No abstract

Broadband electrostatic noise (BEN) bursts whose amplitude sometimes reaches about 100 m V m−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 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.

Abstract. The present paper focuses on the altitude dependence of oxygen ion conics in the dayside cusp/cleft region. Here, combining oxygen data from the Akebono, Interball-2 and Cluster satellites allows, for the first time, one to follow the global development of energetic (up to ∼10 keV) ion outflow over a continuous and broad altitude range up to about 5.5 Earth radii (R E ). According to earlier statistical studies, the results are consistent with a height-integrated energization of ions at altitudes up to 3.5 R E . Higher up, the results inferred from Cluster observations put forward evidence of a saturation of both a transverse energization rate and ion gyroradii. We suggest that such results may be interpreted as finite perpendicular wavelength effects (a few tens of km) in the wave-particle interactions. To substantiate the suggestion, we carry out two-dimensional, Monte Carlo simulations of ion conic production that incorporate such effects and limited residence times due to the finite latitudinal extent of the heating region.

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 ø 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 solitOh gas is calculated in the small-amplitude limit. For low cold to hot electron density ratios and high-mean-s. quare electric fields, a significant number of •olitons with high velocities and amplitudes is produced, implying the generation of an intense broadband electrostatic turbulence. These theoretical results are compared with the propertl'es 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 solitOh 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. 17,415