Electron halo and strahl formation in the solar wind by resonant interaction with whistler waves

Observations of solar wind electron distribution functions (VDFs) reveal considerable deviations from a simple Maxwellian VDF. A thermal core and a suprathermal halo and antisunward, magnetic field-aligned beam, or strahl,'' can be distinguished. At higher energies above 2 keV, a superhalo can even be observed. Akinetic description of electrons in the solar corona and wind, including resonant interaction between electrons and whistler waves, can reproduce an enhancement of suprathermal electron fluxes compared to the core flux. The whistler waves are assumed to be generated below the solar coronal base and propagate through the corona into interplanetary space. However, the resonance condition with these whistlers can only be fulfilled by electrons that move sunward. For antisunward-moving electrons, such a model lacks an efficient diffusion mechanism. The mirror force due to the opening magnetic field geometry of a solar coronal hole and in the solar wind focuses the electrons into a very narrow beam. This expectation of an extremely narrow beam is contradicted by observations of a strahl'' that has a finite width, and of an quasi-isotropic superhalo component. Thus, a diffusion mechanism for antisunward-moving electrons must exist in interplanetary space. In this paper, antisunward-propagating whistler waves are introduced into the kinetic model in order to provide this diffusion. Their wave power is estimated as a small fraction of the total wave power that is measured in interplanetary space. The kinetic results show that the whistler waves are capable of influencing the solar wind electron VDFs significantly, leading to the formation of both the halo and strahl populations and a more isotropic distribution at higher energies, in good agreement with solar wind observations.