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

We investigate the plasma dynamics (outflow speed and turbulence) insidepolar plumes. We compare line profiles (mainly of \ion{O}{6}) observed by theUVCS instrument on SOHO at the minimum of solar cycle 22-23 with modelcalculations. We consider Maxwellian velocity distributions with differentwidths in plume and inter-plume regions. Electron densities are assumed to beenhanced in plumes and to approach inter-plume values with increasing height.Different combinations of the outflow and turbulence velocity in the plumeregions 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 shapesand intensities are reproduced best by a small solar wind speed at lowaltitudes in plumes that increases with height to reach ambient inter-plumevalues above roughly 3-4 $R_\sun$ combined with a similar variation of thewidth of the velocity distribution of the scattering atoms/ions. We also findthat 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 belocated away from the pole. We find that the inclusion of plumes in the modelcomputations provides an improved correspondence with the observations andconfirms previous results showing that published UVCS observations in polarcoronal holes can be roughly reproduced without the need for large temperatureanisotropy. The latitude distributions of plumes and magnetic fluxdistributions are studied by analyzing data from different instruments on SOHOand with SOLIS.Comment: 11 figure