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

• Published in: The Astrophysical journal 2019-11, Vol.885 (2), p.138
• Main Author: Scudder, J. D.
• Format: Article
• Language: English
• Subjects: Astrophysics; Conduction bands; Density; Distribution functions; Electric fields; Electron runaway (plasma physics); Electron velocity distribution; Heat flow; Heat flux; Heat transfer; Heat transmission; Mathematical models; Modelling; Non-Gaussianity; Parameters; Partial pressure; Pitch (inclination); Plasmas (physics); Solar coronal heating; Solar coronal lines; Solar wind; Solar wind velocity; Stellar coronal lines; Velocity distribution; Wind speed
• ISSN: 0004-637X; 1538-4357
• DOI: 10.3847/1538-4357/ab4882

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 .