Alternative high plasma beta regimes of electron heat-flux instabilities in the solar wind

Abstract

The heat transport in the solar wind is dominated by the 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, typically invoked being the self-generated instabilities, or the so-called heat flux instabilities (HFIs). The present paper provides a unified description of the full spectrum of HFIs, as prescribed by the linear kinetic theory for high beta conditions ($\beta_e \gg 0.1$) and different relative drifts ($U$) of the suprathermals. HFIs of different nature are distinguished, 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 to 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 only 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.