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

Abstract

Abstract A Steady Electron Runaway Model (SERM) is formulated describing plasmas in the astrophysical “condition” having finite (rather than infinitesimal) Knudsen number, , 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, , shown to be nearly interchangeable with the pressure Knudsen number, . The eVDF is determined by the total density and pressure, heat flux, and with the Maxwellian as a special case when . The nonthermal part of the eVDF is caused by local and global runaway physics and its density fraction is monotonically dependent on . SERM explains the distinguishable conduction band of suprathermal electrons to be the result of the inhomogeneities of astroplasmas that require to enforce quasi-neutrality. SERM shows that the direction of the heat flow should be that of . 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 , producing a steady-state eVDF, consistent with in situ (i) heat flows, (ii) strahl pitch angle features in high-speed winds, (iii) , and (iv) non-negative probability at all velocities. Because finite is the identified prerequisite for SERM modeling, nonthermal eVDF’s are expected nearly everywhere in astrophysics where .