The Effect of Proton Temperature Anisotropy on the Electron Heat Flux Dissipation in Solar Coronal Holes

Abstract

A possible source of the ion cyclotron waves that energize the protons in the fast solar wind is the electron heat flux launched by reconnection events at the base of coronal holes. A strong enough heat flux becomes unstable and generates cyclotron waves that interact with the protons. When the protons are energized via the cyclotron resonance, they can develop considerable temperature anisotropy in the region of the corona where the solar wind becomes collisionless. The heat flux instability is known to be stabilized by the proton anisotropy, at least in the electrostatic limit. This limits the effect of the proton energization. However, in the collisionless region, the ion cyclotron heat flux instability is dominated by the electromagnetic mode, which is much less studied. We investigate how the electromagnetic mode is affected by the anisotropy and how it interacts with anisotropy-driven instabilities. The latter instabilities will also constrain the anisotropy produced by the cyclotron energization of the protons. We show that, with the temperature anisotropy resulting from the interplay of these processes, the heat-flux instability is strong enough to dissipate most of the heat flux launched at the coronal base and channel it into the kinetic energy flux of the protons within the acceleration region of the solar wind. Since at 1 AU the energy of the solar wind consists primarily of proton kinetic energy, this confirms that the electron heat flux is a viable source of solar wind generation.