Astronomy & Astrophysics (A&A) is an international journal which publishes papers on all aspects of astronomy and astrophysics

In the solar wind electron velocity distributions reveal two counter-moving populations which may induce electromagnetic (EM) beaming instabilities known as heat flux instabilities. Depending on plasma parameters two distinct branches of whistler and firehose instabilities can be excited. These instabilities are invoked in many scenarios, but their interplay is still poorly understood. An exact numerical analysis is performed to resolve the linear Vlasov-Maxwell dispersion and characterize these two instabilities, e.g., growth rates, wave frequencies and thresholds, enabling to identify their dominance for conditions typically experienced in space plasmas. Of particular interest are the effects of suprathermal Kappa-distributed electrons which are ubiquitous in these environments. The dominance of whistler or firehose instability is highly conditioned by the beam-core relative velocity, core plasma beta and the abundance of suprathermal electrons. Derived in terms of relative drift velocity the instability thresholds show an inverse correlation with the core plasma beta for the whistler modes, and a direct correlation with the core plasma beta for the firehose instability. Suprathermal electrons reduce the effective (beaming) anisotropy inhibiting the firehose modes while the whistler instability is stimulated.

Heat transport in the solar wind is dominated by suprathermal electron populations, i.e., a tenuous halo and a field-aligned beam/strahl, with high energies and antisunward drifts along the magnetic field. Their evolution may offer plausible explanations for the rapid decrease of the heat flux with the solar wind expansion, and self-generated instabilities, or so-called “heat flux instabilities” (HFIs), are typically invoked to explain this evolution. This Letter provides a unified description of the full spectrum of HFIs, as prescribed by the linear kinetic theory for high beta conditions (β _e ≫ 0.1) and different relative drifts (U) of the suprathermals. HFIs of different natures are examined, i.e., electromagnetic, electrostatic or hybrid, propagating parallel or obliquely to the magnetic field, etc., as well as their regimes of interplay (co-existence) or dominance. These alternative regimes of HFIs complement each other and may be characteristic of different relative drifts of suprathermal electrons and various conditions in the solar wind, e.g., in the slow or fast winds, streaming interaction regions, and interplanetary shocks. Moreover, these results strongly suggest that heat flux regulation may not involve just one but several HFIs, concomitantly or successively in time. Conditions for a single, well-defined instability with major effects on the suprathermal electrons and, implicitly, the heat flux, seem to be very limited. Whistler HFIs are more likely to occur but only for minor drifts (as also reported by recent observations), which may explain a modest implication in their regulation, shown already in quasilinear studies and numerical simulations.

In collision-poor plasmas from space, e.g. solar wind, terrestrial magnetospheres, kinetic instabilities are expected to play a major role in constraining the temperature anisotropy of plasma particles, but a definitive answer can be given only after ascertaining their properties in these environments. This study describes the full spectrum of electron firehose instabilities in the presence of suprathermal electron populations which are ubiquitous in space plasmas. Suprathermal electrons stimulate both the periodic and aperiodic branches, remarkable being the effects shown by the aperiodic mode propagating obliquely to the ambient magnetic field which markedly exceeds the growth rates of the parallel (periodic) branch reported recently in Lazar et al. Derived exclusively in terms of the plasma parameters, the anisotropy thresholds of this instability are also lowered in the presence of suprathermal electrons, predicting an enhanced effectiveness in the solar wind conditions. These results may also be relevant in various other astrophysical contexts where the firehose instabilities involve, e.g. solar flares, sites of magnetic field reconnection, accretion flows or plasma jets leading to shocks and co-rotating interactions in the heliosphere, interstellar medium, and Galaxy clusters.

The increase of temperature predicted by the solar wind expansion in the direction parallel to the interplanetary magnetic field is already notorious for not being confirmed by the observations. In hot and dilute plasmas from space, particle–particle collisions are not efficient in constraining large deviations from isotropy, but the resulting firehose instability provides itself plausible limitations for the temperature anisotropy of both the electron and proton species. This paper takes into discussion the suprathermal (halo) electrons, which are ubiquitous in the solar wind, and may be highly anisotropic and susceptible to the firehose instability. Suprathermals enhance the high-energy tails of the velocity distributions making them well described by the Kappa distribution functions, with the advantage that these are power laws suitable to reproduce either the entire distribution or only the suprathermal halo tails. New features of the instability are captured from a linear stability analysis of bi-Kappa-distributed electrons with the temperature depending on the power-index κ. This approach enables a realistic interpretation of non-thermal electrons and their effects on the instability: growth rates are systematically stimulated and thresholds are lowered with decreasing the power-index κ. In a zero-order limiting approach of the halo component (minimizing the effects of a cooler and less anisotropic core population), the instability thresholds align to the limits of the temperature anisotropy reported by the observations. These results provide new and valuable support for an extended implication of the firehose instability in the relaxation of temperature anisotropy in collisionless plasmas from space.

We report the first results from particle-in-cell simulations of the fast-growing aperiodic electron firehose instability driven by the anisotropic bi-Kappa distributed electrons. Such electrons characterize space plasmas, e.g., solar wind and planetary magnetospheres. Predictions made by the linear theory for full wave-frequency and wave-vector spectra of instabilities are confirmed by the simulations showing that only the aperiodic branch develops at oblique angles with respect to the magnetic field direction. Angles corresponding to the peak magnetic field fluctuating power spectrum increase with the increase in the anisotropy and with the decrease in the inverse power-law index κ. The instability saturation and later nonlinear evolutions are also dominated by the oblique fluctuations, which are enhanced by the suprathermals and trigger a faster relaxation of the anisotropic electrons. Diffusion in velocity space is stimulated by the growing fluctuations, which scatter the electrons, starting with the more energetic suprathermal populations, as appears already before the saturation. After saturation the fluctuating magnetic field power shows decay patterns in the wave-vector space and a shift toward lower angles of propagation.