The Off-Disk Thermal Structure of a Polar Coronal Hole

The thermal structure of the coronal portion of coronal holes is a key factor in the modeling of the source regions and acceleration mechanisms of the fast solar wind. Studies aimed at this region are best carried out on observations outside the disk, where the chromosphere and transition region are not included in the line of sight. In the past, many off-disk studies measured the line-of-sight temperature using line intensity ratios, a method that cannot provide information on the distribution of material with temperature; a few off-disk DEM determinations are also available, which are limited to a few heights only. No EM loci study has been carried out to the best of our knowledge. In this paper we use SOHO SUMER deep-exposure spectra of a polar coronal hole observed in 1996 to carry out a systematic investigation of the thermal structure of the emitting plasma as a function of distance from the limb, making use of the EM loci technique. Instrument-scattered light limits our investigation to distances in the 1.03-1.17 R_☉ range, where we find that the plasma is close to isothermal along the line of sight, with temperature slowly increasing with distance from the limb.