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Title:
Electron density in the quiet solar coronal transition region from SoHO/SUMER measurements of S VI line radiance and opacity
Authors:
Buchlin, E.; Vial, J.-C.
Affiliation:
AA(Institut d'Astrophysique Spatiale, CNRS & Université Paris Sud, Orsay, France ), AB(Institut d'Astrophysique Spatiale, CNRS & Université Paris Sud, Orsay, France)
Publication:
Astronomy and Astrophysics, Volume 503, Issue 2, 2009, pp.559-568 (A&A Homepage)
Publication Date:
08/2009
Origin:
EDP Sciences
Astronomy Keywords:
Sun: atmosphere, Sun: transition region, Sun: UV radiation
DOI:
10.1051/0004-6361/200811588
Bibliographic Code:
2009A&A...503..559B

Abstract

Context: The steep temperature and density gradients that are measured in the coronal transition region challenge the model interpretation of observations.
Aims: We derive the average electron density < ne > in the region emitting the S vi lines. We use two different techniques, which allow us to derive linearly-weighted (opacity method) and quadratically-weighted (emission measure method) electron density along the line-of-sight, to estimate a filling factor or derive the layer thickness at the formation temperature of the lines.
Methods: We analyze SoHO/SUMER spectroscopic observations of the S vi lines, using the center-to-limb variations in radiance, the center-to-limb ratios of radiance and line width, and the radiance ratio of the 93.3-94.4 nm doublet to derive the opacity. We also use the emission measure derived from radiance at disk center.
Results: We derive an opacity τ0 at S vi 93.3 nm line center of the order of 0.05. The resulting average electron density < {ne}>, under simple assumptions concerning the emitting layer, is 2.4 × 1016 m-3 at T = 2 × 105 K. This value is higher than (and inconsistent with) the values obtained from radiance measurements (2 × 1015 m-3). The last value corresponds to an electron pressure of 10-2 Pa. Conversely, taking a classical value for the density leads to a too high value of the thickness of the emitting layer.
Conclusions: The pressure derived from the emission measure method compares well with previous determinations. It implies a low opacity of between 5 × 10-3 and 10-2. It remains unexplained why a direct derivation leads to a much higher opacity, despite tentative modeling of observational biases. Further measurements in S vi and other lines emitted at a similar temperature should be completed, and more realistic models of the transition region need to be used.
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