Temperature isotropization in solar flare plasmas due to the electron firehose instability

The isotropization process of a collisionless plasma with an electron temperature anisotropy along an external magnetic field ($T_\| ^{\rm e}\gg T_\perp^{\rm e}$, $\|$ and $\perp$ with respect to the background magnetic field) and isotropic protons is investigated using a particle-in-cell (PIC) code. Restricting wave growth mainly parallel to the external magnetic field, the isotropization mechanism is identified to be the Electron Firehose Instability (EFI). The free energy in the electrons is first transformed into left-hand circularly polarized transverse low-frequency waves by a non-resonant interaction. Fast electrons can then be scattered towards higher perpendicular velocities by gyroresonance, leading finally to a complete isotropization of the velocity distribution. During this phase of the instability, Langmuir waves are generated which may lead to the emission of radio waves. A large fraction of the protons is resonant with the left-hand polarized electromagnetic waves, creating a proton temperature anisotropy $T_\|^{\rm p} < T_\perp^{\rm p}$. The parameters of the simulated plasma are chosen compatible to solar flare conditions. The results indicate the significance of this mechanism in the particle acceleration context: the EFI limits the anisotropy of the electron velocity distribution, and thus provides the necessary condition for further acceleration. It enhances the pitch-angle of the electrons and heats the ions.