Steady Electron Runaway Model SERM: Astrophysical Alternative for the Maxwellian Assumption
Abstract
A Steady Electron Runaway Model (SERM) is formulated describing plasmas in the astrophysical “condition” having finite (rather than infinitesimal) Knudsen number, {K}}}Pe}, 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, {E}}}\parallel , shown to be nearly interchangeable with the pressure Knudsen number, {K}}}Pe}. The eVDF is determined by the total density and pressure, heat flux, and {E}}}\parallel with the Maxwellian as a special case when {E}}}\parallel =0. The nonthermal part of the eVDF is caused by local and global runaway physics and its density fraction is monotonically dependent on {E}}}\parallel . SERM explains the distinguishable conduction band of suprathermal electrons to be the result of the inhomogeneities of astroplasmas that require {E}}}\parallel \ne 0 to enforce quasi-neutrality. SERM shows that the direction of the heat flow should be that of {E}\parallel \hat{{\boldsymbol{b}}}. 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 0.2< {E}}}\parallel (1 {au})< 0.65, producing a steady-state eVDF, consistent with in situ (i) heat flows, (ii) strahl pitch angle features in high-speed winds, (iii) {J}\parallel =0, and (iv) non-negative probability at all velocities. Because finite {K}}}Pe} is the identified prerequisite for SERM modeling, nonthermal eVDF’s are expected nearly everywhere in astrophysics where {K}}}Pe}> 0.01.
- Publication:
-
The Astrophysical Journal
- Pub Date:
- November 2019
- DOI:
- 10.3847/1538-4357/ab4882
- Bibcode:
- 2019ApJ...885..138S
- Keywords:
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- Non-Gaussianity;
- Stellar coronal lines;
- Solar coronal heating;
- Solar coronal lines;
- Solar wind;
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