Electron Heating in the Solar Wind

Abstract:

We discuss a recent theoretical model of electron heating in the solar wind developed in [1]. In our model, we assume that a plasma freely expands from a collisional thermal source into space such that outside the source the plasma flow is nearly collisionless. As a result, the electron distribution function forms two distinct components, an electron core, and an electron strahl. The strahl velocity distribution function is non-trivial, it does not resemble a shited Maxwellian often assumed in analytic studies. Rather, this function monotonically declines at high energies, which makes it stable to electron-scale kinetic plasma instabilities. The electrons scattered from the strahl due to week Coulomb collisions, populate the core distribution. As a result, the temperature of the quasi-isotropic core is predicted to decline with the heliospheric distance as $T(r)\sim r^{-2/5}$, which is close to the solar wind observations in the inner heliosphere.

[1] S. Boldyrev, C. Forest, & J. Egedal, Electron temperature of the solar wind, Proceedings of the National Academy of Sciences, 117, 9232-9240 (2020).

The work is supported by NSF under Grant NSF PHY-1707272, by NASA under Grant NASA 80NSSC18K0646, and by the Wisconsin Plasma Physics Laboratory (US Department of Energy Grant DE-SC0018266).