An empirical model of a polar coronal hole at solar minimum

We present a comprehensive and self-consistent empirical model for several plasma parameters in the extended solar corona above a polar coronal hole. The model is derived from observations with the SOHO Ultraviolet Coronagrap h Spectrometer (UVCS/SOHO) during the period between 1996 November and 1997 April. We compare observations of H I Ly alpha and O VI lambda lambda 1032 , 1037 emission lines with detailed three-dimensional models of the plasma parameters and iterate for optimal consistency between measured and synthes ized observable quantities. Empirical constraints are obtained for the radi al and latitudinal distribution of density for electrons, H-0, and O5+, as well as the outflow velocity and unresolved anisotropic most probable speed s for H-0 and O5+. The electron density measured by UVCS/SOHO is consistent with previous solar minimum determinations of the white-light coronal stru cture; we also perform a statistical analysis of the distribution of polar plumes using a long time series. From the emission lines we find that the u nexpectedly large line widths of H-0 atoms and O5+ ions at most heights are the result of anisotropic velocity distributions. These distributions are not consistent with purely thermal motions or the expected motions from a c ombination of thermal and transverse wave velocities. Above 2 R-., the obse rved transverse most probable speeds for O5+ are significantly larger than the corresponding motions for H-0, and the outflow velocities of O5+ are al so significantly larger than the corresponding velocities of H-0. Also, the latitudinal dependence of intensity constrains the geometry of the wind ve locity vectors, and superradial expansion is more consistent with observati ons than radial flow. We discuss the constraints and implications on variou s theoretical models of coronal heating and acceleration.