Steady Electron Runaway Model SERM: Astrophysical Alternative for the Maxwellian Assumption

A Steady Electron Runaway Model (SERM) is formulated describing plasmas in the astrophysical ?condition? having finite (rather than infinitesimal) Knudsen number, <i, suggesting an omnipresent leptokurtic, nonthermal, and heat-conducting electron velocity distribution function (eVDF) as the replacement for the Maxwellian ansatz typically made. The shape parameters of SERM?s eVDFs are functionals of the local dimensionless electric field, <i, shown to be nearly interchangeable with the pressure Knudsen number, <i. The eVDF is determined by the total density and pressure, heat flux, and <i with the Maxwellian as a special case when <i. The nonthermal part of the eVDF is caused by local and global runaway physics and its density fraction is monotonically dependent on <i. SERM explains the distinguishable conduction band of suprathermal electrons to be the result of the inhomogeneities of astroplasmas that require <i to enforce quasi-neutrality. SERM shows that the direction of the heat flow should be that of <i. Almost all reported space age correlations among the shape parameters of the solar wind eVDF are reproduced by this modeling, including scaling of: (i) nonthermal spectral break energy, and (ii) partition of suprathermal density and partial pressure, with solar wind speed. SERM, together with eVDF observations, indirectly bracket <i, producing a steady-state eVDF, consistent with in situ (i) heat flows, (ii) strahl pitch angle features in high-speed winds, (iii) <i, and (iv) non-negative probability at all velocities. Because finite <i is the identified prerequisite for SERM modeling, nonthermal eVDF?s are expected nearly everywhere in astrophysics wher K-Pe > 0.01.