Variation of line widths in polar off-limb regions

Abstract

Using measurement from the Coronal Diagnostic Spectrometer (CDS) on board SOHO we seek to examine the relative roles of radiative and Collisional excitation in regions far off-limb at the Northern pole of the sun. We study the line width variation with height. It is found that above 1150 arcsec the second ratio of two coronal Mg X resonance lines reduces to values that we might expect for a radiatively dominant excitation mode. A comparison of line widths with the Mg X ratios shows that the line widths start to show a decrease in their values at the same location where the dominant excitation changes from being collisionally to radiatively dominant. This is considered to be evidence for a sudden drop in the electron density at this location, and, therefore, the reported tributed to a reduction in the measurable contribution of waves/turbulence to the non-thermal velocity component of the line widths. This result suggests that the reported decrease of line widths in coronal ions at large altitudes off-limb may not simply be related to the dissipation of wave energy. Implication to the acceleration of the fast solar wind will be discussed.