Variation of coronal line widths on and off the disk

We present observations of a Mg X 625 ? coronal line, obtained with the CDS instrument on SoHO, extending from the disk part of the coronal hole to ~90 000 km above the limb in the north polar coronal hole. Observations were performed in polar plumes and inter-plume lanes. To obtain a sufficiently high signal-to-noise ratio the observations were made over long periods of time and subsequent time frames were summed up. For the off-limb observations we notice a turnover point, around 65 000 km above the limb, where the line widths seem to suddenly decrease or level-off. The initial linear increase of line width with altitude supports previous observations and is consistent with an interpretation of linear undamped Alfvèn waves propagating outwards in open field regions. The turnover point seems to indicate the location where a change of physics takes place. We find that this turnover occurs at approximately the same line width value for each of the datasets examined, suggesting that the turnover occurs whenever the non-thermal velocity reaches a certain key velocity. For the on-disk data we find larger widths in the deep coronal hole as compared to the adjacent quiet Sun regions, suggesting the presence of additional waves and/or turbulence in the coronal hole.