Local generation of the strahl in the solar wind electrons via whistler instability

Abstract:

The present study put forth a possible explanation for the formation of the strahl component in the measured solar wind electron velocity distribution functions (VDFs). Based upon the observational fact that in the collisionless plasmas, the halo electrons possess a net drift speed with respect to the core, the present study carries out one-dimensional particle-in-cell simulation of the whistler instability driven by the anisotropic core in the presence of the isotropic drifting halo electrons. Pitch angle scattering of the halo electrons occurs by the enhanced whistler waves. The pitch angle scattering primarily affects the halo moving in the field-aligned direction, resulting in pitch angle diffusion across 90^o gap in velocity space through nonlinear scattering process. This non-resonant interaction is accompanied by the reduction in a net drift speed of the halo, distinguishing the strahl feature from the halo electrons. Consequently, the remaining portion of the field-aligned moving halo, which is not affected by pitch angle scattering, simply appears to form the strahl in the electron velocity distribution. The core-halo-strahl feature shown in the numerical result is consistent with the observed electron VDF in the solar wind. The present scenario of local generation of the strahl suggests the new idea that the halo and strahl components might actually be one and the same, except that the strahl might simply be the remnant of pitch angle scattering of the anti-sunward moving halo by the whistler instability.