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Title: INTENSITY ENHANCEMENT OF O VI ULTRAVIOLET EMISSION LINES IN SOLAR SPECTRA DUE TO OPACITY

Opacity is a property of many plasmas. It is normally expected that if an emission line in a plasma becomes optically thick, then its intensity ratio to that of another transition that remains optically thin should decrease. However, radiative transfer calculations undertaken both by ourselves and others predict that under certain conditions the intensity ratio of an optically thick to an optically thin line can show an increase over the optically thin value, indicating an enhancement in the former. These conditions include the geometry of the emitting plasma and its orientation to the observer. A similar effect can take place between lines of differing optical depths. While previous observational studies have focused on stellar point sources, here we investigate the spatially resolved solar atmosphere using measurements of the I(1032 Å)/I(1038 Å) intensity ratio of O VI in several regions obtained with the Solar Ultraviolet Measurements of Emitted Radiation instrument on board the Solar and Heliospheric Observatory satellite. We find several I(1032 Å)/I(1038 Å) ratios observed on the disk to be significantly larger than the optically thin value of 2.0, providing the first detection (to our knowledge) of intensity enhancement in the ratio arising from opacity effects in the solar atmosphere. Themore » agreement between observation and theory is excellent and confirms that the O VI emission originates from a slab-like geometry in the solar atmosphere, rather than from cylindrical structures.« less
Authors:
;  [1] ; ;  [2] ; ; ;  [3] ;  [4]
  1. Astrophysics Research Centre, School of Mathematics and Physics, Queen's University Belfast, Belfast BT7 1NN (United Kingdom)
  2. Armagh Observatory, College Hill, Armagh BT61 9DG (United Kingdom)
  3. Physics Department, Imperial College, London SW7 2BZ (United Kingdom)
  4. Centre for Statistical Science and Operational Research, School of Mathematics and Physics, Queen's University Belfast, Belfast BT7 1NN (United Kingdom)
Publication Date:
OSTI Identifier:
22365934
Resource Type:
Journal Article
Resource Relation:
Journal Name: Astrophysical Journal Letters; Journal Volume: 784; Journal Issue: 2; Other Information: Country of input: International Atomic Energy Agency (IAEA)
Country of Publication:
United States
Language:
English
Subject:
79 ASTROPHYSICS, COSMOLOGY AND ASTRONOMY; ASTROPHYSICS; CYLINDRICAL CONFIGURATION; DETECTION; ION EMISSION; OPACITY; ORIENTATION; OXYGEN IONS; PLASMA; RADIANT HEAT TRANSFER; SATELLITES; SOLAR ATMOSPHERE; SUN; ULTRAVIOLET RADIATION