Abstract
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. The 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.
Original language | English |
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Article number | L39 |
Pages (from-to) | 1-4 |
Journal | The Astrophysical Journal Letters |
Volume | 784 |
Early online date | 19 Mar 2014 |
DOIs | |
Publication status | Published - 01 Apr 2014 |
Keywords
- opacity
- radiative transfer
- Sun: transition region
- Sun: UV radiation
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Lisa McFetridge
- School of Mathematics and Physics - Senior Lecturer
- Mathematical Sciences Research Centre
Person: Academic