- Email: [email protected]
Wake phenomena observed by an ionospheric sounding rocket
Spare
Planer. Sci., Vol. 38, No. 3, pp. 395-405, 1990 Printed in Chat Britain.
00324633po s3.oo+o.M) Pcrgamon Pms plc
WAKE PHENOMENA OBSERVED BY AN IONOSPHERIC SOUNDING ROCKET K. R. SVRNKS, J. TR0IM
and B. N. MAKHLUM
Norwegian Dcfence Research Institute, P.O. Box 25, N-2007, Kjeller, Norway and
Max-Planck-Institut
K. WILHELM fiir Aeronomie, P.O. Box 20, Max-Planck-Stral3e 2, D-341 1 Katlenburg-Lindau, F.R.G.
(Received 10 July 1989) Abstract-Measurements from the ionospheric sounding rocket POLEWARD LEAP have been analyzed in order to study the nature of wake phenomena. Some outstanding events, which apparently resulted from the interaction between the incoming particles and the F-layer plasma, are discussed. It is shown that the suprathermal electron fluxes observed during the auroral particle precipitation were clearly spin modulated, and that they usually occurred in the wake region of the rocket. In addition, background measurements of both the aurora1 particle precipitation and the ambient plasma are presented.
1. INTRODUCTION The
existence of a region of depleted plasma density behind a vehicle travelling through space has long since been recognized. Even from the measurements carried out to monitor the ambient plasma during such comparatively early space flights as that of Gemini-Agena 10 and 11 (Medved, 1969) and Explorer 31 (Samir and Wrenn, 1969), this phenomenon was readily evident. Since then a fair amount of data has been obtained, related to the nature of the plasma distribution around an orbiting spacecraft. In order to explain these measurements, it has been necessary to take into consideration the typical speed of a space vehicle moving through the magnetospheric plasma. For rockets this speed is about l-3 km s-‘, whereas satellite speeds may approach 10 km s-‘. In both cases it usually means that the vehicle moves through space with a speed comparable to the thermal speed of the ambient ion population. This speed ranges from N 1 km s-’ for an ion temperature of 1500 K (O:), typical of the lower ionosphere, to w 16 km s-’ for an ion temperature of 10,000 K (H+), typical of the outer magnetosphere. The corresponding thermal speeds of the electrons are in the region of _ 25&- 700 km s-‘. Accordingly, the ambient ions are not capable of filling in behind the vehicle before some finite time after its passage. The electrons, however, will instantaneously attempt to fill in this void. Since a charge imbalance is then created, polarization fields are consequently set up to impede the movement of the
electrons. In this way a region of depletion in both the electron and the ion population arises in a direction nearly antiparallel to the velocity vector. This region is called the wake. Computer simulations of the ion density depletions in the wake region have been carried out by e.g. Katz et al. (1984). Such studies have been compared with in situ observations, e.g. by Samir and Fontheim (1981), and found to be in good qualitative agreement. In addition, measurements from the Space Shuttle have shown that the electron density in the wake region close to the vehicle might be orders of magnitude below the ambient density ; see e.g. Ingsoy et al. (1986) and Murphy et al. (1986). Indeed, the ratio of the wake density to the ambient density has, in the case of the space Shuttle, been found to be as small as 10m4according to Siskind et al. (1984). The wake region is also characterized by an enhanced electron temperature when compared with the ambient plasma. This has been shown by Samir and Wrenn (1972) from satellite measurements and by Murphy et al. (1986) in the case of the Space Shuttle. Several mechanisms have been proposed to explain this. Samir and Wrenn (1972), Troy et al. (1975) and Stone (1981) have all suggested selective effects due to potential wells near the vehicle, plasma instabilities and/or turbulence in the wake region, as causes for such temperature enhancements. Murphy et al. (1986) proposed adiabatic compression and, consequently, .heating of the electrons in the wake region as a possible source. Singh et al. (1987) have explained the increase by counterstreaming electron
395
K. R. SVENES et al.
396
populations interacting with each other to produce a single “hot” electron population in the wake. However, this question still remains unresolved. Finally, it should be noted that Siskind et al. (1984) observed an electron temperature increase in the ram direction (parallel to the velocity vector) during the third Space Shuttle flight. It was suggested that this temperature enhancement might in some way have been connected to the generation of local plasma turbulence due to the interaction of the ambient ionosphere and neutrals outgassing from the Shuttle. However, these measurements have been challenged by Murphy et al. (1986), and the issue consequently remains unsettled. In any case it serves as a useful illustration of the dynamical and intricate nature of the interaction between a moving space vehicle and the ambient plasma. 2. DESCRIFWON OF THE INSTRUMENTS
As is evident from the preceding text not many observations of rocket wakes have been reported in the literature. Berthelier and Sturges (1967) reported spin modulations in both electron density and temperature measurements, which were ascribed to wake effects, during a rocket flight in the aurora1 zone. Enhanced electron temperatures near the apogee of some rocket flights have also been reported. These measurements have been attributed to wake effects which becomes prominent near apogee where the rocket moves subsonically and the velocity vector is nearly normal to the rocket axis (Klyueva, 1973 ; Gupta, 1988). Bering (1983) also suggested a wake generated parallel electric field as explanation of measurements carried out during a sounding rocket flight in the aurora1 zone in 1978. During the Norwegian rocket programme, enhanced electron temperatures in the wake zone have been observed in the flight of MAIMIK (Svenes and Tr&m, 1987). In addition, analysis of data from the POLEWARD LEAP flight have previously revealed other interesting effects apparently connected to the wake region (Maehlum, 1988). These effects will be further discussed following a brief description of the instrumentation. POLEWARD LEAP was launched on 11 November 1983 at 21:08 U.T., and it reached an altitude at apogee of 454 km. The launch occurred during the recovery phase of a large substorm which had commenced about one and a half hours earlier. Indeed, the purpose of the flight was just to study this phase ofpoleward movement in the aurora1 arc system accompanying the recovery phase of the magnetospheric substorm. A more detailed description of the flight is given in Stadsnes et al. (1986).
In the present context, data from only two of the instruments on board are utilized in addition to the altitude measurements. Those were the suprathermal particle spectrometer and the electron tem~rature probe provided by the Mu-Planck-Institute for Aeronomy and the Norwegian Defence Research Establishment, respectively. Both of these instruments performed flawlessly during the whole flight. The Suprathermal Electron and Proton Spectrometer, later referred to as the SEPS, provided particle m~s~ments in the energy range from 15 eV to 30 keV for both electrons and protons. The sensors were designed as nested electrostatic analyser systems followed by continuous channel electron multipliers (CEM), and were mounted at 25” and 115” with respect to the rocket axis. Each sensor could detect both species. The conversion factors of the sensors were approx. 3+10e4 E cm2 sr eV for electrons and 810-’ E cm2 sr eV for protons, where E is the instantaneous particle energy during the instrument sweep. The energy resolution was 12-13%, and the entire energy sweep lasted 850 ms. The Electron Tem~rature Probe, later referred to as the ETP, measured the electron part of the current towards the payload. It consisted of a spherical grid 5 cm in diameter surrounding a collector with a bias of + 10 V relative to the payload ground. The amplifier was linear with automatic decade switching, enabling current measurements in the range from about lop9 to low5 A. During operation the potential of the outer grid of the probe was continuously changed. Each such potential sweep was aimed at obtaining an integral electron spectrum. This was done by sweeping the potential of the grid relative to the payload ground from -2.0 to +3.0 V. This procedure was then repeated, yielding the characteristic “saw-tooth” appearance of the measurements shown in Fig. 1. The slope of the linear part of the curve is proportional to the ambient electron temperature, and the point on the voltage axis corresponding to the point where the curve breaks off from the straight line approximation gives the potential difference between the vehicle and the ambient plasma. Finally, the current measured at that point is proportional to the ambient electron density. This standard analysis technique has been applied to all data obtained during the flight. Returning again to Fig. 1, the upper part shows a sweep obtained during a period without any appreciable particle precipitation. The slope of the sweep indicates an ambient electron temperature of about 2400 K. In the lower part, a sweep is shown which was obtained during a period with substantial particle
Wake phenomena observed by an ionospheric sounding rocket
397
3. AMBIENT PLASMA MEAS-
3 $ i
l.OOE-06
l.WE-07 i.ooEaS
l.OOE~
r.ees-lo’.
I ! t ! . . . . ! r . . , ! e . . . ! . !i4!i.978 546.428 !i46.128 344.278
FIG. I. Two mmps OF THE ETP. The sweep in the upper part was carried out during unperturbed conditions, whereas the sweep in the lower part was carried out during a period with significant suprathermal electron production (shaded area),
precipitation. Since the slope of this sweep is much less steep, the ambient electron temperature at that time was much higher, about 3200 K. In addition a population of suprathermal electrons was then present, resulting in the rise of the lower level in the left part of the sweep (shaded area). In order to obtain a measure of the amount of such suprathermal electrons present at any time, the currents measured during each of the first 75 voltage steps of each sweep (a complete sweep consisted of 5 12 voltage steps) were integrated into a single parameter. This parameter then represented the amount of suprathermal electrons, roughly in the energy range between 3.5 and 4 eV, measured during each sweep. Using this number as an ordinate and plotting it against the time of flight, it was possible to quantify the variation of suprathermal electrons measured during the flight.
In the present paper we are mostly concerned with wake phenomena, but we include here a short discussion of the ambient plasma rn~~~ents in the ionospheric F-layer in order to complete the picture. These data are plotted in Fig. 2 which covers the period from 100 to 600 s time of flight. The corresponding altitude interval was from about 205 km on the upleg to 170 km on the downleg. In the upper part of this figure the data from the upward pointing channel of the SEPS are plotted with electron energy as the ordinate and the intensity of the fluxes coded in the colour scale on top of the figure. The second part of the figure shows the ambient electron temperature as estimated from the data of the ETP, and in the third part the ambient electron density (likewise obtained) is displayed. Returning again to the upper part of the figure, it is seen that electron precipitation was observed in strong bursts during the whole flight, extending in energy range from below 100 eV to some keV. This is normal for aurora1 particle pr~ipitation. As is clear there was a very good correspondence between the electron fluxes, especially below about 1 keV, and variations in the ambient electron temperature and density, i.e. every increase of the electron flux was accompanied by an enhancement of the ambient electron temperature and a s~ult~eous decrease in the electron density. Some of these events were also associated with the creation of suprathermal electron populations as illustrated in the lower part of the figure. Here the ordinate of the plot is integrated current measured during the first part of the sweep of the ETP, as previously explained, representing a measure of the amount of suprathermal electrons observed. As seen from the figure, four instances of significant suprathermal electron production were detected during the flight. All of these occurred inside periods of strong particle precipitation and major temperature enhancements. These events wilt be discussed further later on in the text. Perhaps the most conspicuous feature in this set of data is the excellent agreement between the bursts of precipitating electrons, electron density depletions and electron temperature enhancements. Since this remarkable consistency in the m~s~ements persisted during the whole flight it seems reasonable to assume a common cause, for all events, which is in some way connected to the aurora1 precipitation. It is well known that regions of plasma cavity are observed in the aurora1 areas at high aititudes during periods of geomagnetic activity (e.g. Benson and Cal-
398
K. R. Sm
vert, 1979 ; Calvert, 1981; Persoon et al., 1988). Such regions are believed to be associated with the acceleration of aurora1 electrons and the generation of aurora1 kilometric radiation (Calvert, 1981). These observations admittedly refer to an altitude range of a few Earth radii, but lately observations of regions of plasma depletions have also been reported at heights below 1000 km (Gussenhoven et al., 1985; Yeh and Gussenhoven, 1987 ; Haerendel, 1988). A model describing such observations has been proposed by Winglee et al. (1988). They assumed that the energy input of the events is provided by processes in the magnetotail, and mediated through energetic ion beams originating in the plasma sheet boundary layer. These beams are accompanied by costreaming low energy electrons. However, numerical simulations have shown that due to the great difference in the gyroradius of the particles charge separation will occur, leaving the electrons on separate field lines surrounded by the ions. This charge separation will then lead to downward acceleration of the electrons on the field lines with excessive negative charges, whereas a return current of electrons from the ionosphere will be created on the field lines with excessive positive charges. In such circumstances, the plasma depletions observed during the flight of POLEWARD LEAP might be explained as the residual effects of plasma cavity formations in the return current regions at higher altitudes. The observed precipitating low energy electrons would then probably he due to particle scattering from other regions. As for the temperature enhancements these could be the result of either turbulent processes in the ionospheric plasma due to the great energy input, or beam-plasma interactions between the precipitating electrons and the ambient plasma. This view is supported by the “map” of the precipitation region obtained by X-ray and photometer observations from POLEWARD LEAP (Stadsnes et al., 1986). It was concluded here that a “hole” in the precipitation region existed north of the launch site roughly in the direction where the rocket was headed. By “hole” the authors referred to a region of low electron precipitation which contained some proton precipitation. It must, however, be pointed out that these observations might also be explained by another model. Since the aurora1 precipitation is observed to be filamentary by nature at low altitudes, this “hole” may just have constituted a region of unusually low precipitation. Still, that precipitation may have been enough to form a “cold” electron beam moving through the ionospheric plasma. This pr~ipitation
et al. could have been initiated and sustained through a variety of mechanisms previously suggested in the literature such as anomalous resistivity, double layers, magnetic mirror effects, AlfvCn waves or other more exotic models. For a review of these mechanisms see e.g. FPlthammer (1977), Papadopoulos (1977) and Kan (1982). All of these mechanisms are able to account for an electron stream which in effect will constitute a “cold” beam with respect to the thermal F-layer plasma. In such a case this beam would have been likely to interact with the ambient plasma and to transfer energy through wave-particle processes. This energy could have been deposited both as internal energy of the ambient plasma and as pressure-volume work on the s~ro~dings. Thereby both the electron temperature enhan~ments and the electron density depletions would have been accounted for. 4.WAKE EFFFXXS
The suprathermal electron data displayed in the bottom part of Fig. 2 are plotted again in greater detail in Fig. 3. It is evident that the fluxes of suprathermal electrons were strongly spin modulated in all four cases. However, the suprathermal population detected in the period between 540 and 560 s was still outstanding since the spin modulation was then superposed on a general increase of suprathe~al electrons during part of this particular period. Figure 4 shows the latter data again superposed on a plot of the angle between the velocity vector and the direction vector of the ETP. From this plot it is clear that the suprathermal electron production was enhanced in the wake area of the rocket. (The largest angle between the direction vector of the ETP and velocity vector is obtained when the probe is situated in the wake region.) It is also seen that the presence of suprathermal electrons was increasing both in the ram and the wake direction in the period from 540 to about 547 s. The production rate than decreased generally, but the decrease was especially rapid in the ram direction. Indeed, from about 555 s almost all the suprathermal electrons were detected in the wake direction. Closer inspection of the other events shown in Fig. 3 revealed that the suprathermal electrons were observed only in the wake area in all three of these cases. Further examination of the observations also indicates that the maximum amount of suprathermal electrons was observed just before the probe entered the deepest nominal wake position. (The nominal wake position corresponds at any time to a direction antiparallel to the velocity vector of the rocket.) In
Wake phenomena observed by an ionospheric sounding rocket
125 I
00E+*7
250
375
“__^ ..7__.. __._...,.. _ . :-... .. . ...__._._.,,_,_. __” _._..... i.__..-...
399
Wake phenomena observed by an ionospheric sounding rocket
FIG. 3. SPIN
MODULATED SUPRA TIilXMAL
ELECTRON MEASURE-
MENTS FROM FOUR DlTFT.REN’C PERIODS.
401
the case shown in Fig. 4 the angle of difference could amount to as much as 30-40”. As previously mentioned, the temperature of the main electron population also increased during these events. Further examination of the measurements from the period 540-560 s reveals that the enhancements of the suprathermal electron population and the increase in the temperature of the main electron population occurred simultaneously. This is illustrated in Fig. 5. The top part shows the temperature of the ambient electron population, whereas the middle plot shows the amount of suprathermal electrons detected. Evidently the suprathermal electron production peaked simultaneously with the maximum obtained temperature in the ambient plasma population. The lower part of this figure shows electron flux observations from the SEPS at some selected energies. It is seen that during this period the particle precipitation gradually increased in energy, and that the peak in the two curves above occurred at about the time of peak electron precipitation at around 500 eV. As far as the proton data are concerned, very low fluxes were measured during most of the flight. However, in the period 540-549 s the fluxes of protons in the energy range between 80 and 800 eV increased by roughly a factor of 10. Indeed, this was the only period of the flight during which the proton fluxes deviated appreciably from the usual background measurements. It was also due to these low fluxes that the measurements had to be integrated for such a long time, leaving us with a somewhat coarse time resolution for the ion distributions. All the above-mentioned data indicate that beamplasma interactions occur in the ionospheric F-layer during typical aurora1 precipitation events. The notion of heating a plasma by passing a “cold” electron beam through it is fairly well supported by both theory (e.g. Pierce, 1948 ; Buneman, 1959 ; Kainer et al., 1972) and laboratory experiments (e.g. Getty and Smullin, 1963 ; Jost et al., 1980). Indeed, injections of artificial electron beams from rockets in the ionosphere itself have repeatedly lead to observations of electron temperature enhancements (e.g. Kaneko et al., 1978; Winckler, 1980; Duprat et al., 1983; Jacobsen, 1982 ; Ingsoy and Friedrich, 1983 ; Svenes et al., 1988 ; Managadze et al., 1988). The reason that the events occurred only after the rocket had passed through apogee was probably that the ETP then entered deeper into the wake zone. This is due to the typical geometric relationship of rocket flights. In the latter half of the flight, the angle between the velocity vector and the rocket body axis becomes greater, leaving the probes deeper in the “shadow” of
K. R. SVENES et al.
402
A
R
!i
Ii
ii
!I I
ii
I i i i
ii
I_
!
fi i 1 !
IL ! ntlL. i
67.5
i i i !’
!i
!!
I\ ii
I i .
I i .
! !
i
!
i j i !
i i ! I i i i i
!
i
!
i J
*i 11
i i !!
i_i
‘\i
I
00 5.0
FIG. 4. THEANGLE
BETWEEN THE DIRECTION VECTOR OF THE
ETP AND
BROKEN LINE, AND THE MEASUREMENTS OF SUPRATHERMAL
the rocket for part of the spin. Thus, in this case it meant that the observations of suprathermal electrons were made only when the ETP entered deep into the wake zone. It should also be mentioned here that no spin modulation could be detected in the electron temperature or density measurements. However, this was not expected either since the time resolution of those measurements was rather poor. As explained previously the electron temperature and density were estimated from the sweep of the ETP. During the POLEWARD LEAP flight though, the sweep rate was such that the rocket revolved through an angle of 83” during one sweep of the ETP. Thus, on the basis of these measurements it was not possible to resolve the wake region from the ambient ionospheric plasma. On the other hand, the suprathermal electron population was just estimated from an integral of the first part of the sweep as previously explained. During this period the rocket revolved through an angle of about 12”, thereby yielding a much better time resolution of this measurement. In fact, as seen from Fig. 3, the time resolution was good enough to enable detection of the wake region. From theoretical considerations it is well known that an underdense electron beam passing through a thermal plasma will produce suprathermal electrons
I
564
boo
THE VELOCITY VECTOR PLQTTED WITH
ELECTRONS PLQTTBD WITH FULL LINE.
in inverse proportion to the difference between the respective densities (see e.g. Kainer et al., 1972 ; Papadopoulos and Szuszczewicz, 1988). Thus, the smaller the difference between the beam and the ambient plasma densities the more effective the suprathermal electron production. This is entirely in accordance with the observations from the POLEWARD LEAP flight. Since the electron density was probably lower inside the wake zone of the rocket than in the ambient plasma, but still higher than in the beam, the production of suprathermal electrons was much more effective here. As seen from Fig. 3, this is exactly what the observations implied. Thus, it seems likely that the low energy electron precipitation observed can be modelled by an electron beam heating the ambient ionospheric plasma through waveparticle interactions. In addition, this beam would also produce suprathermal electrons when it passed through the low density plasma in the wake of the rocket. Another interesting factor to note here is that the maximum suprathermal electron population in some instances seemed to occur somewhat outside the geometrical wake zone. If indeed the region of maximum suprathermal electron production corresponds to the most pronounced wake region, then this means that the wake had been shifted relative to the geometric
Wake phenomena observed by an ionospheric sounding rocket 3.505*03
1
,
3.251+03 g
!
3.OOE+03
d 2.751+03
2SOE+O3.~i 540.000
541Om
550.m
555.wO
550.000
1.Om-O5
s
1.00&05
I a
1.00547
l~ooE-oo,,..,.-..,....,...., 55o.ooo
5
s
403
km s-’ would have shifted the wake axis some 30”. Such a drift velocity could have been caused by a 60 mV m-r electric field pointing due North, which is a field strength well within the range normally measured in the aurora1 ionosphere (see e.g. Maynard, 1972). However, there are unfortunately no electric field measurements available from POLEWARD LEAP to confirm this assumption, Finally, from Fig. 5 it is seen that during the period between 540 and about 550 s the augmented suprathermal electron population in the wake region was superposed on a general increase of the suprathermal electron population. The maximum of this population occurred simultaneously with the maximum of the ambient electron temperature, and also simultaneously with the maximum flux of 500 eV electrons. It has to be added though, that this period also was the only period during which a significant amount of ions were detected. The ion measurements did not have a sufficient time resolution to enable a direct comparison with the parameters plotted in Fig. 5, but a distinct flux of ions in the energy range 80-800 eV was definitely present during this period. Since this event was also unique with respect to the total suprathermal electron production it is reasonable to assume that the resulting plasma response was due to the flux of ions. Thus, this might be an indication of an interaction between an ion beam and the ionospheric plasma.
10
;i
5.
f, =: X
Summarizing the above measurements and discussion yields the following empirical conclusions on the results from the POLEWARD LEAP flight :
2 lo,
-In
I
104
540
CONCLUSION
545
550
555
560
FROM THE PEXIOD 56560 S. FIG. 5. &Xl-RON The ambient electron temperature is shown in the upper part, the suprathermal electron measurements in the middle part and electron flux observations at various selected energies in the lower part.
wake position, In the period 540-560 s this difference amounted to more than 30” as seen from Fig. 4. This phenomenon may have been caused by a convection of the whole plasma surrounding the rocket. The most likely agent of such a plasma drift is the E x B force. Since the rocket was moving at a velocity of about 2 km SC’ at this time, a drift velocity of 1.2
the descending part of the flight some periods occurred during which spin modulated suprathermal electron populations were detected. These suprathermal electron populations were observed predominantly in the wake region of the rocket. -During one of these periods the augmented suprathermal electron population in the wake region appeared to be superposed on a general enhancement of the suprathermal electron population. This period coincided with the only period in the l-light during which significant fluxes of precipitating ions were detected. -Measurements of the ambient electron temperature and density and of the precipitating aurora1 electrons revealed a close correlation between electron temperature enhancements,
K. R. SVENESet al.
404
electron density depletions and fluxes of precipitating electrons below 1 keV during the whole passage through the ionospheric F-layer. It has been suggested that all these measurements might be related to various types of beam-plasma interactions. This view is supported both by theoretical considerations and previous experiments. However, since complete measurements of all relevant parameters are not available, this assumption cannot be confirmed conclusively. The effect of the wake region on the measurements is quite definite. Evidence has even been found of a shift of the wake position in relation to the geometrical position of the “shadow” zone “behind” the vehicle. This might have been caused by background electric fields which are often present in the ionosphere during aurora1 conditions. Thus, it has been shown that even a small space vehicle like a rocket modifies the ionospheric plasma to an extent which cannot be entirely disregarded during the interpretation of the low energy plasma measurements. AcknowledgementsThe authors acknowledge the excellent work of the project scientist Finn Ssrls throughout the whole project. We are also grateful to Magne Hgv&g for his help in providing us with the altitude data. This experiment was financially supported by the Royal Norwegian Council for Scientific and industrial Research (NTNF), Space Activity Division. The MPAE instrument was supported by the MaxPlanck-Gesellschaft.
REFERENCES
Benson, B. F. and Calve& W. (1979) ISIS-1 observations at the source of aurora1 kilometric radiation. Geophys. Res. L.&c. 6, 479. Bering, E. A. (1983) A sounding rocket observation of an apparent wake generated parallel electric field. J. geophys. Res. 88, 96 1. Berthelier, J. J. and Sturges, D. J. (1967) Simultaneous measurements of electron density and temperature in the aurora1 zone. Planet. Space Sci. 15, 1049. Buneman, 0. (1959) Dissipation ofcurrents in ionized media. Phys. Rev. llS, 503. Calvert, W. (198 1) The aurora1 plasma cavity. Geophys. Res. Lecc. 8, 919. Duprat, G. R. J., Bernstein, W., McNamara, A. G. and Whalen, B. A. (1983) Measurements of the stability of energetic electron beams in the ionosphere. J. geophys. Res. 88, 3095. F&hammer, C.-G. (1977) Problems related to macroscopic electric fields in the magnetosphere. Rev. Geophys. Space Phys. 15,457. Getty, W. D. and Smullin, L. D. (1963) Beam-plasma discharge: buildup of oscillations. J. appl. Phys. 34, 3421. Gupta, S. P. (1988) Expansion of plasma in the wake region of moving rockets-evidence of enhanced electron temperature. Adv. Space Res. 8, 225. Gussenhoven, M. S., Hardy, D. A., Rich, F., Burke, W. J. and Yeh, H.-C. (1985) High-level spacecraft charging in
the low-altitude polar aurora1 environment. J. geophys. Res. 90, 11009. Haerendel, G. (1988) Cosmic linear accelerators, in Proceedings of the Joint Varenna-Abastumani International School and Workshop on Plasma Astrophysics, Varenna,
Italy (24 Aug.-3 Sept. 1988),pp. 374. ESA-SP 285 (Vol. I). Ingssy, P. and Friedrich, M. (1983)Plasma heating generated by a rocketborne electron accelerator, in Active Experiments in Space (Compiled by Burke, W. R.), pp. 129-136. ESA SP 195. Ingsery P., Lebreton, J. P., Maehlum, B. N. and Trsim, J. (1986) Plasma energization in the Shuttle wake region during beam injection from Spaceiab 1. Planet. Space Sci. 34,555.
Jacobsen, T. A. (1982) Observations of plasma heating effects in the ionosphere by a rocket borne electron accelerator, in ArtiJicial Particle Beams in Space Plasma Studies (Edited by Grandal, B.), pp. 175-197. Plenum Press, London. Jost, R. J., Anderson, H. R. and McGarity, J. 0. (1980) Electron energy distributions measured during electron beam/plasma interaction. Geophys. Res. Left. 7, 509. Kainer, S., Dawson, J. and Shanny, R. (1972) Interaction of a highly energetic electron beam with a dense plasma. Phys. Fluids 15,493.
Kan, J. R. (1982) Towards a unified theory of discrete auroras. &ace &i. Rev. 31,71. Kaneko. 0.. Sasaki. S. and Kawashima. N. (1978) Active experiment in spa& by an electron beam. IS‘AS desearch Note, No. 85. Katz, I., Cooke, D. L., Mandell, M. J., Parks, D. E. and Rubin, A. G. (1984) Three-dimensional wake model for low earth orbit. J. Spacecraji 21, 125. Klyueva, N. M. (1973) Rocket measurements of electron concentration and electron temperature in the polar ionosphere, in Space Research XIII, p. 1043.Akademie Verlag,
Berlin. Maehlum, B. N. (1988) Beam-plasma experiments. Comp. Phys. Commun. 49, 119. Managadze, G. G., Riedler, W., Balebanov, B. M., Friedrich, M.. Gaeua. T. T.. Kloz. Z.. Laliashivili. N. A.. Leonov. N. A., fyadhov, S: B., Ma&son, A. A.,Mayo&v, A. D.; Buruchudladze, A. A., Torkar, K. M. and Zbyszynski, Z. (1988) Potential observations of an electron-emitting rocket payload and other related plasma measurements. Planet.;sdace
Sci. 36, 399.
_
Mavnard. < , N. C. (1972) Electric fields in the ionosuhere and magnetosphere, in Magnetosphere-ionosphere Interactions (Edited by Folkestad, K.), pp. 155. Universitetsforlaget, Oslo. Medved, D. B. (1969) Measurements of ion wakes and body effects with the GeminilAgena satellite. RareJied Gas Dyn. 1, 1525. Murphy, G., D’An.gelo, N., Kurth, W. S. and Picket& J. (1586) Measurem&ts of plasma parameters in the vicinity of the Space Shuttle. Planet. Space Sci. 34, 993. Papadopoulos, K. (1977) A review of anomalous resistivity for the ionosphere. Rev. Geophys. Space Phys. 15, 113. Papadopoulos, K. and Szuszczewicz, E. P. (1988) Current understanding and issues on electron beam injection in space. Adv. Space Res. 8, 101. Persoon, A. M., Gurnett, D. A., Peterson, W. K., Waite, J. H., Jr. Burch, J. L. and Green, J. L. (1988) Electron density depletions in the nightside aurora1 zone. J. geophys. Res.
93, 1871.
Wake phenomena observed by an ionospheric sounding rocket Pierce, J. R. (1948) Possible tluctuations in electron streams due to ions. J. appl. Phys. 19,231. Samir, U. and Fontheim, E. G. (1981) Comparison oftheory and in situ observations for electron and ion distributions in the near wake of the Explorer 31 and AE-C satellites. Planet. Space Sci. 29,975.
Samir, U. and Wrenn, G. L. (1969) The dependence ofcharge and potential distribution around a spacecraft on ionic composition. Planer. Space Sci. 17,693. Samir, U. and Wrenn, G. L. (1972) Experimental evidence of an electron temperature enhancement in the wake of an ionospheric satellite. Planet. Space Sci. 20, 899. Singh, N., Samir, U., Stone, N. H. and Wright, K. H., Jr. (1987) A possible explanation of the electron temperature enhancement in the wake of a satellite. J. geophys. Res. 92,610O.
Siskind, D. E., Banks, P. M., Raitt, W. J. and Williamson, P. R. (1984) Interactions between the orbiting Space Shuttie and the ionosphere. Planet. Space Sci. 32: 8s 1. Stadsnes. J.. Tooohol. K. R. and Mbeide. K. f 1986) Coordinated rockki measurements of aurora1 ‘X-rays and optical emissions, in Proceedings ofthe 13th Annual Meering on Upper Atmosphere
Studies by Optical Methocis
(Edited by MBseide, K.), p. 91. University of Oslo Report 8628.
405
Stone, N. H. (1981) The aerodynamics of bodies in a rarefied ionized gas with the applications to spacecraft environmental dynamics. NASA Technical Paper 1933. Svenes, K. and T&m, J. (1987) Preliminary results from the electron temoerature probe on MAZMZK. FFI/ RAPPORT-87/7093. Svenes. K.. Troim. J.. Friedrich. M.. Torkar. K. M. and Holmgren, G. (1988) Ionospheric plasma measurements from the accelerator rocket MAZMZK. Planer. Space Sci. 36,1509.
Troy, B. E. Jr., Maier, E. J. and Samir, U. (1975) Electron temperatures in the wake of an ionospheric satellite. J. geophys. Res. 80,993.
Winckler, J. R. (1980) The application of artificial electron beams to magnetospheric research. Rev. Geophys. Space Phys. 18, 659. Winglee, R. M., Pritchett, P. L., Dusenbery, P. B., Persoon, A. M., Waite, J. H., Jr. Moore, T. E., Burch, J. L., Collin, H. L., Slavin, J. A. and Sugiura, M. (1988) Particle acceleration and wave emissions associated with the formation of aurora1 cavities and enhancements. J. geophys. Res. 93, 14567. Yeh, H.-C. and Gussenhoven, M. S. (1987) The statistical electron environment for defense meteorological satellite program eclipse charging. J. geophys. Res. 92,7705.
Planer. Sci., Vol. 38, No. 3, pp. 395-405, 1990 Printed in Chat Britain.
00324633po s3.oo+o.M) Pcrgamon Pms plc
WAKE PHENOMENA OBSERVED BY AN IONOSPHERIC SOUNDING ROCKET K. R. SVRNKS, J. TR0IM
and B. N. MAKHLUM
Norwegian Dcfence Research Institute, P.O. Box 25, N-2007, Kjeller, Norway and
Max-Planck-Institut
K. WILHELM fiir Aeronomie, P.O. Box 20, Max-Planck-Stral3e 2, D-341 1 Katlenburg-Lindau, F.R.G.
(Received 10 July 1989) Abstract-Measurements from the ionospheric sounding rocket POLEWARD LEAP have been analyzed in order to study the nature of wake phenomena. Some outstanding events, which apparently resulted from the interaction between the incoming particles and the F-layer plasma, are discussed. It is shown that the suprathermal electron fluxes observed during the auroral particle precipitation were clearly spin modulated, and that they usually occurred in the wake region of the rocket. In addition, background measurements of both the aurora1 particle precipitation and the ambient plasma are presented.
1. INTRODUCTION The
existence of a region of depleted plasma density behind a vehicle travelling through space has long since been recognized. Even from the measurements carried out to monitor the ambient plasma during such comparatively early space flights as that of Gemini-Agena 10 and 11 (Medved, 1969) and Explorer 31 (Samir and Wrenn, 1969), this phenomenon was readily evident. Since then a fair amount of data has been obtained, related to the nature of the plasma distribution around an orbiting spacecraft. In order to explain these measurements, it has been necessary to take into consideration the typical speed of a space vehicle moving through the magnetospheric plasma. For rockets this speed is about l-3 km s-‘, whereas satellite speeds may approach 10 km s-‘. In both cases it usually means that the vehicle moves through space with a speed comparable to the thermal speed of the ambient ion population. This speed ranges from N 1 km s-’ for an ion temperature of 1500 K (O:), typical of the lower ionosphere, to w 16 km s-’ for an ion temperature of 10,000 K (H+), typical of the outer magnetosphere. The corresponding thermal speeds of the electrons are in the region of _ 25&- 700 km s-‘. Accordingly, the ambient ions are not capable of filling in behind the vehicle before some finite time after its passage. The electrons, however, will instantaneously attempt to fill in this void. Since a charge imbalance is then created, polarization fields are consequently set up to impede the movement of the
electrons. In this way a region of depletion in both the electron and the ion population arises in a direction nearly antiparallel to the velocity vector. This region is called the wake. Computer simulations of the ion density depletions in the wake region have been carried out by e.g. Katz et al. (1984). Such studies have been compared with in situ observations, e.g. by Samir and Fontheim (1981), and found to be in good qualitative agreement. In addition, measurements from the Space Shuttle have shown that the electron density in the wake region close to the vehicle might be orders of magnitude below the ambient density ; see e.g. Ingsoy et al. (1986) and Murphy et al. (1986). Indeed, the ratio of the wake density to the ambient density has, in the case of the space Shuttle, been found to be as small as 10m4according to Siskind et al. (1984). The wake region is also characterized by an enhanced electron temperature when compared with the ambient plasma. This has been shown by Samir and Wrenn (1972) from satellite measurements and by Murphy et al. (1986) in the case of the Space Shuttle. Several mechanisms have been proposed to explain this. Samir and Wrenn (1972), Troy et al. (1975) and Stone (1981) have all suggested selective effects due to potential wells near the vehicle, plasma instabilities and/or turbulence in the wake region, as causes for such temperature enhancements. Murphy et al. (1986) proposed adiabatic compression and, consequently, .heating of the electrons in the wake region as a possible source. Singh et al. (1987) have explained the increase by counterstreaming electron
395
K. R. SVENES et al.
396
populations interacting with each other to produce a single “hot” electron population in the wake. However, this question still remains unresolved. Finally, it should be noted that Siskind et al. (1984) observed an electron temperature increase in the ram direction (parallel to the velocity vector) during the third Space Shuttle flight. It was suggested that this temperature enhancement might in some way have been connected to the generation of local plasma turbulence due to the interaction of the ambient ionosphere and neutrals outgassing from the Shuttle. However, these measurements have been challenged by Murphy et al. (1986), and the issue consequently remains unsettled. In any case it serves as a useful illustration of the dynamical and intricate nature of the interaction between a moving space vehicle and the ambient plasma. 2. DESCRIFWON OF THE INSTRUMENTS
As is evident from the preceding text not many observations of rocket wakes have been reported in the literature. Berthelier and Sturges (1967) reported spin modulations in both electron density and temperature measurements, which were ascribed to wake effects, during a rocket flight in the aurora1 zone. Enhanced electron temperatures near the apogee of some rocket flights have also been reported. These measurements have been attributed to wake effects which becomes prominent near apogee where the rocket moves subsonically and the velocity vector is nearly normal to the rocket axis (Klyueva, 1973 ; Gupta, 1988). Bering (1983) also suggested a wake generated parallel electric field as explanation of measurements carried out during a sounding rocket flight in the aurora1 zone in 1978. During the Norwegian rocket programme, enhanced electron temperatures in the wake zone have been observed in the flight of MAIMIK (Svenes and Tr&m, 1987). In addition, analysis of data from the POLEWARD LEAP flight have previously revealed other interesting effects apparently connected to the wake region (Maehlum, 1988). These effects will be further discussed following a brief description of the instrumentation. POLEWARD LEAP was launched on 11 November 1983 at 21:08 U.T., and it reached an altitude at apogee of 454 km. The launch occurred during the recovery phase of a large substorm which had commenced about one and a half hours earlier. Indeed, the purpose of the flight was just to study this phase ofpoleward movement in the aurora1 arc system accompanying the recovery phase of the magnetospheric substorm. A more detailed description of the flight is given in Stadsnes et al. (1986).
In the present context, data from only two of the instruments on board are utilized in addition to the altitude measurements. Those were the suprathermal particle spectrometer and the electron tem~rature probe provided by the Mu-Planck-Institute for Aeronomy and the Norwegian Defence Research Establishment, respectively. Both of these instruments performed flawlessly during the whole flight. The Suprathermal Electron and Proton Spectrometer, later referred to as the SEPS, provided particle m~s~ments in the energy range from 15 eV to 30 keV for both electrons and protons. The sensors were designed as nested electrostatic analyser systems followed by continuous channel electron multipliers (CEM), and were mounted at 25” and 115” with respect to the rocket axis. Each sensor could detect both species. The conversion factors of the sensors were approx. 3+10e4 E cm2 sr eV for electrons and 810-’ E cm2 sr eV for protons, where E is the instantaneous particle energy during the instrument sweep. The energy resolution was 12-13%, and the entire energy sweep lasted 850 ms. The Electron Tem~rature Probe, later referred to as the ETP, measured the electron part of the current towards the payload. It consisted of a spherical grid 5 cm in diameter surrounding a collector with a bias of + 10 V relative to the payload ground. The amplifier was linear with automatic decade switching, enabling current measurements in the range from about lop9 to low5 A. During operation the potential of the outer grid of the probe was continuously changed. Each such potential sweep was aimed at obtaining an integral electron spectrum. This was done by sweeping the potential of the grid relative to the payload ground from -2.0 to +3.0 V. This procedure was then repeated, yielding the characteristic “saw-tooth” appearance of the measurements shown in Fig. 1. The slope of the linear part of the curve is proportional to the ambient electron temperature, and the point on the voltage axis corresponding to the point where the curve breaks off from the straight line approximation gives the potential difference between the vehicle and the ambient plasma. Finally, the current measured at that point is proportional to the ambient electron density. This standard analysis technique has been applied to all data obtained during the flight. Returning again to Fig. 1, the upper part shows a sweep obtained during a period without any appreciable particle precipitation. The slope of the sweep indicates an ambient electron temperature of about 2400 K. In the lower part, a sweep is shown which was obtained during a period with substantial particle
Wake phenomena observed by an ionospheric sounding rocket
397
3. AMBIENT PLASMA MEAS-
3 $ i
l.OOE-06
l.WE-07 i.ooEaS
l.OOE~
r.ees-lo’.
I ! t ! . . . . ! r . . , ! e . . . ! . !i4!i.978 546.428 !i46.128 344.278
FIG. I. Two mmps OF THE ETP. The sweep in the upper part was carried out during unperturbed conditions, whereas the sweep in the lower part was carried out during a period with significant suprathermal electron production (shaded area),
precipitation. Since the slope of this sweep is much less steep, the ambient electron temperature at that time was much higher, about 3200 K. In addition a population of suprathermal electrons was then present, resulting in the rise of the lower level in the left part of the sweep (shaded area). In order to obtain a measure of the amount of such suprathermal electrons present at any time, the currents measured during each of the first 75 voltage steps of each sweep (a complete sweep consisted of 5 12 voltage steps) were integrated into a single parameter. This parameter then represented the amount of suprathermal electrons, roughly in the energy range between 3.5 and 4 eV, measured during each sweep. Using this number as an ordinate and plotting it against the time of flight, it was possible to quantify the variation of suprathermal electrons measured during the flight.
In the present paper we are mostly concerned with wake phenomena, but we include here a short discussion of the ambient plasma rn~~~ents in the ionospheric F-layer in order to complete the picture. These data are plotted in Fig. 2 which covers the period from 100 to 600 s time of flight. The corresponding altitude interval was from about 205 km on the upleg to 170 km on the downleg. In the upper part of this figure the data from the upward pointing channel of the SEPS are plotted with electron energy as the ordinate and the intensity of the fluxes coded in the colour scale on top of the figure. The second part of the figure shows the ambient electron temperature as estimated from the data of the ETP, and in the third part the ambient electron density (likewise obtained) is displayed. Returning again to the upper part of the figure, it is seen that electron precipitation was observed in strong bursts during the whole flight, extending in energy range from below 100 eV to some keV. This is normal for aurora1 particle pr~ipitation. As is clear there was a very good correspondence between the electron fluxes, especially below about 1 keV, and variations in the ambient electron temperature and density, i.e. every increase of the electron flux was accompanied by an enhancement of the ambient electron temperature and a s~ult~eous decrease in the electron density. Some of these events were also associated with the creation of suprathermal electron populations as illustrated in the lower part of the figure. Here the ordinate of the plot is integrated current measured during the first part of the sweep of the ETP, as previously explained, representing a measure of the amount of suprathermal electrons observed. As seen from the figure, four instances of significant suprathermal electron production were detected during the flight. All of these occurred inside periods of strong particle precipitation and major temperature enhancements. These events wilt be discussed further later on in the text. Perhaps the most conspicuous feature in this set of data is the excellent agreement between the bursts of precipitating electrons, electron density depletions and electron temperature enhancements. Since this remarkable consistency in the m~s~ements persisted during the whole flight it seems reasonable to assume a common cause, for all events, which is in some way connected to the aurora1 precipitation. It is well known that regions of plasma cavity are observed in the aurora1 areas at high aititudes during periods of geomagnetic activity (e.g. Benson and Cal-
398
K. R. Sm
vert, 1979 ; Calvert, 1981; Persoon et al., 1988). Such regions are believed to be associated with the acceleration of aurora1 electrons and the generation of aurora1 kilometric radiation (Calvert, 1981). These observations admittedly refer to an altitude range of a few Earth radii, but lately observations of regions of plasma depletions have also been reported at heights below 1000 km (Gussenhoven et al., 1985; Yeh and Gussenhoven, 1987 ; Haerendel, 1988). A model describing such observations has been proposed by Winglee et al. (1988). They assumed that the energy input of the events is provided by processes in the magnetotail, and mediated through energetic ion beams originating in the plasma sheet boundary layer. These beams are accompanied by costreaming low energy electrons. However, numerical simulations have shown that due to the great difference in the gyroradius of the particles charge separation will occur, leaving the electrons on separate field lines surrounded by the ions. This charge separation will then lead to downward acceleration of the electrons on the field lines with excessive negative charges, whereas a return current of electrons from the ionosphere will be created on the field lines with excessive positive charges. In such circumstances, the plasma depletions observed during the flight of POLEWARD LEAP might be explained as the residual effects of plasma cavity formations in the return current regions at higher altitudes. The observed precipitating low energy electrons would then probably he due to particle scattering from other regions. As for the temperature enhancements these could be the result of either turbulent processes in the ionospheric plasma due to the great energy input, or beam-plasma interactions between the precipitating electrons and the ambient plasma. This view is supported by the “map” of the precipitation region obtained by X-ray and photometer observations from POLEWARD LEAP (Stadsnes et al., 1986). It was concluded here that a “hole” in the precipitation region existed north of the launch site roughly in the direction where the rocket was headed. By “hole” the authors referred to a region of low electron precipitation which contained some proton precipitation. It must, however, be pointed out that these observations might also be explained by another model. Since the aurora1 precipitation is observed to be filamentary by nature at low altitudes, this “hole” may just have constituted a region of unusually low precipitation. Still, that precipitation may have been enough to form a “cold” electron beam moving through the ionospheric plasma. This pr~ipitation
et al. could have been initiated and sustained through a variety of mechanisms previously suggested in the literature such as anomalous resistivity, double layers, magnetic mirror effects, AlfvCn waves or other more exotic models. For a review of these mechanisms see e.g. FPlthammer (1977), Papadopoulos (1977) and Kan (1982). All of these mechanisms are able to account for an electron stream which in effect will constitute a “cold” beam with respect to the thermal F-layer plasma. In such a case this beam would have been likely to interact with the ambient plasma and to transfer energy through wave-particle processes. This energy could have been deposited both as internal energy of the ambient plasma and as pressure-volume work on the s~ro~dings. Thereby both the electron temperature enhan~ments and the electron density depletions would have been accounted for. 4.WAKE EFFFXXS
The suprathermal electron data displayed in the bottom part of Fig. 2 are plotted again in greater detail in Fig. 3. It is evident that the fluxes of suprathermal electrons were strongly spin modulated in all four cases. However, the suprathermal population detected in the period between 540 and 560 s was still outstanding since the spin modulation was then superposed on a general increase of suprathe~al electrons during part of this particular period. Figure 4 shows the latter data again superposed on a plot of the angle between the velocity vector and the direction vector of the ETP. From this plot it is clear that the suprathermal electron production was enhanced in the wake area of the rocket. (The largest angle between the direction vector of the ETP and velocity vector is obtained when the probe is situated in the wake region.) It is also seen that the presence of suprathermal electrons was increasing both in the ram and the wake direction in the period from 540 to about 547 s. The production rate than decreased generally, but the decrease was especially rapid in the ram direction. Indeed, from about 555 s almost all the suprathermal electrons were detected in the wake direction. Closer inspection of the other events shown in Fig. 3 revealed that the suprathermal electrons were observed only in the wake area in all three of these cases. Further examination of the observations also indicates that the maximum amount of suprathermal electrons was observed just before the probe entered the deepest nominal wake position. (The nominal wake position corresponds at any time to a direction antiparallel to the velocity vector of the rocket.) In
Wake phenomena observed by an ionospheric sounding rocket
125 I
00E+*7
250
375
“__^ ..7__.. __._...,.. _ . :-... .. . ...__._._.,,_,_. __” _._..... i.__..-...
399
Wake phenomena observed by an ionospheric sounding rocket
FIG. 3. SPIN
MODULATED SUPRA TIilXMAL
ELECTRON MEASURE-
MENTS FROM FOUR DlTFT.REN’C PERIODS.
401
the case shown in Fig. 4 the angle of difference could amount to as much as 30-40”. As previously mentioned, the temperature of the main electron population also increased during these events. Further examination of the measurements from the period 540-560 s reveals that the enhancements of the suprathermal electron population and the increase in the temperature of the main electron population occurred simultaneously. This is illustrated in Fig. 5. The top part shows the temperature of the ambient electron population, whereas the middle plot shows the amount of suprathermal electrons detected. Evidently the suprathermal electron production peaked simultaneously with the maximum obtained temperature in the ambient plasma population. The lower part of this figure shows electron flux observations from the SEPS at some selected energies. It is seen that during this period the particle precipitation gradually increased in energy, and that the peak in the two curves above occurred at about the time of peak electron precipitation at around 500 eV. As far as the proton data are concerned, very low fluxes were measured during most of the flight. However, in the period 540-549 s the fluxes of protons in the energy range between 80 and 800 eV increased by roughly a factor of 10. Indeed, this was the only period of the flight during which the proton fluxes deviated appreciably from the usual background measurements. It was also due to these low fluxes that the measurements had to be integrated for such a long time, leaving us with a somewhat coarse time resolution for the ion distributions. All the above-mentioned data indicate that beamplasma interactions occur in the ionospheric F-layer during typical aurora1 precipitation events. The notion of heating a plasma by passing a “cold” electron beam through it is fairly well supported by both theory (e.g. Pierce, 1948 ; Buneman, 1959 ; Kainer et al., 1972) and laboratory experiments (e.g. Getty and Smullin, 1963 ; Jost et al., 1980). Indeed, injections of artificial electron beams from rockets in the ionosphere itself have repeatedly lead to observations of electron temperature enhancements (e.g. Kaneko et al., 1978; Winckler, 1980; Duprat et al., 1983; Jacobsen, 1982 ; Ingsoy and Friedrich, 1983 ; Svenes et al., 1988 ; Managadze et al., 1988). The reason that the events occurred only after the rocket had passed through apogee was probably that the ETP then entered deeper into the wake zone. This is due to the typical geometric relationship of rocket flights. In the latter half of the flight, the angle between the velocity vector and the rocket body axis becomes greater, leaving the probes deeper in the “shadow” of
K. R. SVENES et al.
402
A
R
!i
Ii
ii
!I I
ii
I i i i
ii
I_
!
fi i 1 !
IL ! ntlL. i
67.5
i i i !’
!i
!!
I\ ii
I i .
I i .
! !
i
!
i j i !
i i ! I i i i i
!
i
!
i J
*i 11
i i !!
i_i
‘\i
I
00 5.0
FIG. 4. THEANGLE
BETWEEN THE DIRECTION VECTOR OF THE
ETP AND
BROKEN LINE, AND THE MEASUREMENTS OF SUPRATHERMAL
the rocket for part of the spin. Thus, in this case it meant that the observations of suprathermal electrons were made only when the ETP entered deep into the wake zone. It should also be mentioned here that no spin modulation could be detected in the electron temperature or density measurements. However, this was not expected either since the time resolution of those measurements was rather poor. As explained previously the electron temperature and density were estimated from the sweep of the ETP. During the POLEWARD LEAP flight though, the sweep rate was such that the rocket revolved through an angle of 83” during one sweep of the ETP. Thus, on the basis of these measurements it was not possible to resolve the wake region from the ambient ionospheric plasma. On the other hand, the suprathermal electron population was just estimated from an integral of the first part of the sweep as previously explained. During this period the rocket revolved through an angle of about 12”, thereby yielding a much better time resolution of this measurement. In fact, as seen from Fig. 3, the time resolution was good enough to enable detection of the wake region. From theoretical considerations it is well known that an underdense electron beam passing through a thermal plasma will produce suprathermal electrons
I
564
boo
THE VELOCITY VECTOR PLQTTED WITH
ELECTRONS PLQTTBD WITH FULL LINE.
in inverse proportion to the difference between the respective densities (see e.g. Kainer et al., 1972 ; Papadopoulos and Szuszczewicz, 1988). Thus, the smaller the difference between the beam and the ambient plasma densities the more effective the suprathermal electron production. This is entirely in accordance with the observations from the POLEWARD LEAP flight. Since the electron density was probably lower inside the wake zone of the rocket than in the ambient plasma, but still higher than in the beam, the production of suprathermal electrons was much more effective here. As seen from Fig. 3, this is exactly what the observations implied. Thus, it seems likely that the low energy electron precipitation observed can be modelled by an electron beam heating the ambient ionospheric plasma through waveparticle interactions. In addition, this beam would also produce suprathermal electrons when it passed through the low density plasma in the wake of the rocket. Another interesting factor to note here is that the maximum suprathermal electron population in some instances seemed to occur somewhat outside the geometrical wake zone. If indeed the region of maximum suprathermal electron production corresponds to the most pronounced wake region, then this means that the wake had been shifted relative to the geometric
Wake phenomena observed by an ionospheric sounding rocket 3.505*03
1
,
3.251+03 g
!
3.OOE+03
d 2.751+03
2SOE+O3.~i 540.000
541Om
550.m
555.wO
550.000
1.Om-O5
s
1.00&05
I a
1.00547
l~ooE-oo,,..,.-..,....,...., 55o.ooo
5
s
403
km s-’ would have shifted the wake axis some 30”. Such a drift velocity could have been caused by a 60 mV m-r electric field pointing due North, which is a field strength well within the range normally measured in the aurora1 ionosphere (see e.g. Maynard, 1972). However, there are unfortunately no electric field measurements available from POLEWARD LEAP to confirm this assumption, Finally, from Fig. 5 it is seen that during the period between 540 and about 550 s the augmented suprathermal electron population in the wake region was superposed on a general increase of the suprathermal electron population. The maximum of this population occurred simultaneously with the maximum of the ambient electron temperature, and also simultaneously with the maximum flux of 500 eV electrons. It has to be added though, that this period also was the only period during which a significant amount of ions were detected. The ion measurements did not have a sufficient time resolution to enable a direct comparison with the parameters plotted in Fig. 5, but a distinct flux of ions in the energy range 80-800 eV was definitely present during this period. Since this event was also unique with respect to the total suprathermal electron production it is reasonable to assume that the resulting plasma response was due to the flux of ions. Thus, this might be an indication of an interaction between an ion beam and the ionospheric plasma.
10
;i
5.
f, =: X
Summarizing the above measurements and discussion yields the following empirical conclusions on the results from the POLEWARD LEAP flight :
2 lo,
-In
I
104
540
CONCLUSION
545
550
555
560
FROM THE PEXIOD 56560 S. FIG. 5. &Xl-RON The ambient electron temperature is shown in the upper part, the suprathermal electron measurements in the middle part and electron flux observations at various selected energies in the lower part.
wake position, In the period 540-560 s this difference amounted to more than 30” as seen from Fig. 4. This phenomenon may have been caused by a convection of the whole plasma surrounding the rocket. The most likely agent of such a plasma drift is the E x B force. Since the rocket was moving at a velocity of about 2 km SC’ at this time, a drift velocity of 1.2
the descending part of the flight some periods occurred during which spin modulated suprathermal electron populations were detected. These suprathermal electron populations were observed predominantly in the wake region of the rocket. -During one of these periods the augmented suprathermal electron population in the wake region appeared to be superposed on a general enhancement of the suprathermal electron population. This period coincided with the only period in the l-light during which significant fluxes of precipitating ions were detected. -Measurements of the ambient electron temperature and density and of the precipitating aurora1 electrons revealed a close correlation between electron temperature enhancements,
K. R. SVENESet al.
404
electron density depletions and fluxes of precipitating electrons below 1 keV during the whole passage through the ionospheric F-layer. It has been suggested that all these measurements might be related to various types of beam-plasma interactions. This view is supported both by theoretical considerations and previous experiments. However, since complete measurements of all relevant parameters are not available, this assumption cannot be confirmed conclusively. The effect of the wake region on the measurements is quite definite. Evidence has even been found of a shift of the wake position in relation to the geometrical position of the “shadow” zone “behind” the vehicle. This might have been caused by background electric fields which are often present in the ionosphere during aurora1 conditions. Thus, it has been shown that even a small space vehicle like a rocket modifies the ionospheric plasma to an extent which cannot be entirely disregarded during the interpretation of the low energy plasma measurements. AcknowledgementsThe authors acknowledge the excellent work of the project scientist Finn Ssrls throughout the whole project. We are also grateful to Magne Hgv&g for his help in providing us with the altitude data. This experiment was financially supported by the Royal Norwegian Council for Scientific and industrial Research (NTNF), Space Activity Division. The MPAE instrument was supported by the MaxPlanck-Gesellschaft.
REFERENCES
Benson, B. F. and Calve& W. (1979) ISIS-1 observations at the source of aurora1 kilometric radiation. Geophys. Res. L.&c. 6, 479. Bering, E. A. (1983) A sounding rocket observation of an apparent wake generated parallel electric field. J. geophys. Res. 88, 96 1. Berthelier, J. J. and Sturges, D. J. (1967) Simultaneous measurements of electron density and temperature in the aurora1 zone. Planet. Space Sci. 15, 1049. Buneman, 0. (1959) Dissipation ofcurrents in ionized media. Phys. Rev. llS, 503. Calvert, W. (198 1) The aurora1 plasma cavity. Geophys. Res. Lecc. 8, 919. Duprat, G. R. J., Bernstein, W., McNamara, A. G. and Whalen, B. A. (1983) Measurements of the stability of energetic electron beams in the ionosphere. J. geophys. Res. 88, 3095. F&hammer, C.-G. (1977) Problems related to macroscopic electric fields in the magnetosphere. Rev. Geophys. Space Phys. 15,457. Getty, W. D. and Smullin, L. D. (1963) Beam-plasma discharge: buildup of oscillations. J. appl. Phys. 34, 3421. Gupta, S. P. (1988) Expansion of plasma in the wake region of moving rockets-evidence of enhanced electron temperature. Adv. Space Res. 8, 225. Gussenhoven, M. S., Hardy, D. A., Rich, F., Burke, W. J. and Yeh, H.-C. (1985) High-level spacecraft charging in
the low-altitude polar aurora1 environment. J. geophys. Res. 90, 11009. Haerendel, G. (1988) Cosmic linear accelerators, in Proceedings of the Joint Varenna-Abastumani International School and Workshop on Plasma Astrophysics, Varenna,
Italy (24 Aug.-3 Sept. 1988),pp. 374. ESA-SP 285 (Vol. I). Ingssy, P. and Friedrich, M. (1983)Plasma heating generated by a rocketborne electron accelerator, in Active Experiments in Space (Compiled by Burke, W. R.), pp. 129-136. ESA SP 195. Ingsery P., Lebreton, J. P., Maehlum, B. N. and Trsim, J. (1986) Plasma energization in the Shuttle wake region during beam injection from Spaceiab 1. Planet. Space Sci. 34,555.
Jacobsen, T. A. (1982) Observations of plasma heating effects in the ionosphere by a rocket borne electron accelerator, in ArtiJicial Particle Beams in Space Plasma Studies (Edited by Grandal, B.), pp. 175-197. Plenum Press, London. Jost, R. J., Anderson, H. R. and McGarity, J. 0. (1980) Electron energy distributions measured during electron beam/plasma interaction. Geophys. Res. Left. 7, 509. Kainer, S., Dawson, J. and Shanny, R. (1972) Interaction of a highly energetic electron beam with a dense plasma. Phys. Fluids 15,493.
Kan, J. R. (1982) Towards a unified theory of discrete auroras. &ace &i. Rev. 31,71. Kaneko. 0.. Sasaki. S. and Kawashima. N. (1978) Active experiment in spa& by an electron beam. IS‘AS desearch Note, No. 85. Katz, I., Cooke, D. L., Mandell, M. J., Parks, D. E. and Rubin, A. G. (1984) Three-dimensional wake model for low earth orbit. J. Spacecraji 21, 125. Klyueva, N. M. (1973) Rocket measurements of electron concentration and electron temperature in the polar ionosphere, in Space Research XIII, p. 1043.Akademie Verlag,
Berlin. Maehlum, B. N. (1988) Beam-plasma experiments. Comp. Phys. Commun. 49, 119. Managadze, G. G., Riedler, W., Balebanov, B. M., Friedrich, M.. Gaeua. T. T.. Kloz. Z.. Laliashivili. N. A.. Leonov. N. A., fyadhov, S: B., Ma&son, A. A.,Mayo&v, A. D.; Buruchudladze, A. A., Torkar, K. M. and Zbyszynski, Z. (1988) Potential observations of an electron-emitting rocket payload and other related plasma measurements. Planet.;sdace
Sci. 36, 399.
_
Mavnard. < , N. C. (1972) Electric fields in the ionosuhere and magnetosphere, in Magnetosphere-ionosphere Interactions (Edited by Folkestad, K.), pp. 155. Universitetsforlaget, Oslo. Medved, D. B. (1969) Measurements of ion wakes and body effects with the GeminilAgena satellite. RareJied Gas Dyn. 1, 1525. Murphy, G., D’An.gelo, N., Kurth, W. S. and Picket& J. (1586) Measurem&ts of plasma parameters in the vicinity of the Space Shuttle. Planet. Space Sci. 34, 993. Papadopoulos, K. (1977) A review of anomalous resistivity for the ionosphere. Rev. Geophys. Space Phys. 15, 113. Papadopoulos, K. and Szuszczewicz, E. P. (1988) Current understanding and issues on electron beam injection in space. Adv. Space Res. 8, 101. Persoon, A. M., Gurnett, D. A., Peterson, W. K., Waite, J. H., Jr. Burch, J. L. and Green, J. L. (1988) Electron density depletions in the nightside aurora1 zone. J. geophys. Res.
93, 1871.
Wake phenomena observed by an ionospheric sounding rocket Pierce, J. R. (1948) Possible tluctuations in electron streams due to ions. J. appl. Phys. 19,231. Samir, U. and Fontheim, E. G. (1981) Comparison oftheory and in situ observations for electron and ion distributions in the near wake of the Explorer 31 and AE-C satellites. Planet. Space Sci. 29,975.
Samir, U. and Wrenn, G. L. (1969) The dependence ofcharge and potential distribution around a spacecraft on ionic composition. Planer. Space Sci. 17,693. Samir, U. and Wrenn, G. L. (1972) Experimental evidence of an electron temperature enhancement in the wake of an ionospheric satellite. Planet. Space Sci. 20, 899. Singh, N., Samir, U., Stone, N. H. and Wright, K. H., Jr. (1987) A possible explanation of the electron temperature enhancement in the wake of a satellite. J. geophys. Res. 92,610O.
Siskind, D. E., Banks, P. M., Raitt, W. J. and Williamson, P. R. (1984) Interactions between the orbiting Space Shuttie and the ionosphere. Planet. Space Sci. 32: 8s 1. Stadsnes. J.. Tooohol. K. R. and Mbeide. K. f 1986) Coordinated rockki measurements of aurora1 ‘X-rays and optical emissions, in Proceedings ofthe 13th Annual Meering on Upper Atmosphere
Studies by Optical Methocis
(Edited by MBseide, K.), p. 91. University of Oslo Report 8628.
405
Stone, N. H. (1981) The aerodynamics of bodies in a rarefied ionized gas with the applications to spacecraft environmental dynamics. NASA Technical Paper 1933. Svenes, K. and T&m, J. (1987) Preliminary results from the electron temoerature probe on MAZMZK. FFI/ RAPPORT-87/7093. Svenes. K.. Troim. J.. Friedrich. M.. Torkar. K. M. and Holmgren, G. (1988) Ionospheric plasma measurements from the accelerator rocket MAZMZK. Planer. Space Sci. 36,1509.
Troy, B. E. Jr., Maier, E. J. and Samir, U. (1975) Electron temperatures in the wake of an ionospheric satellite. J. geophys. Res. 80,993.
Winckler, J. R. (1980) The application of artificial electron beams to magnetospheric research. Rev. Geophys. Space Phys. 18, 659. Winglee, R. M., Pritchett, P. L., Dusenbery, P. B., Persoon, A. M., Waite, J. H., Jr. Moore, T. E., Burch, J. L., Collin, H. L., Slavin, J. A. and Sugiura, M. (1988) Particle acceleration and wave emissions associated with the formation of aurora1 cavities and enhancements. J. geophys. Res. 93, 14567. Yeh, H.-C. and Gussenhoven, M. S. (1987) The statistical electron environment for defense meteorological satellite program eclipse charging. J. geophys. Res. 92,7705.