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Issue A&A
Volume 436, Number 2, June III 2005
Page(s) L35 - L38
Section Letters
DOI http://dx.doi.org/10.1051/0004-6361:200500120



A&A 436, L35-L38 (2005)
DOI: 10.1051/0004-6361:200500120

Letter

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

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 $\ion{Mg}{x}$ 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 $\approx$1150´´ the ratio of the two coronal $\ion{Mg}{x}$ 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 $\approx$1150´´. We suggest that the decrease in the line widths above $\approx$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.


Key words: Sun: UV radiation -- Sun: corona -- Sun: atmosphere




© ESO 2005


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