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Very low frequency waves stimulated by an electron accelerator in the auroral ionosphere
~‘i. Space Res. Vol. I, PP. 117—122. OCOSPAR, 1981. Printed in Great Britain.
O2731l77/81/020~0hI7$O5.OO/O
VERY LOW FREQUENCY WAVES STIMULATED BY AN ELECTRON ACCELERATOR IN THE AURORAL IONOSPHERE J. A. Holtet,* B. Grandal,** T. A. Jacobsen,** B. N. Maehlum,** J. Tr$im,~B. K. Pran* and A. Egeland* *fr~tituteof Physics, University of Oslo, Norway **Norwegian Defence Research Establishment, Kjeller, Norway ABSTRACT The sounding rocket POLAR 5 carried a 10 keV electron accelerator and various diagnostic instruments in a mother—daughter configuration. Onboard wave receivers recorded several types of VLF wave phenomena directly associated with the operation of the accelerator, with delays from 5 to 50 ins after the injection of the electrons. These delayed after—effects range from broadband noise, f > 3 kHz, observed above 170 kin, through narrow band emissions at 2 and 5.6 kHz which appeared when the rocket crossed a region with precipitation of energetic electrons, to emissions covering frequencies from 3—4 to well above 100 kHz observed within the E—region (150—95 km). The latter was also associated with apparent changes in electron density. The observed emission properties indicate that the region perturbed by the beam and the neutralizing return current to the daughter may be a favoured generation region. INTRODUCTION The sounding rocket POLAR 5 was launched from Andøya Rocket Range, Northern Norway, February 1, 1976, at 1929 UT into a slightly disturbed ionosphere with weak auroral activity. The rocket trajectory, being close to a magnetic meridian plane, crossed two auroral regions, one, the main one, between 86 and 111 s flight time, and a secondary region between 230 and 330 s. The rocket carried an electron accelerator and various diagnostic instruments in a mother—daughter payload configuration. The daughter, with the accelerator, was separated axially from the mother in a forward direction at an altitude of 90 km. A slight offset of the velocity vector from the mother’s plane of trajectory introduced an east—west separation between the two payloads. The accelerator emitted 10 key electrons at a flux of 8 x 1017 s—i, corresponding to a current of —130 mA. The accelerator was operated in a pulsed mode at a repetition rate of -‘2.5 Hz. Within each main pulse the beam was modulated at 250 Hz. A main pulse would thus consist of five 2 ins sub—pulses separated by 2 ins spacing, and last for 18 ms. The accelerator was turned on at 105 s (148 km altitude). An off period of 15.5 s from 237.4 s was included. The VLF experiment was carried by the mother payload. The receiving antenna was an electric dipole, 9.3 m tip—to—tip, oriented 900 to the rocket spin axis. The experiment would thus record both electromagnetic and electrostatic waves. The instrumentation did not provide additional information which made it possible to distinguish between the two wave modes. The receiver covered the frequency band up to 100 kHz
117
J.A. Holtet eL~ a~.
118
in broadband mode. The payload also included a HF wave experiment, covering frequencies up to 5 MHz. Stimulated waves observed at these high frequencies were in general not associated with the VLF waves [i) and will not be dealt with here. The oriboard particle detectors recorded increased electron fluxes in the two auro— ral regions. Of special interest is a double peaked structure in the fluxes of 4—5 and 12—27 keV electrons observed within the northern auroral form [2]. The number 3, with maxidensity of thermal varied during the flight lO~aurora cm mum density within plasma the main auroral region. To the from north106of tothis a slow, steady decrease in the density was observed, with no enhancement in the region of the second aurora. The background DC electric field stayed at a very low value (<20 mV/rn) throughout the flight. The background emission activity in the VLF range was dominated by a strong band of auroral hiss, above -~7kHz. This noise—band hampered detailed studies of beam related wave effects from this frequency up to ~5O kHz. For more detailed information about instrumentation, ionospheric conditions and background observations see Maehlum et al (2) VLF WAVES ASSOCIATED WITH ELECTRON INJECTION The injection of the artificial electrons into the ionospheric plasma stimulated several different wave effects at various frequencies in the VLF range. Such waves were observed during the entire operation of the accelerator, but the character of the emissions varied through the flight. ______
20
_________________
-~
x
_________
_____________________________
Figure 1: Spectrogram showing the direct radiation as vertical lines, and lower hybrid and ELF radiation at 5—6 kHz and 1—3 kHz following the direct radiation (excv~ples encircled). An amplifier gain shift appeared at pulse No. 404.
~ 10
PULSE NO. 40 FLT. TIME (i) 205.054 ALTITUDE (kin) 219
4~5
215.4*1 201
Direct radiation. Most easily detected were the strong, spike—like transients directly associated with the beam injection (Fig. 1). These signals, which covered a wide frequency range (>2 MHz) were observed through most of the flight. Attenuation of the wave appeared, however, at low altitudes (below 140 krn) and the waves disappeared around 105 km. The signature of the five sub—pulses can be recognized in the emission (Fig. 2). The time delay from beam injection to the arrival of the wave— train at the mother was very short, less than 250 us. We will therefore refer to this as direct radiation. The short delay indicates that the waves were electromagnetic. This emission may resemble reported observations of beam—plasma discharge [3), and could thus be taken as evidence of such processes. In the spectrogram representation (Fig. I) the direct radiation appears as vertical lines, which resemble the sub— ionospheric VLF phenomenon known as “atmospherics’. (An atmospheric is a wave—train which is radiated from an electric discharge in the atmosphere.) The spectral appea—
VLF Waves Stimulated by Electron Accelerator
19
rance may therefore evoke the more simple suggestion that the direct radiation may have its origin in an electric discharge in the accelerator when this was activated. Delayed radiation. The direct radiation faded away shortly after the last electron sub—pulse and the signal returned to the background level. However, during most of the flight waves were also observed with considerably longer delays. We will in the following refer to this as (wave) after—effects. From the spectrograms (Figs. 1 and 3) the various after—effects can be divided in the following four groups: a) Short delay after—effects, b) Lower hybrid radiation, c) ELF—radiation, and d) Broadband long delay radiation. Short delay after—effects, appear in the spectrograms as a broadening of the trace of the direct radiation, extending from 3—4 kHz up into the hiss band. In an amplitude—time plot this after—effect is seen to grow up between the “fingers” of the direct radiation and decay slowly after the last sub—pulse (Fig. 2). The delay from the electron injection to the arrival of the wave was usually in the range 4—8 ms, with maximum amplitude around 18—22 ms, i.e. at, or shortly after the last sub—pulse. The radiation appeared at the very first gun pulses and lasted till approximately 340 s (altitude’-~l7Okin), when it disappeared after a gradual decay. It was not possible to relate spesific amplitude variations to the beam injection angle or the orientation of the receiving antenna.
A
PULSE
342
TIME
B
259906,
.cu.s
-
0
PULSE
349
TIME
262363,
..u..
20
40 TIME
(ms)
0
20
40
TIME
(ms)
Figure 2: Amplitude variations observed in the frequency band 5. 2 — 6. 0 kllz associated with injection of electrons. Panel A shows direct radiation and short delay radiation and panel B direct radiation and, separated from this at the peak between 30—40 ma, lower hybrid radiation. The five dots above the curves indicate the time of injection of the five beam sub—pulses. Lower hybrid radiation. Below the natural hiss, long delayed emissions were found in a relatively narrow band centered around 5.6 kHz. This is close to the local lower hybrid frequency, ~LH’ which for a mean ion mass of 20 is 7.3 kHz. In the spectrograms this radiation appears as “blobs” after the direct radiation (Fig. 1), and in amplitude—time plots at a fixed frequency it can be seen as peaks growing up clearly separated from the direct radiation (Fig. 2b). The start of after—effects was usually observed at a delay of 22—26 ms from the beam injection, and maximum amplitude was reached in the delay range 30—40 ms. This radiation was observed sporadically between 150 and 200 s. The main emission region appeared, however, from the gun off period till 320 s, i.e. in the second auroral region. The emission thus showed a spatial correlation with the precipitated auroral electrons. The stimulated amplitude showed some dependence on the angle of injection of the electron beam with a tendency of having maximum amplitudes for upward injections. This was, however, not consistent for the whole data set, and large amplitudes were also found to be generated for downward injections.
J.A. Holtet et a7.
20
ELF radiation. Narrow band stimulated emissions were also observed between 1 and 3 kllz. The emission trace on the spectrograms showed a tendency of dispersion with the low frequency waves preceding those at high frequencies (Fig. 1). These aftereffects were only observed inside the second auroral region. No clear dependence on the direction of the injected beam could be found.
30
25
I
‘~J~
~
‘I’
V
1
‘~
‘~
Figure 3: long Spectrogram showing broadband delay after—effects
•
-
as broad vertical lines (examples indicated by arrows). The dark area around 15 kHz is spin modula—
-
20
Altitud. (km)
“
.
ted cruroral hiss
373.774 129.04
377.370
Broadband long delay radiation. In the last part of the flight, (i.e. after~355 s, below 150 kin) stimulated after—effects which extended over a frequency range from 2—3 kHz to well above 100 kHz were observed. On the spectrogram (Fig. 3) the emissions appear as broad diffuse traces, well separated from the direct radiation. The delay from beam injection to detection of the wave was around 30 ins, and the stimulated wave would typically last for 50—60 ins. The observed amplitude showed a periodic variation. The majority of the high amplitudes appeared for injections perpendicular to the geomagnetic field. This picture was, however, somewhat disturbed by a second modulation introducted by the rotation of the receiving antenna. Maximum field strength seemed to appear at a direction perpendicular to the magnetic field. It should be noted that this after—effect appeared in a period when the amplitude
A
10
fV~ •
TIME 1.) ALTITUDE (km)
360 146.14
40
$S~~
370 133.97
380 120.88
Figure 4: Cold plasma density measured by the NDRE capasitance probe in the Eregion. The spikes on the curve are associated with the injection of beam electrons. The structure of these density after—effects is shown in more detail in the enlarged example.
VLF Waves Stimulated by Electron Accelerator
121
of the direct radiation was decreasing. During the last part of the observations, i.e. from 390 to 398 s (from 105 to 95 km altitude) when this after—effect was still observed, the direct radiation was even vanishing. In the same period as these long—delayed wave after—effects were present, the electron density probe detected large gun—related changes in the ambient thermal plasma. These after—effects showed up as apparent increases in the electron density lasting 60—80 ins after the start of the electron injection (Fig. 4). Maximum effect was detected for injections perpendicular to the geomagnetic field, and also in other respects a reasonably good connection was found between wave and density aftereffects. It seems thus as if these two data sets reflect two different features of one and the same phenomenon. The physical relation behind this is, however, not clear. DISCUSSION With exception of the E—region after—effects the other types of stimulated waves showed no clear relation between stimulated wave amplitude and beam injection angle. This, together with no systematic change in delays for upward and downward injection makes a generation well above the rocket unlikely. Processes including e.g. electrostatic double layers above the rocket or backscatter from the atmosphere below could give the same geometry for upward and downward directed beams, but this would result in differences in time delay. The observed emissions are therefore believed to be generated in the vicinity of the daughter. Furthermore, the combination of the short distance between the generation region and the receiver and the relatively long delay requires a slow wave, making it natural to suggest that the waves were electrostatic. During the injection of the electrons and shortly thereafter the plasma around the daughter would be perturbed by the beam itself and the return current flowing to the daughter in order to neutralize the body. This perturbation and heating of the ambient plasma will give rise to a wide range of wave emissions. The spatial correlation with auroral electrons may be due to an energy transfer from the particle resulting in an amplification of certain waves. In a laboratory simultation of a rocket experiment Tsutsui et al (4) found that waves could be generated near the lower hybrid frequency due to an interaction between a low energy electron beam (the return current) and the plasma. In the same experiment waves were generated around 2 kHz by a beam—plasma interaction. The phase velocity of both were of the order of the ion—acoustic velocity. Assuming T — 2000 K, t.-’ 1000 K and a mean ion mass of 20~ the group velocity of ion_acousticewaves is f~und to be of the order of 1.4 x l0~mis at 6 kHz [5). For propagation over a distance of 80 m, this corresponds to a time delay of “~50ms which is much greater than what was observed. However, if we assume that the generation is streched out over some distance along a gyro—column, and that the waves are propagating perpendicularly to the magnetic field, the distance would typically be 30 in and the corresponding de— lay “2l ins, which is reasonably close to the observed value. This would also be in agreement with laboratory observations by Gekelnian and Stenzel (6) . It is therefore suggested that the narrow—band after—effects at 2 and 5.6 kHz are ion—acoustic waves generated in the return current region. When it comes to the E—region wave— and plasma after—effects a local source region seems also here to be most likely. In this case the beam electrons themselves seem to be involved in the generation, since there is a clear relation between injection angle and amplitude of the stimulated effect. Since the process takes place in the E—region, a mechanism which builds on differential mobilities between electrons and ions, i.e. a current driven instability, may be suggested.
122
J.A.
Holtet et ci.
ACKNOWLEDGEMENTS We are very grateful to Dr N.C. Maynard, NASA GSFC, for his cooperation in the wave and field experiment. The project was sponsored jointly by the National Aeronautics and Space Administration, USA, and the Norwegian Council for Scientific and Industrial Research. REFERENCES 1.
B. Grandal, J.A. Holtet, B.N. Mahiurn, B.K. Pran and J. TrØim, Observations of waves artificially stimulated by an electron beam inside a region with auroral precipitation, Planet Space Sci, in press, 1980.
2.
B.N. Mmhlum, K. M~seide,K. Aarsnes, A. Egeland, B. Grandal, J.A. Holtet, T.A. Jacobsen, N.C. Maynard, F. SØraas, J. Stadsnes, E.V. Thrane and J. Tr~im, Planet Space Sci, 28, 259, 1980.
3.
E.V. Mishin and Yu.Ya. Ruzhim, Preprint No 21 a and b, IZMIRAN/Akademi NAUK USSR, 1978.
4.
N. Tsutsui, H. Hiramato, H. Matsumoto and K. Kimura, J Geophys Res, 84, 4217, 1979.
5.
N. BjØrn~ and J. Trulsen, Report 22—76, The Auroral Observatory, TromsØ, Norway, 1976.
6.
W. Gekelman and R.L. Stenzel, Phys Fluids, 21, 2014, 1978.
O2731l77/81/020~0hI7$O5.OO/O
VERY LOW FREQUENCY WAVES STIMULATED BY AN ELECTRON ACCELERATOR IN THE AURORAL IONOSPHERE J. A. Holtet,* B. Grandal,** T. A. Jacobsen,** B. N. Maehlum,** J. Tr$im,~B. K. Pran* and A. Egeland* *fr~tituteof Physics, University of Oslo, Norway **Norwegian Defence Research Establishment, Kjeller, Norway ABSTRACT The sounding rocket POLAR 5 carried a 10 keV electron accelerator and various diagnostic instruments in a mother—daughter configuration. Onboard wave receivers recorded several types of VLF wave phenomena directly associated with the operation of the accelerator, with delays from 5 to 50 ins after the injection of the electrons. These delayed after—effects range from broadband noise, f > 3 kHz, observed above 170 kin, through narrow band emissions at 2 and 5.6 kHz which appeared when the rocket crossed a region with precipitation of energetic electrons, to emissions covering frequencies from 3—4 to well above 100 kHz observed within the E—region (150—95 km). The latter was also associated with apparent changes in electron density. The observed emission properties indicate that the region perturbed by the beam and the neutralizing return current to the daughter may be a favoured generation region. INTRODUCTION The sounding rocket POLAR 5 was launched from Andøya Rocket Range, Northern Norway, February 1, 1976, at 1929 UT into a slightly disturbed ionosphere with weak auroral activity. The rocket trajectory, being close to a magnetic meridian plane, crossed two auroral regions, one, the main one, between 86 and 111 s flight time, and a secondary region between 230 and 330 s. The rocket carried an electron accelerator and various diagnostic instruments in a mother—daughter payload configuration. The daughter, with the accelerator, was separated axially from the mother in a forward direction at an altitude of 90 km. A slight offset of the velocity vector from the mother’s plane of trajectory introduced an east—west separation between the two payloads. The accelerator emitted 10 key electrons at a flux of 8 x 1017 s—i, corresponding to a current of —130 mA. The accelerator was operated in a pulsed mode at a repetition rate of -‘2.5 Hz. Within each main pulse the beam was modulated at 250 Hz. A main pulse would thus consist of five 2 ins sub—pulses separated by 2 ins spacing, and last for 18 ms. The accelerator was turned on at 105 s (148 km altitude). An off period of 15.5 s from 237.4 s was included. The VLF experiment was carried by the mother payload. The receiving antenna was an electric dipole, 9.3 m tip—to—tip, oriented 900 to the rocket spin axis. The experiment would thus record both electromagnetic and electrostatic waves. The instrumentation did not provide additional information which made it possible to distinguish between the two wave modes. The receiver covered the frequency band up to 100 kHz
117
J.A. Holtet eL~ a~.
118
in broadband mode. The payload also included a HF wave experiment, covering frequencies up to 5 MHz. Stimulated waves observed at these high frequencies were in general not associated with the VLF waves [i) and will not be dealt with here. The oriboard particle detectors recorded increased electron fluxes in the two auro— ral regions. Of special interest is a double peaked structure in the fluxes of 4—5 and 12—27 keV electrons observed within the northern auroral form [2]. The number 3, with maxidensity of thermal varied during the flight lO~aurora cm mum density within plasma the main auroral region. To the from north106of tothis a slow, steady decrease in the density was observed, with no enhancement in the region of the second aurora. The background DC electric field stayed at a very low value (<20 mV/rn) throughout the flight. The background emission activity in the VLF range was dominated by a strong band of auroral hiss, above -~7kHz. This noise—band hampered detailed studies of beam related wave effects from this frequency up to ~5O kHz. For more detailed information about instrumentation, ionospheric conditions and background observations see Maehlum et al (2) VLF WAVES ASSOCIATED WITH ELECTRON INJECTION The injection of the artificial electrons into the ionospheric plasma stimulated several different wave effects at various frequencies in the VLF range. Such waves were observed during the entire operation of the accelerator, but the character of the emissions varied through the flight. ______
20
_________________
-~
x
_________
_____________________________
Figure 1: Spectrogram showing the direct radiation as vertical lines, and lower hybrid and ELF radiation at 5—6 kHz and 1—3 kHz following the direct radiation (excv~ples encircled). An amplifier gain shift appeared at pulse No. 404.
~ 10
PULSE NO. 40 FLT. TIME (i) 205.054 ALTITUDE (kin) 219
4~5
215.4*1 201
Direct radiation. Most easily detected were the strong, spike—like transients directly associated with the beam injection (Fig. 1). These signals, which covered a wide frequency range (>2 MHz) were observed through most of the flight. Attenuation of the wave appeared, however, at low altitudes (below 140 krn) and the waves disappeared around 105 km. The signature of the five sub—pulses can be recognized in the emission (Fig. 2). The time delay from beam injection to the arrival of the wave— train at the mother was very short, less than 250 us. We will therefore refer to this as direct radiation. The short delay indicates that the waves were electromagnetic. This emission may resemble reported observations of beam—plasma discharge [3), and could thus be taken as evidence of such processes. In the spectrogram representation (Fig. I) the direct radiation appears as vertical lines, which resemble the sub— ionospheric VLF phenomenon known as “atmospherics’. (An atmospheric is a wave—train which is radiated from an electric discharge in the atmosphere.) The spectral appea—
VLF Waves Stimulated by Electron Accelerator
19
rance may therefore evoke the more simple suggestion that the direct radiation may have its origin in an electric discharge in the accelerator when this was activated. Delayed radiation. The direct radiation faded away shortly after the last electron sub—pulse and the signal returned to the background level. However, during most of the flight waves were also observed with considerably longer delays. We will in the following refer to this as (wave) after—effects. From the spectrograms (Figs. 1 and 3) the various after—effects can be divided in the following four groups: a) Short delay after—effects, b) Lower hybrid radiation, c) ELF—radiation, and d) Broadband long delay radiation. Short delay after—effects, appear in the spectrograms as a broadening of the trace of the direct radiation, extending from 3—4 kHz up into the hiss band. In an amplitude—time plot this after—effect is seen to grow up between the “fingers” of the direct radiation and decay slowly after the last sub—pulse (Fig. 2). The delay from the electron injection to the arrival of the wave was usually in the range 4—8 ms, with maximum amplitude around 18—22 ms, i.e. at, or shortly after the last sub—pulse. The radiation appeared at the very first gun pulses and lasted till approximately 340 s (altitude’-~l7Okin), when it disappeared after a gradual decay. It was not possible to relate spesific amplitude variations to the beam injection angle or the orientation of the receiving antenna.
A
PULSE
342
TIME
B
259906,
.cu.s
-
0
PULSE
349
TIME
262363,
..u..
20
40 TIME
(ms)
0
20
40
TIME
(ms)
Figure 2: Amplitude variations observed in the frequency band 5. 2 — 6. 0 kllz associated with injection of electrons. Panel A shows direct radiation and short delay radiation and panel B direct radiation and, separated from this at the peak between 30—40 ma, lower hybrid radiation. The five dots above the curves indicate the time of injection of the five beam sub—pulses. Lower hybrid radiation. Below the natural hiss, long delayed emissions were found in a relatively narrow band centered around 5.6 kHz. This is close to the local lower hybrid frequency, ~LH’ which for a mean ion mass of 20 is 7.3 kHz. In the spectrograms this radiation appears as “blobs” after the direct radiation (Fig. 1), and in amplitude—time plots at a fixed frequency it can be seen as peaks growing up clearly separated from the direct radiation (Fig. 2b). The start of after—effects was usually observed at a delay of 22—26 ms from the beam injection, and maximum amplitude was reached in the delay range 30—40 ms. This radiation was observed sporadically between 150 and 200 s. The main emission region appeared, however, from the gun off period till 320 s, i.e. in the second auroral region. The emission thus showed a spatial correlation with the precipitated auroral electrons. The stimulated amplitude showed some dependence on the angle of injection of the electron beam with a tendency of having maximum amplitudes for upward injections. This was, however, not consistent for the whole data set, and large amplitudes were also found to be generated for downward injections.
J.A. Holtet et a7.
20
ELF radiation. Narrow band stimulated emissions were also observed between 1 and 3 kllz. The emission trace on the spectrograms showed a tendency of dispersion with the low frequency waves preceding those at high frequencies (Fig. 1). These aftereffects were only observed inside the second auroral region. No clear dependence on the direction of the injected beam could be found.
30
25
I
‘~J~
~
‘I’
V
1
‘~
‘~
Figure 3: long Spectrogram showing broadband delay after—effects
•
-
as broad vertical lines (examples indicated by arrows). The dark area around 15 kHz is spin modula—
-
20
Altitud. (km)
“
.
ted cruroral hiss
373.774 129.04
377.370
Broadband long delay radiation. In the last part of the flight, (i.e. after~355 s, below 150 kin) stimulated after—effects which extended over a frequency range from 2—3 kHz to well above 100 kHz were observed. On the spectrogram (Fig. 3) the emissions appear as broad diffuse traces, well separated from the direct radiation. The delay from beam injection to detection of the wave was around 30 ins, and the stimulated wave would typically last for 50—60 ins. The observed amplitude showed a periodic variation. The majority of the high amplitudes appeared for injections perpendicular to the geomagnetic field. This picture was, however, somewhat disturbed by a second modulation introducted by the rotation of the receiving antenna. Maximum field strength seemed to appear at a direction perpendicular to the magnetic field. It should be noted that this after—effect appeared in a period when the amplitude
A
10
fV~ •
TIME 1.) ALTITUDE (km)
360 146.14
40
$S~~
370 133.97
380 120.88
Figure 4: Cold plasma density measured by the NDRE capasitance probe in the Eregion. The spikes on the curve are associated with the injection of beam electrons. The structure of these density after—effects is shown in more detail in the enlarged example.
VLF Waves Stimulated by Electron Accelerator
121
of the direct radiation was decreasing. During the last part of the observations, i.e. from 390 to 398 s (from 105 to 95 km altitude) when this after—effect was still observed, the direct radiation was even vanishing. In the same period as these long—delayed wave after—effects were present, the electron density probe detected large gun—related changes in the ambient thermal plasma. These after—effects showed up as apparent increases in the electron density lasting 60—80 ins after the start of the electron injection (Fig. 4). Maximum effect was detected for injections perpendicular to the geomagnetic field, and also in other respects a reasonably good connection was found between wave and density aftereffects. It seems thus as if these two data sets reflect two different features of one and the same phenomenon. The physical relation behind this is, however, not clear. DISCUSSION With exception of the E—region after—effects the other types of stimulated waves showed no clear relation between stimulated wave amplitude and beam injection angle. This, together with no systematic change in delays for upward and downward injection makes a generation well above the rocket unlikely. Processes including e.g. electrostatic double layers above the rocket or backscatter from the atmosphere below could give the same geometry for upward and downward directed beams, but this would result in differences in time delay. The observed emissions are therefore believed to be generated in the vicinity of the daughter. Furthermore, the combination of the short distance between the generation region and the receiver and the relatively long delay requires a slow wave, making it natural to suggest that the waves were electrostatic. During the injection of the electrons and shortly thereafter the plasma around the daughter would be perturbed by the beam itself and the return current flowing to the daughter in order to neutralize the body. This perturbation and heating of the ambient plasma will give rise to a wide range of wave emissions. The spatial correlation with auroral electrons may be due to an energy transfer from the particle resulting in an amplification of certain waves. In a laboratory simultation of a rocket experiment Tsutsui et al (4) found that waves could be generated near the lower hybrid frequency due to an interaction between a low energy electron beam (the return current) and the plasma. In the same experiment waves were generated around 2 kHz by a beam—plasma interaction. The phase velocity of both were of the order of the ion—acoustic velocity. Assuming T — 2000 K, t.-’ 1000 K and a mean ion mass of 20~ the group velocity of ion_acousticewaves is f~und to be of the order of 1.4 x l0~mis at 6 kHz [5). For propagation over a distance of 80 m, this corresponds to a time delay of “~50ms which is much greater than what was observed. However, if we assume that the generation is streched out over some distance along a gyro—column, and that the waves are propagating perpendicularly to the magnetic field, the distance would typically be 30 in and the corresponding de— lay “2l ins, which is reasonably close to the observed value. This would also be in agreement with laboratory observations by Gekelnian and Stenzel (6) . It is therefore suggested that the narrow—band after—effects at 2 and 5.6 kHz are ion—acoustic waves generated in the return current region. When it comes to the E—region wave— and plasma after—effects a local source region seems also here to be most likely. In this case the beam electrons themselves seem to be involved in the generation, since there is a clear relation between injection angle and amplitude of the stimulated effect. Since the process takes place in the E—region, a mechanism which builds on differential mobilities between electrons and ions, i.e. a current driven instability, may be suggested.
122
J.A.
Holtet et ci.
ACKNOWLEDGEMENTS We are very grateful to Dr N.C. Maynard, NASA GSFC, for his cooperation in the wave and field experiment. The project was sponsored jointly by the National Aeronautics and Space Administration, USA, and the Norwegian Council for Scientific and Industrial Research. REFERENCES 1.
B. Grandal, J.A. Holtet, B.N. Mahiurn, B.K. Pran and J. TrØim, Observations of waves artificially stimulated by an electron beam inside a region with auroral precipitation, Planet Space Sci, in press, 1980.
2.
B.N. Mmhlum, K. M~seide,K. Aarsnes, A. Egeland, B. Grandal, J.A. Holtet, T.A. Jacobsen, N.C. Maynard, F. SØraas, J. Stadsnes, E.V. Thrane and J. Tr~im, Planet Space Sci, 28, 259, 1980.
3.
E.V. Mishin and Yu.Ya. Ruzhim, Preprint No 21 a and b, IZMIRAN/Akademi NAUK USSR, 1978.
4.
N. Tsutsui, H. Hiramato, H. Matsumoto and K. Kimura, J Geophys Res, 84, 4217, 1979.
5.
N. BjØrn~ and J. Trulsen, Report 22—76, The Auroral Observatory, TromsØ, Norway, 1976.
6.
W. Gekelman and R.L. Stenzel, Phys Fluids, 21, 2014, 1978.