Properties of Solar Polar Coronal Plumes Constrained by Ultraviolet Coronagraph Spectrometer Data

Abstract

We investigate the plasma dynamics (outflow speed and turbulence) inside polar plumes. We compare line profiles (mainly of \ion{O}{6}) observed by the UVCS instrument on SOHO at the minimum of solar cycle 22-23 with model calculations. We consider Maxwellian velocity distributions with different widths in plume and inter-plume regions. Electron densities are assumed to be enhanced in plumes and to approach inter-plume values with increasing height. Different combinations of the outflow and turbulence velocity in the plume regions are considered. We compute line profiles and total intensities of the \ion{H}{1} Ly$\alpha$ and the \ion{O}{6} doublets. The observed profile shapes and intensities are reproduced best by a small solar wind speed at low altitudes in plumes that increases with height to reach ambient inter-plume values above roughly 3-4 $R_\sun$ combined with a similar variation of the width of the velocity distribution of the scattering atoms/ions. We also find that plumes very close to the pole give narrow profiles at heights above 2.5 $R_\sun$, which are not observed. This suggests a tendency for plumes to be located away from the pole. We find that the inclusion of plumes in the model computations provides an improved correspondence with the observations and confirms previous results showing that published UVCS observations in polar coronal holes can be roughly reproduced without the need for large temperature anisotropy. The latitude distributions of plumes and magnetic flux distributions are studied by analyzing data from different instruments on SOHO and with SOLIS.