SPRG Seminars
March 1, 2011:
" Solar Wind Electrons "
Chadi Salem, Space Science Lab, University of California at Berkeley
Solar wind electrons are known to be out of thermodynamic equilibrium. The electron velocity distribution functions (eVDFs) consist of three main populations: a cold and dense core, a hotter and more tenuous halo, and a field-aligned strahl beam. They also display non-Maxwellian features, such as temperature anisotropies, suprathermal tails as well as heat fluxes along the local magnetic field direction. These non-thermal features appear to be strongly regulated by microscopic processes such as Coulomb collisions and/or turbulence and wave-particle interactions. The origin of such distributions and more generally the physical processes that transport energy in the solar wind are not well understood. The non-equilibrium characteristics of the eVDFs at 1 AU are of great importance in many aspects, for instance in understanding heat conduction, plasma microinstabilities and transport in weakly collisional plasma, as well as in the scenario at the origin of the solar wind.
We present here a comprehensive analysis of the structure of the eVDFs from the WIND 3DP experiment up to energies of 2-3 keV (energy above which the super-halo population exists). We will describe the technique used to model the eVDFs, and how we deal with spacecraft potential issues. We will discuss the properties of the different electron populations, and structure of the strahl in the slow and fast wind flows. We are in the process of applying our analysis to the entire > 15 year Wind data set, generating an accurate database of core, halo and strahl parameters as well as s/c potential. The resulting database will be an important tool in probing electron thermodynamics and universal plasma processes observable in the solar wind, more specifically, the formation of the suprathermal halo (and tails) through the role of electron-whistler interactions and the growth of anisotropy-driven electron instabilities and their contribution to the overall short wavelength power.