R. Pottelette

Generation of broadband electrostatic noise by electron acoustic solitons

Broadband electrostatic noise (BEN) bursts whose amplitude sometimes reaches about 100 mV m{sup {minus}1} have been observed by the Viking satellite in the dayside auroral zone. These emissions have been shown to be greatly influenced by nonlinear effects and to occur simultaneously with the observation of particle distributions favouring the destabilization of the electron acoustic mode. It is shown that electron acoustic solitons passing by the satellite would generate spectra that can explain the high-frequency part of BEN, above the electron plasma frequency.

Electron-acoustic solitons in an electron-beam plasma system

Electron-acoustic solitons exist in a two electron temperature plasma (with “cold” and “hot” electrons) and take the form of negative electrostatic potential pulses. They develop on a spatial scale of a few Debye lengths and propagate at the electron-acoustic velocity which is intermediate between the two electron thermal velocities. They correspond to local enhancement of the cold electron density. It is shown that the introduction of an electron beam in such a plasma allows the existence of new electron-acoustic solitons with velocity related to the beam velocity. Depending on the beam density and temperature and below a critical velocity of the electron beam, they often have a positive potential signature. In such conditions they correspond to electron density holes for the cold electron population. The properties of these solitons are studied in detail. These results suggest that further analysis of recent observations of electron density holes might provide the means to identify these structures in the magnetospheric plasma.Electron-acoustic solitons exist in a two electron temperature plasma (with “cold” and “hot” electrons) and take the form of negative electrostatic potential pulses. They develop on a spatial scale of a few Debye lengths and propagate at the electron-acoustic velocity which is intermediate between the two electron thermal velocities. They correspond to local enhancement of the cold electron density. It is shown that the introduction of an electron beam in such a plasma allows the existence of new electron-acoustic solitons with velocity related to the beam velocity. Depending on the beam density and temperature and below a critical velocity of the electron beam, they often have a positive potential signature. In such conditions they correspond to electron density holes for the cold electron population. The properties of these solitons are studied in detail. These results suggest that further analysis of recent observations of electron density holes might provide the means to identify these structures in t...

Modulated electron-acoustic waves in auroral density cavities : FAST observations

We report on FAST observations of large amplitude (up to 500 mV m−1) envelope solitary waves at the edges of the AKR source region. These edges are characterized by the presence of two electron populations: a dominant hot (∼keV) component and a minority cold (<60 eV) component. The nonlinear waves are recorded when the spacecraft passes the base of the parallel auroral acceleration region. They form intense packets of electron acoustic waves. The modulation is due to ion acoustic waves. These structures are electrostatic and propagate along the magnetic field at speeds of a few 100 km s−1. They may play a crucial role in the acceleration processes taking place in these regions.

Turbulence generated by a gas of electron acoustic solitons

The characteristics of the electrostatic turbulence generated by a gas of electron acoustic solitons are investigated. Electron acoustic solitons are shown to propagate in magnetized plasmas up to about 30° off the parallel direction without significant changes in their properties relative to the nonmagnetized case. Using the conservation properties of the Korteweg-deVries equation, the velocity distribution function of the soliton gas is calculated in the small-amplitude limit. For low cold to hot electron density ratios and high-mean-square electric fields, a significant number of solitons with high velocities and amplitudes is produced, implying the generation of an intense broadband electrostatic turbulence. These theoretical results are compared with the properties of the broadband electrostatic noise (BEN) emissions observed by the Viking satellite in the dayside auroral zone. The electric spectra generated by the electron acoustic soliton gases which can be derived from Viking plasma and wave observations are calculated numerically. These spectra are shown to agree with experimental data and in particular to explain the high-frequency part of BEN emissions, which lies in a range forbidden for linear electrostatic waves.

Solitary waves and weak double layers in a two‐electron temperature auroral plasma

We show that the main characteristics of ion acoustic solitary waves and weak double layers observed by the Swedish satellite Viking can be well reproduced assuming the presence of two electron components in the auroral plasma. The characteristics of the ion acoustic solitons excited in such a plasma are derived with the help of the Sagdeev potential. The results show that the interactions between the hot and the cold electron component in the presence of a finite ion temperature produce rarefactions of the localized density. Such nonlinear structures exist in a more extended range of plasma parameters than the one previously studied in the small amplitude limit case using the Korteweg-de-Vries equation. We find that the density of the cold population must be always smaller than the hot one, while the hot to cold temperature ratio must be greater than ∼ 10. The characteristics of these structures are quite different from those obtained in the small amplitude limit case and better reproduce the Viking observations in terms of their velocity, width, and amplitude scales.

Auroral plasma turbulence and the cause of auroral kilometric radiation fine structure

Electron holes excited in the auroral kilometric radiation (AKR) source region in presence of a very dilute cool electron background are interpreted as causing the observed fine structure in AKR radiation. Using high time and frequency resolution measurements of the FAST wave tracker, we demonstrate that a substantial part of the AKR emission consists of a large number of elementary radiation events that we interpret as traveling electron holes that may have resulted from the nonlinear evolution of electron acoustic waves and have the properties of Bernstein-Green-Kruskal modes. Estimates of the propagation velocity of these structures are in good agreement with theory. Power estimates show that each elementary radiation event may contribute ≃ 103–4 W of power to AKR implying that a moderately large number of elementary radiators is required to reproduce the total AKR emission. The elementary radiation structures are sometimes reflected from the acceleration potential or become trapped in larger structures like ion acoustic waves or ion holes. The observations indicate that the radiation efficiency is highest at the turning point where the velocity of the elementary radiators with respect to the reflector system vanishes. Monitoring the time variation of the frequency drift of the elementary radiators allows to qualitatively infer about the mesoscale motion of the AKR source region and the spatial extension of the mesoscale field-aligned electric potential drops.

Plasma diffusion at the magnetopause? The case of lower hybrid drift waves

We have recalculated the diffusion expected from the quasi-linear theory of the lower hybrid drift instability at the Earths magnetopause. The resulting diffusion coefficient is marginally large enough to explain the thickness of the boundary layer under quiet conditions, based on observational upper limits for the wave intensities. Thus one possible model for the boundary layer could involve equilibrium between the diffusion arising from lower hybrid waves and various loss processes.

Parametric study of kinetic Alfvén solitons in a two electron temperature plasma

The nonlinear regime of the kinetic Alfven wave is studied in a collisionless low-β plasma that is composed of a cold ion population and of two electron populations. In the one electron population plasma, it is known that solitary kinetic Alfven waves (SKAW) propagate either at the Alfven velocity υA or at the ion-acoustic velocity cs. In this latter case, only plasma compressions exist. In the two electron population plasma, rarefactions, as well as compressions, may exist at this velocity. Furthermore, when the electron-acoustic mode exists and when υA>υea (vea is the electron-acoustic velocity), the inertia of the cold electron component allows for the existence of compressive SKAW which propagate with a velocity lying in a limited interval above υea. When υA<vea, these solitons are rarefactive and continuously evolve from classical SKAW moving at υA to electron-acoustic-like SKAW moving at υea. The possibility of applying these results to some astrophysical plasma observations is investigated.

Observations of electron conies by the Viking satellite

Electron angular distributions peaked at oblique angles to the magnetic field, electron conies, are frequently found in the Viking data at all magnetic local times, but with a maximum in the dusk sector. Several types of electron conies have been observed by Viking as well as by other satellites. One type is frequently seen below the acceleration region. It is caused by adiabatic motion of electrons with a field-aligned distribution that has a broader angular width than the loss cone. The atmospheric loss of particles will result in a distribution peaked at pitch angles close to the loss cone. Another type is due to heating or acceleration processes at low altitudes and is seen above the acceleration region. Particle and wave characteristics and possible generation mechanisms for this type of electron conic are discussed. The ion arid electron angular distributions observed by Viking indicate that a parallel electric field is present below Viking during this type of event. Ion observations give important information on the processes that produce the electron conies. Almost 200 electron conic events have been detected in the Viking data, that is, in about every third orbit that was studied. Their frequency of occurrence in altitude maximizes in the upper part of the acceleration region, between 10,000 and 11,000 km. The number of events are found to decrease at higher altitudes. This might indicate that diffusion processes, associated with wave generation, remove the anisotropy in the electron distribution function in and above the acceleration region. Wave observations show the presence of both low-frequency waves and waves close to the electron gyro frequency. Acceleration in a fluctuating (approximately 1 Hz) parallel electric field is suggested as a likely mechanism to create the electron conies observed by Viking. The period of the fluctuations is comparable to the travel time, below the acceleration region, of the electrons forming the conic distribution.

Impulsive broadband electrostatic noise in the cleft: A signature of dayside reconnection

Magnetosheath plasma injection events are frequently observed in the dayside oval. From the time dispersion characteristics of the injected magnetosheath ions it is possible to deduce the injection time of magnetosheath electrons. It is shown by means of specific examples taken from the Swedish Viking spacecraft data set, that this electron injection time occurs together with the generation of burst of broadband electrostatic noise (BEN). The overall duration of the BEN emissions lasts typically of the order of 1 min, probably representing the total duration of the injection events. This is within the same time domain as that inferred for flux transfer events (FTE). On a shorter timescale, BEN consists of a succession of very impulsive bursts which last a few seconds; this reveals the characteristic timescales of the elementary injection process. The BEN emissions appear to be the “messengers” of the reconnection process occurring along the magnetic field lines which are connected to the reconnection sites. The information is transmitted via solitary waves, probably of electron-acoustic type, and consisting of isolated potential wells. From the observations, the overall extension of the injection region at the magnetopause is estimated to be of the size of a large fraction of an Earth radius. However, plasma injection appears as a noncontinuous process that takes place in narrow spatially localized regions of the size of merely more than an ion gyroradius at the magnetopause.