UVCS SOHO EMPIRICAL DETERMINATIONS OF ANISOTROPIC VELOCITY DISTRIBUTIONS IN THE SOLAR CORONA/

We present a self-consistent empirical model for several plasma parame ters of a polar coronal hole near solar minimum, derived from observat ions with the Solar and Heliospheric Observatory Ultraviolet Coronagra ph Spectrometer. The model describes the radial distribution of densit y for electrons, H-0, and O5+ and the outflow velocity and unresolved most probable velocities for H-0 and O5+ during the period between 199 6 November and 1997 April. In this Letter, we compare observations of H I Ly alpha and O VI lambda lambda 1032, 1037 emission lines with spa tial models of the plasma parameters, and we iterate for optimal consi stency between measured and synthesized observable quantities. The une xpectedly large line widths of H-0 atoms and O5+ ions at most radii ar e the result of anisotropic velocity distributions, which are not cons istent with purely thermal motions or the expected motions from a comb ination of thermal and transverse wave velocities. Above 2 R., the obs erved transverse, most probable speeds for O5+ are significantly large r than the corresponding motions for H-0, and the outflow velocities o f O5+ are also significantly larger than the corresponding velocities of H-0. We discuss the constraints and implications on various theoret ical models of coronal heating and acceleration.