Generation of Suprathermal Electrons by Resonant Wave‐Particle Interaction in the Solar Corona and Wind

Abstract

Observations of solar wind electron velocity distribution functions (VDFs) reveal pronounced deviations from a Maxwellian with enhanced numbers of suprathermal electrons. It is shown that these suprathermal tails of the electron VDFs can originate from the solar corona. A model for the acceleration of suprathermal electrons based on resonant interaction with whistler waves is presented. Quasi-linear theory describes this interaction as pitch-angle diffusion in the reference frame of the waves. The waves are assumed to be generated below the coronal base and to propagate antisunward through the corona. Under plasma conditions with high whistler wave phase speeds, the resonant interaction causes electrons to be accelerated from relatively small sunward velocities parallel to the background magnetic field to high speeds perpendicular to the magnetic field. Such plasma conditions are found in the solar corona. A kinetic model is developed to study this acceleration mechanism and the evolution of an electron VDF from the coronal base up into interplanetary space. The kinetic model includes not only the resonant interaction with whistler waves but also Coulomb collisions and the mirror force the electrons experience in the opening magnetic structure of a coronal funnel. The wave absorption of the electrons is also considered to guarantee energy conservation. Kinetic results for the coronal funnel and solar wind are presented. The electron VDFs show deviations from a Maxwellian that are in coincidence with theoretical expectations. The whistler waves generate suprathermal electrons. Toward interplanetary space, the mirror force focuses the electrons toward a narrow "strahl." A comparative study without whistler waves shows that the waves considerably enhance the suprathermal electron fluxes in interplanetary space at 1 AU, as they are observed in the solar wind.