Ubiquitous non-thermals in astrophysical plasmas: Restating the difficulty of maintaining maxwellians

Abstract

This paper outlines the rather narrow conditions on a radiatively decoupled plasma where a Maxwell-Boltzmann (MB) distribution can be assumed with confidence. The complementary non-thermal distribution with non-perturbative kurtosis is argued to have a much broader purview than has previously been accepted. These conditions are expressed in terms of the electron Knudsen number, Ke , the ratio of the electron mean free path to the scale length of electron pressure. Rather generally, f(v < v 2(K e )) will be Gaussian, so that MB atomic or wave particle effects controlled by speeds v 0.01 is common in all main-sequence stellar atmospheres above approximately 0.05 stellar radii from the surface. The entire solar corona and wind are included in this regime where non-thermal distributions with kurtosis are shown to be ubiquitous, heat flux is not well modeled by Spitzer-Braginskii closure, and fluid modeling is qualitative at best. © 2013. The American Astronomical Society. All rights reserved..