D. Le Quéau

Auroral kilometric radiation sources: In situ and remote observations from Viking

The present paper is a summary of studies carried out from Viking measurements on the propagation and the generation of the auroral kilometric radiation (AKR hereafter). Advantage is taken of the spin modulation of the AKR observed as Viking was rotating in the cartwheel mode. This, together with the study of the cutoff of the various spectral components, confirms that low-amplitude Z and O modes are generated at the same time as a larger-amplitude X mode. Hence Z, O and X mode AKR is all generated by the same sources. The spectrum of the dominant polarization, the X mode, usually contains several spectral peaks. An AKR source crossing is characterized by a minimum in the frequency of the lowest-frequency peak (fpeak) and by a maximum of its amplitude. About 50 AKR source crossings are used to demonstrate that fpeak approaches fce, the electron gyrofrequency: (fpeak - fce)/fce ≈ 0.025 in AKR sources. Similarly, the low-frequency cutoff of the AKR is found, on average, to coincide with fce. The density inside AKR sources is determined from four sets of independent measurements, namely (1) the upper frequency cutoff of the hiss, (2) the relaxation sounder, (3) the Langmuir probe, and (4) particle measurements. It is shown that an AKR source coincides with a strong depletion in the density of the cold/cool electrons that becomes comparable to or less than the density of energetic electrons (E ≥ 1 keV). The total density inside AKR sources is of the order of 1 cm−3, typically a factor 5 to 10 below that of the surrounding regions. AKR sources are found to coincide also with an acceleration region characterized by a potential drop of ≥1 kV, both below and above the spacecraft. Evidence for this comes from the observation of electrons accelerated above the spacecraft and ions accelerated below it. In addition to a strong depletion in the density of the cool electrons, particle measurements on Viking give evidence of several possible free energy sources that could drive unstable the AKR, namely (1) a loss cone, (2) a hole for parallel velocities smaller than that of the observed downgoing electron beam, and (3) a trapped electron component for a pitch angle a ≈ 90°. The trapped electron component, bounded at low perpendicular energies (a few keVs) by an enhanced loss cone, is observed inside, and only inside, AKR sources. It is therefore concluded that the corresponding ∂f/∂v⊥ > 0, for small parallel velocities, is the free energy source that drives unstable the cyclotron maser.

A study of the propagation of Alfvén waves in the auroral density cavities

From the Viking and, more recently, the FAST spacecraft observations, it is known that the auroral acceleration regions correspond to relatively small-scale density cavities (a few kilometers to a few tens of kilometers in a direction perpendicular to the magnetic field). In order to study how Alfven waves can contribute to the auroral acceleration, we analyze their propagation in the presence of the sharp density gradients that characterize the edges of these cavities. It is shown that an Alfven wave packet entering into the cavity quickly develops short perpendicular length scales in the region of density inhomogeneity. Transverse scales of the order of c/ω pe are reached after a propagation of a few tenths of second, which corresponds to ∼ 2000 km in the cavity. This contributes to the creation of significant parallel electric field in the region of density gradient. Its amplitude is enhanced by the formation, on the gradients, of strong space charges due to the ion polarization drift. As the auroral cavities are known to be strong current regions, the density gradients would thus be the sites of particularly powerful wave/particle energy transfer and consequently, if the incident flux of Alfven waves coming from the magnetosphere is high enough, of the strongest particle acceleration.

The eigenmode of solitary kinetic Alfvén waves observed by Freja satellite

The properties of the solitary kinetic Alfven waves (SKAWs) observed by Freja satellite are studied statistically for a typical event on March 7, 1994. The eigenmode of the SKAWs observed is still recognized by their anisotropic structure (1000 km x 1 km), their waveform which is similar to that of a wave packet and their frequency close to the local oxygen ion cyclotron frequency. If these, spatial structures really exist, they may have resulted from the evolution of the SKAWs or some envelope solitons of ion cyclotron and magnetosonic waves, which is supported by four statistical results: (1) The phase velocity is directly proportional to the Alfven velocity. (2) The amplitudes of the electric filed, magnetic field and density fluctuations are directly proportional to the timescales of the pulsations. (3) The amplitudes as well as the timescales are directly proportional to the phase velocity. (4) The polarization sense is both of right circular and left circular mode with different phase relation among the electric field, magnetic field, and density fluctuations. Moreover, the SKAWs are often superposed by a strong electrostatic fluctuations at several hundred hertz, the causal relation between the SKAWs and superposed waves is still unknown.

Measurement of the direction of the auroral kilometric radiation electric field inside the sources with the Viking satellite

The spin modulation of the high-frequency noise from one electric antenna of the Viking satellite has been used to determine for the first time the direction of the polarization plane of the auroral kilometric radiation (AKR) inside the sources of the radiation (i.e., at the local frequency of generation). The observations show a clear difference between the modulation patterns obtained inside the AKR sources and the ones obtained far from them. The chi-square goodness of fit test has been used to compare the measurements made during 36 AKR source crossings with a theoretical model accounting for the specific Viking antenna equipment. This statistical study shows that, inside the sources, the wave electric field is directed perpendicular to the static magnetic field, within 10°. This observation, consistent with a wave generation by the cyclotron maser instability, is also a fundamental constraint on any model attempting to relate the wave observations far from the sources to the physical characteristics of the sources.

Heating of oxygen ions by resonant absorption of Alfvén waves in a multicomponent plasma

Conical distributions of oxygen ions are commonly observed at high altitudes above the auroral zone and in the cusp and cleft regions. The occurrence of these oxygen conics is well correlated with the intensity of waves in the ULF frequency range (of the order of a few hertz). It is shown that the inhomogeneity along the geomagnetic field lines allows a mode coupling between downgoing Alfven waves (either proton cyclotron or magnetosonic) and upgoing oxygen cyclotron waves. Thus the electromagnetic energy flux carried downward with a large phase velocity can be transferred into a low phase velocity left-handed mode that resonates with oxygen ions and heats them. We estimate the corresponding rate of absorption of the incident Alfven waves as a function of the incidence angle and of the parameter ηO+, which is essentially the product of the O+ relative concentration and the characteristic length of inhomogeneity of the magnetic field, normalized to the typical wavelength of the Alfven waves, at the O+ gyrofrequency. For an incident magnetosonic wave (type II branch) the rate of absorption can be quite large (up to 20%), for ηO+ < 10−2. This peak in the absorption coefficient, however, is obtained for a relatively narrow angular range, when the incidence angle is close to 90°. An incident proton cyclotron wave (type III branch) is more likely to lead to efficient O+ heating because (1) two regimes, either at small or at large incidence angle, lead to an absorption rate up to 20%, (2) these maxima are only slightly dependent on the incidence angle, and (3) large absorption rates are expected over a relatively broad range of values of ηO+ (5 × 10−3 < ηO+ < 10−1), compatible with those values that are expected throughout the magnetosphere.

Resonant absorption of downward propagating electromagnetic hiss

Electromagnetic hiss is regularly detected by electric and magnetic wave receiver on board low-altitude satellites at high magnetic latitudes. We solve the full wave equation for electromagnetic magnetosonic waves (MSWs) initially launched earthward at high altitudes above the satellite. Owing to both the inhomogeneity of the medium (variation along the geomagnetic field) and the bi-ionic composition of the plasma (H+, O+), the wave field undergoes a singularity at the altitude where its frequency matches the local gyrofrequency of H+. This phenomenon is accompanied by strong modifications of the wave properties (for example, a polarization reversal) and by a resonant absorption that contributes to the heating of the protons. It is shown that a WKB point of view still provides a qualitative description of the full wave.propagation. The resonant absorption can then be viewed as a three-step process as follows: (1) downward propagation of MSWs followed by (2) a reflection and by (3) a mode conversion into electromagnetic ion cyclotron waves, which are appropriate to heat the protons. During this process wave polarization reverses near the crossover point, where the mode conversion mainly occurs. The amplitude of the component B⊥ is found to undergo a moderate increase at the point where the wave frequency matches the proton gyrofrequency. The analysis is extended to include kinetic effects resulting from the longitudinal motions of the electrons Te. This aims at explaining the strong enhancement of the parallel electric field recorded onboard AUREOL 3. If Te ≫ Ti, it is shown that this E‖. can be interpreted as being due to ion acoustic waves excited by upward propagating electromagnetic ion cyclotron waves. The estimate of the size of E‖ (and of B⊥) allows a precise diagnosis of the mode conversion of the incident electromagnetic hiss and therefore helps understanding how its energy is used to heat the protons.

Kinetic model of Alfvén wave light-ion gyroresonance heating

The absorption of Alfven waves by gyroresonant interaction with the light-ions in an ionospheric oxygen-hydrogen plasma with a parallel magnetic field gradient is reconsidered, taking into account simultaneously the plasma inhomogeneity and the finite temperature of the resonant ions (Ti≠0). A kinetic full wave equation is derived, that is valid in the vicinity where the wave frequency matches the local proton gyrofrequency. It is analytically solved for the case of a Lorentzian distribution function. The energy transmission, reflection and absorption coefficients, for waves incident from the high magnetic field side onto the gyroresonant interaction region, are found to be the same as for the cold plasma case (Ti=0). Conversely, the absorption of waves incident from the low magnetic field side is found to be enhanced and strongly depends on the ionic temperature, whereas their transmission to the high magnetic field side occurs still in the same proportion as for the cold plasma case. Elaborating local d...