The Role and Implications of Non-Thermal Distributions in the Solar Corona/Wind

Abstract:

Astrophysical plasmas contain ubiquitous non-thermal velocity distribution functions (VDF’s) down to the mean energies of the plasma. These non-thermal particles represent a non-negligible part of the solar wind density, gas pressure, and heat flow. The usual fluid treatments ignore the information in non-Gaussian VDF’s, while kinetic treatments suggest they play an essential role for the observed wind expansion. The existence of the such kurtotic VDF’s contradict the very Spitzer-Braginskii (SB) approximations made to motivate low order closures approximations of the quasi-neutral fluid equations for the plasma (1). Their routine observation implies the usual continuum description of the plasma must change from the SB framework. The origin of such kurtotic distributions has been traced (1) to the consequences of gravity’s violating the assumptions of the SB framework, giving any astrophysical plasma fluid description a non-local character. The quasi-steady character of the observed solar wind implies that this non-local character is resolved by spatial variation of the kurtotic distribution functions moving along the tubes of magnetic force as they enforce the higher goals of quasi-neutrality and zero current. This insight suggests an unusual equation of state for the plasma.

A minimal fluid model will be introduced for the coronal expansion that (i) suggests the origin and strength of kurtotic VDF’s, (ii) explains their ubiquity; (iii) provides a continuum description, (iv) allows variations of kurtosis and velocity filtration-like variations of T; and (v) retains coulomb collisions. The non-thermal electrons of the solar wind VDF’s will be shown to closely resemble the conduction band in a metal, being the global regulators of quasi-neutrality, electric and heat currents and behaving as a second electron fluid which, together with the protons forms the minimal 3 fluid description.

(1) Scudder, J.D. and H. Karimabadi, Ap J., 770:26, 2013