Letter to the Editor

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

¹ Armagh Observatory, College Hill, Armagh BT61 9DG, N. Ireland e-mail: [eos;jgd]@arm.ac.uk
² Indian Institute of Astrophysics, II Block, Koramangala, Bangalore 560 034, India e-mail: dipu@iiap.res.in

Received: 9 February 2005
Accepted: 30 April 2005

Abstract

Using measurements of 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  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 Alfvén waves.