Particle-In-Cell Simulations of Whistler Heat Flux Instabilities in the Solar Wind: Heat Flux Regulation and Electron Halo Formation.

Abstract:

To shed light on the role of whistler waves in the heat flux regulation (e.g. Scime et al. 1994) and the scattering of strahl electrons in the solar wind (e.g. Maksimovic et al. 2005), we model electron velocity distribution functions composed of core and strahl populations as typically encountered in the near-Sun solar wind as observed by Parker Solar Probe. We demonstrate with two-dimensional Particle-In-Cell simulations that, depending on the initial drift velocities of the two electron populations, a parallel or oblique whistler heat flux instability can be excited. We confirm that the interaction between parallel whistler waves and strahl electrons can hardly explain the scattering of strahl electrons into the halo (e.g. Lopez et al. 2019; Verscharen et al. 2019). On the contrary, we show for the first time that the oblique whistler waves produce enhanced pitch-angle scattering of suprathermal electrons, resulting in the transferring of a significant fraction of strahl electrons into the halo.
Both instabilities are accompanied by a decrease of the heat flux carried by the strahl electrons, which is more efficient for the oblique instability in comparison with the parallel one.