KINETIC ELECTRONS IN HIGH-SPEED SOLAR-WIND STREAMS - FORMATION OF HIGH-ENERGY TAILS

We study the evolution of the electron velocity distribution function in high-speed solar wind streams from the collision-dominated corona a nd into the collisionless interplanetary space. The model we employ so lves the kinetic transport equation with the Fokker-Planck collision o perator to describe Coulomb collisions between electrons. We use a tes t particle approach, where test electrons are, injected into a prescri bed solar wind background. The density, temperature, and electric fiel d associated with the background are computed from fluid models. The t est electrons are in thermal equilibrium with the background at the ba se of the corona, and we study the evolution of the velocity distribut ion of the test electrons as a function of altitude. We find that velo city filtration, due to the energy dependence of the Coulomb cross sec tion, is a small effect and is not capable of producing significant be ams in the distribution or a temperature moment that increases with al titude. The distribution function is mainly determined by the electric held and the expanding geometry and consists of a population with an almost isotropic core which is bound in the electrostatic potential an d a beam-like high-energy tail which escapes. The trapped electrons co ntribute significantly to the even moments of the distribution functio n but almost nothing to the odd moments; the drift speed and energy fl ux moments are carried solely by the tail. In order to describe the hi gh-speed solar wind observed near 0.3 AU by the Helios spacecraft, we use a multifluid model where ions are heated preferentially. The resul ting test electron distribution at 0.3 AU, in this background, is in v ery good agreement with the velocity distributions observed by the Hel los spacecraft.