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Astron. Astrophys. 342, 785-798 (1999)


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Statistical characteristics of radiation formed in atmosphere with randomly distributed inhomogeneities

A.G. Nikoghossian 1,3, S. Pojoga 2,3 and Z. Mouradian 3

1 Byurakan Astrophysical Observatory, 378433 Byurakan, Armenia
2 Astronomical Institute of the Romanian Academy, RO-75212 Bucharest, Romania
3 Observatoire de Paris-Meudon, DASOP, F-92195 Meudon Cedex, France

Received 18 August 1998 / Accepted 25 November 1998

Abstract

In this paper we continue studying the spatial brightness variations of solar quiescent prominences. The theory is developed to determine the mean intensity and the relative mean square deviation (RelMSD) for the line radiation emerging from an one-dimensional atmosphere with randomly distributed inhomogeneities. Both the local thermodynamic equilibrium (LTE) and Non-LTE cases are considered. The results previously obtained for the LTE atmosphere are extended to encompass the more realistic situation when the random parameters describing the physical properties of structural elements may take an arbitrary number of possible values. The profiles of spectral lines constructed by solving the stochastic problem of radiative transfer in the LTE atmosphere allow one to check the accuracy of the commonly used approximations.

The problem of determining the statistical characteristics of the line radiation formed in a multicomponent Non-LTE atmosphere is treated for the case of conservative scattering. We derive a closed-form analytical expression for the RelMSD of the outgoing intensity for an arbitrarily large number of structural elements in the medium.

The theoretical results are employed for interpreting some of the features specific to the spatial brightness fluctuations of prominences in extreme ultraviolet (EUV) lines. The estimates obtained for the mean value of the line-of-sight number of threads are in agreement with those inferred by other authors. Some important conclusions on the multithread structure of prominences are drawn.

Key words: radiative transfer – Sun: prominences – Sun: transition region

Send offprint requests to: Z. Mouradian

© European Southern Observatory (ESO) 1999

Online publication: February 23, 1999

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