Radial Evolution of the Electron Velocity Distribution Functions in the Heliosphere: Role of Collisions

he electron velocity distribution function in the solar wind has often been described as the superposition of an isotropic core, a moderately energetic and slightly anisotropic halo and a high energy field aligned beam. The relative weight of these components depends on the characteristics of the solar wind streams (fast and slow) and on the heliocentric distance. We present kinetic simulations using the model given in [1] to study the effects of electron-electron and electron-proton collisions on the electron velocity distribution function in the interplanetary space beyond ∼0.3 AU from the Sun. We show that collisions do naturally generate a two population electron velocity distribution with an isotropic and cold "core" and a hot and collimated "halo." The temperature profiles and temperature anisotropies observed in our simulations are consistent with spacecraft observations [2, 3, 4]. Since waves are not included in our simulations we suggest that Coulomb collisions are an essential ingredient which should be included in any model of the evolution of the electron velocity distribution function in interplanetary space. © 2010 American Institute of Physics.