The nearly isotropic velocity distributions of energetic electrons in the solar wind

Observations with Wind and other spacecraft of the solar wind have shown that the velocity distributions of energetic electrons associated with solar impulsive electron events are often nearly isotropic, as shown by Lin et al. and Ergun et al. It is believed that in each case the electrons are originally associated with a flare-associated beam. The issue of interest is: how can a beam of fast electrons evolve into such a final state? This paper offers a theoretical explanation. We attribute the isotropization of the electron beam distribution to nonresonant pitch-angle scattering by enhanced Alfven waves in the corona and the solar wind. The proposed scenario is demonstrated with test particle calculations.