.
19.
T.J.
Hallinan,
H. Leinbach,
G. Mantjoukis
and W. Bernstein,
J.
Geoohvs.
Res.
89, 23335-
2347 (1984). 20.
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P.J. Kellogg, S.J. Monson, R.H. Holzworth and R.J. Jost, IEEE Trans. on Plasma Science, Vol. PS-l4, 891-901 (1986). P.J. Kellogg, H.R. Anderson, W. Bernstein, T.J. Hallinan, R.W. Holzworth, R.J. Jost, H. Leinbach and E.P. Szuszczewicz, Laboratory simulation of injection of particle beams in the ionosphere, in Artificial Particle Beams in Space Plasma Studies, NATA Advanced Study Inst. Series B, Vol. 79, Plenum Press, New York, 1982. X. Llobet, W. Bernstein and A. Konradi, J. Geophys. Res, 90, 5187-5196 (1985). T.J. Hallinan, H. Leinbach and W. Bernstein, Visible signatures of the multipatep transition to a beam-plasma-discharge, in Artificial Particle Beams in Space Plasma Studies, NATA Advanced Study Inst. Series B, Vol. 79, Plenum Press, New York 1982. P.J. Kellogg and R.W. Roswell, Phys. Fluids 29, 1669-1674 (1986). W.E. Sharp, Geophys. Rca. Lett. 9, 869-872 (1982). E.P. Szuszczewicz, and C.S. Lin, Time-dependent plasma behavior triggered by a pulsed electron gun under conditions of beam-plasma-discharge, in Artificial Particle Beams in Space Plasma Studies, NATA Advanced Study Inst. Series B, Vol. 79, Plenum Press, New York, 1982. E.P.
Szuszczewicz,
K. Papadopoulos,
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C.S.
Lin and D.N. Walker,
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~ 1565 (1982). D.N. Walker, E.P. Szuszczewicz and C.S. Lin, Ignition of the beam plasma discharge and its dependence on electron density, in Artificial Particle Beams in Space Plasma Studies, NATA Advanced Study Inst. Series B, Vol. 79, Plenum Press, New York, 1982. D.N. Walker and E.P. Szuszczewicz, J. Geophys. Res. 90, 1691-1697 (1985). K. Papadopoulos, Theory of beam plasma discharge, in Artificial Particle Beams in Space Plasma Studies, NATA Advanced Study Inst. Series B, Vol. 79, Plenum Press, New York, 1982. H.L. Rowland, C.L. Chang and K. Papadopoulos, J. Geophys. Res, 86, 9215-9218 (1981). E.P. Szuszczewicz, J. Atmos. Terr. Phys. 47, 1189 (1985). R.J. Nemzek and J.R. Winckler, ‘Prompt’ electron pulses generated by the Echo 7 electron beam, ~ Trans. Am. Geophys. Un. 70, 439 (1989. A.W. Trivelpiece and R.W. Could, J. Aool. Phys. 30, 1784-1793 (1959). R.M. Winglee and P.L. Pritchett, J, Ceophys. Res, 93, 5823-5828 (1988). R.M. Winglee and P.J. Kellogg, J. Geophys. Res. 95, 6167-6190 (1990). R.M. Winglee, J. Ceophys. Res, 95, 6167-6190 (1990). T.M. O’Neil, J.H. Winfrey and J.H. Malmberg, Phys. Fl. 14, 1204-1212 (1971). CC. Managadze, V.M. Balebanov, A.A. Burchudladze, T.I. Cagua, N.A. Leonov, S.B. Lyakhov, A.A. Martinson, A.D. Mayorov, W.K. Riedler, M.F. Friedrich, K.M. Torkar, A.N. Laliashvili, Z. Klos and Z. Zbysznski, Planet. Space Sd. 36, 399-410 (1988). L.W. Parker and B.L. Murphy, J. Ceophys. Res. 72, 1631-1636 (1967).
41.
L.M. Linson,
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