A Gravito‐electrostatic Sheath Model for Surface Origin of Subsonic Solar Wind Plasma

Abstract

Parker's model prediction of an unbounded solution for the supersonic solar wind plasma (SWP) requires a subsonic origin at the solar surface. The transition to supersonic SWP flow is well understood by analogy with the de Laval nozzle. The basic physics, however, of the self-consistent solar surface emission of subsonic SWP is still not very clear. We propose a theoretical model based on a gravito-electrostatic plasma sheath (GES) to investigate the surface emission mechanism of the quasi-neutral SWP. The basic equations for the interior steady state description are solved numerically as an initial-value problem under spherical geometry. A bounded solution for the solar self-gravity distribution is found to exist, meaning that a global quasi-hydrostatic equilibrium is formed at some distance from the heliocentric origin. This defines the solar surface boundary in our model, which lies at radius ξ ~ 3.5 (in units of the Jeans length). A minimum speed of ~3.0 cm s-1, corresponding to Mach number ~10-7, is obtained at this location. Consequently, a subsonic origin of the solar interior plasma flowing radially outward becomes a physical reality. Our model requires a mean electron temperature Te ~ 107 K (=1 keV) for the numerical results to match the standard solar values. The boundary is found to be negatively biased, with a normalized value of the electrostatic potential θs ~ -1 (=1 kV). It is conjectured that our model provides an interesting new physical basis for understanding the collective dynamical coupling processes of solar interior and exterior (heliosphere) regions through local, as well as global, modes of GES-induced collective waves, instabilities, and oscillations.