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Astronomy & Astrophysics manuscript no. eosmgx April 27, 2005 (DOI: will be inserted by hand later)

Summary: Astronomy & Astrophysics manuscript no. eosmgx April 27, 2005
(DOI: will be inserted by hand later)
On the widths and ratios of Mg X 609.79 and 624.94 Å lines in
polar off-limb regions
E. O'Shea1, D. Banerjee2, and J.G.Doyle1
Armagh Observatory, College Hill, Armagh BT61 9DG, N.Ireland
e-mail: eos@arm.ac.uk, e-mail: jgd@arm.ac.uk
Indian Institute of Astrophysics, II Block, Koramangala, Bangalore 560 034, India
e-mail: dipu@iiap.res.in
Abstract. Using measurements of Mg X 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 Mg X 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.
Key words. Sun: UV radiation ­ Sun: corona ­ Sun: atmosphere
1. Introduction


Source: Armagh Observatory


Collections: Physics