On the widths and ratios of MgX 609.79 and 624.94 Å lines in polar off-limb regions

Abstract

Using measurements of MgX 609.78 and 624.94 Å lines from the Coronal Diagnostic Spectrometer (CDS) on board SOHO, we seek to examine the variation of line width and line ratio in regions far off-limb at the Northern pole of the Sun. It is found that above ~1150" the ratio of the two coronal MgX resonance lines reduces to values that might be expected for a more radiatively dominant excitation mode. A comparison of the line widths with these ratios indicates that the line widths start to show a decrease in their values at the location where the dominant excitation changes from being collisionally to radiatively dominant, that is, at ~1150". We suggest that the decrease in the line widths above ~1150" is likely to be due to a reduction in the non-thermal component of the line widths caused by a damping of upwardly propagating Alfv´en waves.