[en] Norris et al, [1983] have recently shown that thermal (approx.1 eV) electrons are accelerated along fields lines (up to tens of electron volts) when intense ion cyclotron waves (ICW's) are simultaneously present in the equatorial magnetosphere. In the present paper a mechanism that explains these observations is proposed. It is shown that, owing to the small but finite parallel electric field developed by ICW's in a multicomponent plasma (e.g., containing minor He⁺ ions), electrons having parallel velocities close to the Alfven velocity (at the equator) can be trapped in the potential troughs of this ICW. Then, taking into account the inhomogeneity along geomagnetic field lines, it is shown that the increase of the parallel phase velocity of the ICW as it leaves the equator implies a parallel acceleration of these trapped electrons. The maximum parallel energy that is reached (a few tens of electron volts) is in agreement with observations; it is obtained by matching the trapping force (proportional to E/sub parallel/) to the detrapping force due to the inhomogeneity along field lines

[en] Large-amplitude (10 to 100 μV m^-1 Hz^-1/2) natural radio emissions in a wide frequency range (100 kHz up to 2 MHz) are frequently observed on board the AUREOL/ARCAD 3 satellite at high latitude and at altitudes between 400 and 2,000 km. The simultaneous measurement of the local cold plasma density allows the identification of cutoff and resonance frequencies. Three different kinds of wave are observed: (1) electrostatic emissions near the local value of the plasma frequency (f_p), (2) electromagnetic whistler mode emissions, sometimes associated with type (1) emissions, and (3) electromagnetic Z mode emissions, also associated with type 1 emissions, but occurring more rarely than the whistler mode emissions and then only when f_p is greater than the electron cyclotron frequency (f_ce). These emissions are always associated with high levels of ELF electrostatic turbulence and a high flux of low-energy precipitating electrons, extending in energy down to the lower limit of the detectors (∼ 100 eV). The statistical distribution of the emissions in geomagnetic coordinates shows an occurrence greater than 80% in the polar cusp region and between 25% and 60% in the nightside auroral zone. A generation mechanism for such emissions is proposed, based on the calculation of the growth rate of the kinetic Cherenkov instability, associated with a beamlike suprathermal tail in the parallel distribution of the bulk electron population

[en] A comprehensive set of wave analysis techniques is applied to dayside ELF hiss observed over three typical orbits of the low-altitude AUREOL 3 satellite. Validity domains are established for each technique, whose results indicate the following propagation characteristics. Within the plasmasphere the waves are narrow-band, have a lower cutoff frequency close to the local proton gyrofrequency f_H+, and propagate downward with very oblique k vectors directed toward lower L values. Left-hand mode wave are detected just below f^H+. Within the plasmapause gradient the waves are broad-band, have a lower cutoff frequency at the vicinity of f_H+, and propagate mainly upward with oblique k vectors directed toward lower L values. Within the light-ion trough region and the auroral zones the waves are broad-band, have lower cutoff frequencies that can be below the local f_H+, and propagate downward with k vectors along B₀. Narrow-band emissions detected in the vicinity of the cusp propagate upward. Exceptions are found at frequencies just above f_H+ where, at nearly all invariant latitudes, waves are commonly upgoing. The authors conclude that (1) altitudes and (2) the large density gradients that characterize the plasmapause and the cusp seem to act as traps for waves reflected below the satellite

[en] The detection of left waves in a frequency range where only right waves are predicted by the magneto-ionic theory has raised questions on the validity of the methods used to determine the sense of polarization of electromagnetic emissions. A new method, based on the sign of the imaginary part of the cross-spectrum between two magnetic wave field components is discussed. It is valid for non-plane waves as well as for plane waves. Abnormal polarizations are shown to be correlated with scintillations in the telemetry signal

[en] On board the AUREOL-3 satellite the simultaneous measurement of five field components (three magnetic and two electric components) is carried out for ELF waves in a frequency range from 10Hz to 1.5kHz. At frequencies above the proton gyrofrequency, f_H(H⁺), there is only one mode: the whistler mode. So, we apply our previous direction finding technique to determine the wave normal directions of whistler mode waves in this frequency range. The estimation of the eigenvalues for the spectral matrices enables us to distinguish between the one- and two-direction model. However, the lower frequency of the ELF band is on some occasions, below the proton gyrofrequency, and for this situation the problem becomes very complicated because of the important effect of multiple ion species; H_e⁺ and O⁺ ions in addition to H⁺. The frequency range below f_H(H⁺) can be divided characteristically into four ranges; (1) f_H(H⁺) >f _cr (cross-over frequency), (2) f_cr > f > f_cf (cut-off), (3) f_cf > f > f_H(H_e⁺), and (4) f_H(H_e⁺) > f. In the frequency ranges of (1), (2) and (4), two modes are possible to propagate; R and L mode. While, there is only one mode of R mode in the range of (3). We have developed a direction finding method for the case when there are two possible modes, in which we are able to distinguish the wave mode, i.e. between R and L mode and then find the wave normal direction by means of the maximum likelihood concept. We describe, in some details, the present systematic direction finding system adopted for the ARCAD 3, and some preliminary results are presented. One specific orbit is chosen, for which plasmaspheric ELF hiss is recorded over a wide latitude range from a small L(L=1.3) to the plasmapause region. The variation of the wave normal directions obtained for those ELF emissions is discussed

[en] ELF waves generated by artificial modulation of the auroral electrojet above Tromso have been detected on the AUREOL-3 satellite. The emission is seen on the EH electric antenna at all the modulation frequencies (525, 825, 1125, 1425 Hz) except at 15 Hz where the level of the natural emissions is too high. The amplitude, which increases with the frequency, reaches about 5 microv/m at 1425 Hz. From time to time, a weak signal (about 0.1 mgamma) is detected on the magnetic BX and BZ antennas. The region of observations is very localised but is 3 deg south of the L shell that crosses the expected heating zone

[en] Simultaneous measurements of the fine structure of the topside F-region electron density and the ELF hiss wave characteristics, carried out aboard the AUREOL/ARCAD 3 satellite, give evidence of an efficient ducting mechanism by crests of ionization. Typical horizontal size of these plasma ducts lies in a range 5-50 km. The shape of the density structures as well as wave propagation characteristics exhibit a well marked difference between the inner and outer sides of the equatorward boundary of the mid-latitude ionospheric trough. Moreover, the observation of small scale plasma instabilities and electrostatic turbulence occuring usually on the flanks of density enhancements strongly suggests that the ducts are convecting structures assumed to be created in the auroral zone

[en] A study is made of the formation of the spectra of Langmuir waves excited as a result of the development of beam-plasma instability in a collisionless magnetized plasma with low-frequency turbulence. Equations are derived that describe the dynamics of the formation of spectra in the quasilinear statistical approximation. The equations obtained account for small- and large-angle scattering of the electron-beam-excited waves by given background plasma density fluctuations. The scattering of Langmuir waves leads to the redistribution of their energy in phase space and, under appropriate conditions, to the appearance of a characteristic dent in the wave spectra in the frequency range where the spectral intensity is maximum. Numerical simulations carried out for plasma parameters typical of the polar cap of the Earth's magnetosphere help to explain the shape of the spectra of Langmuir waves that were recorded by the Interball-2 satellite when it was flying through this magnetospheric region

[en] Simultaneous measurements of the fine structure of the topside F-region electron density and the ELF hiss wave characteristics, carried out aboard the Aureol/Arcad 3 satellite, provide evidence of an efficient ducting mechanism produced by crests of ionization. The horizontal size of these plasma ducts lies, typically, in the range 5-50 km. The shape of the density structures as well as the wave propagation characteristics exhibit a well-marked difference between the inner and outer sides of the equatorward boundary of the mid-latitude ionospheric trough. Moreover, the observation of small scale plasma instabilities and electrostatic turbulence, occurring usually on the flanks of density enhancements, strongly suggests that the ducts are convecting structures assumed to be created in the auroral zone. 14 references

[en] The WHISPER relaxation sounder that is onboard the four CLUSTER spacecraft has for main scientific objectives to monitor the natural waves in the 2 kHz - 80 kHz frequency range and, mostly, to determine the total plasma density from the solar wind down to the Earth's plasmasphere. To fulfil these objectives, the WHISPER uses the two long double sphere antennae of the Electric Field and Wave experiment as transmitting and receiving sensors. In its active working mode, the WHISPER works according to principles that have been worked out for topside sounding. A radio wave transmitter sends an almost monochromatic and short wave train. A few milliseconds after, a receiver listens to the surrounding plasma response. Strong and long lasting echoes are actually received whenever the transmitting frequencies coincide with characteristic plasma frequencies. Provided that these echoes, also called resonances, may be identified, the WHISPER relaxation sounder becomes a reliable and powerful tool for plasma diagnosis. When the transmitter is off, the WHISPER behaves like a passive receiver, allowing natural waves to be monitored. The paper aims mainly at the resonance identification process description and the WHISPER capabilities and performance highlighting. (author)