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Astron. Astrophys. 357, 767-776 (2000)

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A study of opacity in SOHO-SUMER and SOHO-CDS spectral observations

II. Atmospheric structure at the solar limb

G.A. Fischbacher, S.D. Loch and H.P. Summers

University of Strathclyde, 107 Rottenrow, Glasgow G4 0NG, UK

Received 31 December 1999 / Accepted 20 January 2000


To model the EUV emission from solar plasma at transition region temperatures it is necessary to take account of both the structure and dynamics of the plasma and the effects of opacity which radiatively couple different points within the plasma. Simple escape probability techniques exist which avoid the need for a full solution of the radiative transfer equation. These have been used to diagnose spectral line opacities, to calculate excited state population structures and have also been used in conjunction with simple atmosphere models to calculate limb brightening curves and the variation of selected line ratios on crossing the limb.

In this work we firstly revisit these techniques and extend them for use with some stratified atmosphere models. The objective is to assess the continuing effectiveness of such methods within more detailed structural and dynamic atmosphere pictures. We find that although the emission characteristics are dominated by inhomogeneities, the observed opacities are less sensitive and may be reasonably well accounted for within a stratified atmosphere model.

Secondly, we develop the escape probability techniques to examine modifications to the population structure and line blending, and use them in conjunction with the stratified atmosphere models to predict both limb brightening and the variation of branching multiplet ratios on crossing the limb. Results are compared with observations by the SUMER instrument on board the SOHO spacecraft.

Key words: radiative transfer – Sun: atmosphere – Sun: transition region – Sun: UV radiation

Send offprint requests to: G.A. Fischbacher

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© European Southern Observatory (ESO) 2000

Online publication: June 5, 2000