Observations of solar wind electrons indicate that their distribution functions show peculiar deviations from thermodynamic equilibrium. The electron distribution function typically consists of a…

This federal contract award, NNX16AI59G, was granted by NASA to The Regents of the University of California, doing business as the University of California, Berkeley. The $287,893.29 grant supports an integrated research program to investigate the microphysical mechanisms that regulate the kinetic, non-thermal features of solar wind electrons. The research focuses on evaluating the role of collisions versus collisionless processes in regulating electron heat flux, analyzing core and halo drifts, and studying instabilities driven by the electron strahl and temperature anisotropies. This research aims to elucidate the importance of these electron-kinetic effects for the global evolution of the solar wind over the solar cycle. The parent organization for the University of California, Berkeley is the State of California Controller's Office, which has experience supporting federal contracts and grants in areas like defense, scientific research, and education.

Observations of solar wind electrons indicate that their distribution functions show peculiar deviations from thermodynamic equilibrium. The electron distribution function typically consists of a core, a halo, and a strahl component. All of these components can show relative drifts with respect to each other and temperature anisotropies with respect to the background magnetic field. We propose an integrated research program to elucidate the importance of microphysical mechanisms that regulate these kinetic, non-thermal, features of solar wind electrons. We focus on collisional versus collisionless heat-flux regulation; core and halo drifts; strahl structure; and instabilities generated by drifts, heat-flux, temperature anisotropies, and strahl, as well as their contribution to turbulence. We will use well calibrated in-situ measurements of the solar wind electron distribution function at 1AU from the wind spacecraft, along with proton measurements, as well as magnetic field and plasma wave data. Data over the entire wind mission will be used to enable statistically significant analyses over a wide range of solar wind conditions. First, we will investigate the validity of the spitzer-h\"arm approximation by evaluating to what degree the distribution function deviates from maxwellian equilibrium depending on the collisionality of the plasma. This test will allow US to determine the role of collisions for the regulation of electron heat flux. We will also analyze large core and halo drifts observed in the distribution function and study their variability with solar wind conditions, as well as their effects on damping rates of characteristic plasma eigenmodes such as kinetic alfven waves or whistler waves at subproton scales. We will then concentrate on instabilities driven by the electron strahl and temperature anisotropy in the electron core. We will investigate the acting physical mechanisms, their parameter dependencies, and their consequences for the evolution of the distribution function. Based on this knowledge about strahl- and anisotropy-driven instabilities, we will generalize our treatment to include additional sources of free energy for micro-instabilities---i.e., strahl anisotropy, halo heat flux and halo anisotropy. These instabilities can be an important mechanism to generate fluctuations on kinetic scales that will contribute to the turbulent spectrum. Finally, we will determine the relevance of these electron-kinetic effects for the global evolution of the solar wind and their variations during the solar cycle.