H. de Feraudy

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.

Wideband bursts of auroral kilometric radiation and their association with UV auroral bulges

Impulsive wideband bursts of auroral kilometric radiation (AKR) observed from the Interball 2 spacecraft are characterized by a rapid rise of intensity ( 6000 km), the bursts show an upward source expansion, often simultaneous with the downward expansion of the same source below 3000 km. We observe a close association of the AKR bursts with the fast expansion of the auroral bulges recorded with the UV imager on Polar. It is concluded that the bursts are triggered during the rapid development of auroral substorms. The bursts show a microstructure, which consists of multiple microbursts lasting sometimes less than 6 s. They indicate that the source regions are filamented. This filamentation may be the counterpart of a filamentary nature of the currents closing the current wedge, flowing parallel to the geomagnetic field.

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.

Toward a consistent picture of the generation of electromagnetic ion cyclotron ELF waves on auroral field lines

Simultanous Freja observations of precipitating keV electrons and electromagnetic emissions around half the local proton gyrofrequency are analyzed. The observed wave fields are used for reconstructing the energy density in wave vector space, the wave distribution function (WDF), and detailed observations of the electron distribution are used to analyze the linear stability of the plasma. We find that a local Landau resonance with precipitating electrons can generate electromagnetic waves having their wave vector (k) pointing obliquely relative to the background magnetic field. However, the observed parallel electron drift energy suggests that the unstable frequencies are located above the peak in the observed power spectrum and that the k spectrum predicted from the linear stability analysis does not overlap with the reconstructed WDF. This apparent inconsistency between the proposed instability mechanism and the available Freja data is resolved by placing the source region within 2400 km above Freja. By using a ray-tracing mapping of an instability at an assumed generation altitude down to Freja altitudes, we show that an electron Landau instability occurring above Freja can be made consistent with the observed wave frequency as well as the reconstructed WDF. The agreement that we obtain between data and theory leads to consistent picture of the generation of electromagnetic ion cyclotron ELF waves on auroral field lines.

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.

AKR bursts and substorm field line excitation

Abstract A new manifestation of the Auroral Kilometric Radiation is put in evidence through the observations of the POLRAD wave experiment of the INTERBALL mission, the Wide Band Bursts of the AKR, or WBB AKR. It consists in bursts of radiation with a very broad bandwidth, typically 100 – 800 kHz. The whole frequency range is excited at once or in less than few minutes. This corresponds to the excitation of long segments of auroral field lines in a time often less that one minute. The sources of the emission are stretched along the field lines between altitudes ranging from 2000 to 20000 km. The relationship of these bursts to the development of auroral bulges, in the UV spectral range, is shown by the comparison of the POLRAD wave observations and those of the UV imager of the POLAR mission. It is shown that these bursts are generated during fast expansion of the auroral bulge. An accurate timing of the burst events is made with the time evolution of the frequency integrated wave energy flux. It shows that the bursts have a rise time of few minutes which is followed by an exponential relaxation with a characteristic time of few tens of minutes. The source of the bursts first expands along the field lines then it shrinks during the relaxation phase. The bursts are triggered few minutes before the maximum intensity of the UV auroral bulge. It is also shown that the bursts actually consist of a large number of individual broad bandwidth elements, each, lasting less than few seconds.

A new instrument for space plasma exploration: The current density coil

This paper presents an instrument aimed at measuring current densities in space plasmas: a current density coil. Such an instrument already exists for the estimation of currents in the laboratory. A special design has been developed and tested for use on board spacecraft. The characteristics of the instrument are explained in details and many tests performed on the ground are presented. It is shown that the current density coil is sensitive enough to measure ionospheric currents.