Variation of line widths in coronal hole off-limb regions: evidence for long-term structuring in plumes

Abstract

A detailed analysis of an off-limb polar region observed over a number of weeks by the Coronal Diagnostic Spectrometer (CDS), on board the SOHO spacecraft, is presented. Form a measurement of a number of coronal lines we find that polar plume regions posses a form of long term fine structuring visible in peak intensity, line-of-sight (L.O.S) velocity and line width measurements. Correlation found between these three measured quantities and Mg X 624/609 flux ratios suggest that small-scale continuous localized heating events are the cause of the fine structuring. A comparison of the line widths star to show a decrease in their values at exactly the same location where the document excitation changes from being collisionally to radiatively dominant. The resulting absence of the Collisional broadening contribution to the line width is likely to be the cause of this and therefore the reported decrease of line widths in coronal ions may not simply be related to the dissipation of wave energy.