The Origin of Persistently Nonthermal Solar Wind Electrons: the Steady Electron Runaway Model's Demonstration of Dreicer Bifurcation using Measured E∥ and Ion–Electron Coulomb Drag

Abstract

Abstract The Steady Electron Runaway Model (SERM) develops the hypothesis that the solar wind’s observed ubiquitous nonthermal electron velocity distribution functions (eVDFs) are caused by Dreicer's velocity space bifurcation in the strong dimensionless E ∥ required by quasi-neutrality. SERM’s predicted partitions for the pressure and density are contrasted with appropriately adapted eVDF properties from the Wind 3DP experiment (1995–1998), based on in situ observations of E ∥ . The observed number fraction of electrons in runaway, δ 3DP, follows a thousandfold decline of Dreicer’s predicted fraction, δ, across the observed tenfold reduction of E ∥ , satisfying δ 3DP ≃ δ 0.89. SERM’s predictions are shown to reproduce the observed variations with E ∥ of the electron partial pressure and excess kurtosis,  e .  e and E ∥ are positively correlated across 4 yr, as expected by the SERM–Dreicer origin of the suprathermals. SERM quantitatively explains the observed 50 yr anticorrelation between δ 3DP and the partition slope temperature ratios. This documentation quantitatively establishes Coulomb runaway physics as the missing determinant of the ubiquitous nonthermal solar wind eVDF. Astrophysical plasmas, like stellar winds, are unavoidably inhomogeneous, requiring E ∥ to enforce quasi-neutrality. Between the stars E ∥ is expected to be sufficiently large that measurable runaway density fractions (0.1%–30%) will occur, producing widespread leptokurtic eVDFs. Using inhomogeneous two-fluid information, SERM predicts spatially dependent leptokurtic eVDF profiles consonant with Coulomb collisions and the fluid’s E ∥(r). SERM can also comment on its eVDFs’ consistency with Maxwellians presumed in the Spitzer–Härm closure. The solar wind profile shows the implied strong radial gradient of the plasma eVDF’s transformation from near thermal to strongly leptokurtic across 1.5–6 R ⊙.