UVCS/[ITAL]SOHO[/ITAL] Empirical Determinations of Anisotropic Velocity Distributions in the Solar Corona

Abstract

We present a self-consistent empirical model for several plasma parameters of a polar coronal hole near solar minimum, derived from observations with the Solar and Heliospheric Observatory Ultraviolet Coronagraph Spectrometer. The model describes the radial distribution of density for electrons, H⁰, and O^{5 +} and the outflow velocity and unresolved most probable velocities for H⁰ and O^{5 +} during the period between 1996 November and 1997 April. In this Letter, we compare observations of H I Lyα and O VI λλ1032, 1037 emission lines with spatial models of the plasma parameters, and we iterate for optimal consistency between measured and synthesized observable quantities. The unexpectedly large line widths of H⁰ atoms and O^{5 +} ions at most radii are the result of anisotropic velocity distributions, which are not consistent with purely thermal motions or the expected motions from a combination of thermal and transverse wave velocities. Above 2 R, the observed transverse, most probable speeds for O^{5 +} are significantly larger than the corresponding motions for H⁰, and the outflow velocities of O^{5 +} are also significantly larger than the corresponding velocities of H⁰. We discuss the constraints and implications on various theoretical models of coronal heating and acceleration.