Electron Firehose instability and acceleration of electrons in solar flares

TL;DR

This paper investigates the Electron Firehose instability in solar flares, showing how electron anisotropies can lead to wave excitation and potential particle acceleration during the impulsive phase.

Contribution

It provides a linear kinetic theory analysis of wave properties and instability thresholds related to electron anisotropy in solar flare conditions.

Findings

Electrons become unstable during acceleration with specific wave excitation.

Left-hand circularly polarized waves near the proton gyrofrequency are excited.

Maximum growth rates occur at oblique propagation with higher frequencies.

Abstract

An electron distribution with a temperature anisotropy T_par/T_perp > 1 can lead to the Electron Firehose instability (Here par and perp denote directions relative to the background magnetic field B_0). Since possible particle acceleration mechanisms in solar flares exhibit a preference of energizing particles in parallel direction, such an anisotropy is expected during the impulsive phase of a flare. The properties of the excited waves and the thresholds for instability are investigated by using linearized kinetic theory. These thresholds were connected to the pre-flare plasma parameters by assuming an acceleration model acting exclusively in parallel direction. For usually assumed pre-flare plasma conditions the electrons become unstable during the acceleration process and lefthand circularly polarized waves with frequencies of about the proton gyrofrequency are excited at parallel propagation. Indications have been found, that the largest growth rates occur at oblique propagation and the according frequencies lie well above the proton gyrofrequency.