Abstract
This review paper compiles recent results obtained by the present group of authors describing the effects of suprathermal populations present in space plasmas (up to a few keVs) on temperature anisotropy instabilities. Of particular interest are the electromagnetic cyclotron and firehose excitations, which play a major role in limiting temperature anisotropy, resulting, for instance, from the adiabatic expansion of the solar wind. Relying on a rigorous modeling and interpretation of the observed velocity distributions, both theoretical models and numerical simulations indicate a systematic stimulation of these excitations in the presence of suprathermal populations of electrons or protons. Moreover, the enhanced fluctuations react back on particles, and determine a faster and deeper relaxation of their anisotropy. The present comparative analysis suggests that previous studies, considering only quasi-thermal low-energy populations, may have significantly underestimated these excitations and their implications in various applications in space plasmas.
Highlights
With the first in-situ explorations of solar wind plasmas in interplanetary space, the kinetic properties of plasma populations became of great interest (Olbert et al, 1968; Vasyliunas, 1968; Gary, 1993)
In order to outline the effects of suprathermals we compare the evolution obtained for the bi-Kappa electrons (κ 3, red lines) with that for bi-Maxwellian limit (κ ∞, blue line). These temporal profiles show a significant stimulation of the instability, with the magnetic wave energy of the O-electron firehose (EFH) fluctuations enhanced in the presence of suprathermal electrons, which in turn leads to a deeper relaxation of the initial temperature anisotropy
In this paper we have reviewed a number of recent results on the instabilities induced by the temperature anisotropy of plasma particles, under conditions typical for the solar wind and planetary environments, e.g., in Refs. (Lazar et al, 2019; López et al, 2019; Shaaban et al, 2021b; Shaaban et al, 2021a)
Summary
With the first in-situ explorations of solar wind plasmas in interplanetary space, the kinetic (microand mesoscopic) properties of plasma populations became of great interest (Olbert et al, 1968; Vasyliunas, 1968; Gary, 1993). In recent years important progresses have been made, both theoretically and numerically, by developing kinetic approaches relying on the anisotropic Kappa distribution functions These models become able to describe the observed suprathermal populations (Pierrard and Lazar, 2010; Lazar et al, 2012), and their contributions to kinetic effects and instabilities of plasma particles (Shaaban et al, 2019a; Lazar et al, 2019; López et al, 2019; López et al, 2021). The free energy of suprathermal particles is expected to stimulate kinetic instabilities, and increase the level of magnetic fluctuations Despite these expectations, a large number of dispersion and stability analyses predict the opposite, especially an inhibition of these instabilities, for many conditions specific to the solar wind (Lazar and Poedts, 2009; Mace and Sydora, 2010; Lazar et al, 2011; Lazar et al, 2013). The last section (Section 5) brings together the main conclusions and a series of perspectives of the present review study
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