Solar Wind Electron Thermodynamics

Solar wind electrons are known to be out of thermodynamic equilibrium. Their velocity distribution functions (VDFs) 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. What is the origin of such distributions and more generally what are the physical processes that transport the energy in the solar wind? The non-equilibrium characteristics of the electron VDFs 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 electron VDFs from the WIND 3DP experiment up to energies of the super-halo population. The properties of the different electron populations, as well as the structure of the strahl in the slow and fast wind flows will be discussed. This is an important step in our study of 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.