Thermodynamics of Solar Wind Core, Halo and Strahl Electrons: Properties, Regulation and Constraints

Abstract:

We present a comprehensive statistical analysis of solar wind electrons using the electron analyzers of the 3D-Plasma instrument on the Wind spacecraft. This work uses a sophisticated algorithm developed to analyze separately the different populations - core, halo and strahl - of the electron velocity distribution function (eVDF) up to 'super-halo' energies (2 keV). The code determines their respective set of parameters through fits to the measured eVDF, taking properly into account spacecraft charging and other instrumental effects. We use here several years (half a solar cycle, approximately 1.5 million of independent measurements) of core, halo and strahl electron parameters to investigate the properties of these different populations and the physical processe(s) that likely act to control and regulate them.

We discuss new results obtained on (1) the electron temperature anisotropies and their variation with collisions and/or solar wind fluctuations and instabilities, (2) the properties of core and halo drifts in the solar wind proton frame, (3) the electron heat flux, and (4) the electron strahl. These new observations emphasize the non-negligible role of Coulomb collisions in shaping the electron distribution function and regulating of the thermal and supra thermal electrons, but that the solar wind electron expansion and compression are limited fundamentally by some instabilities under certain conditions.