Coronal linear threads: W-L radiation of supra-thermal streams

¹ Institut d'Astrophysique de Paris, CNRS, 98bis bd. Arago, 75014 Paris, France e-mail: koutchmy@iap.fr
² Byurakan Astrophysical Observatory, 378433 Byurakan, Armenia e-mail: narthur@bao.sci.am
³ Isaac Newton Institute of Chile, Armenian Branch

Corresponding author: S. Koutchmy, koutchmy@iap.fr

Received: 9 July 2002
Accepted: 28 August 2002

Abstract

The paper aims at revealing the role of the Compton effect when considering the W-L scattering of the solar photospheric radiation by coronal supra-thermal streams. We first document several examples of well observed cases of linear W-L coronal threads extending above flaring active regions which are good candidates to give the signature known from the interpretation of radio type III bursts. The height-dependent model problem of Compton scattering on the beam of fast electrons gyrating around the lines of force of the magnetic field is considered. The resulting change in frequency averaged over the beam and the solid angle, within which the photospheric radiation falls, is computed for both sunward and antisunward directed streams. The dependence of the effect on the height above the solar surface, the speed of electrons and the slope angle is discussed. For illustration, we consider the case, in which the frequency distribution of the incident radiation is Planckian. The effective value of the cross-section of interaction is compared with that for the Thomson scattering. In particular, we conclude that, depending on the angle between the directions of the initial outburst and the magnetic field, the streams moving away from the Sun may produce drifts in frequency to the shortwave, as well as to the longwave domains of the spectrum. The effect may become essential even for moderate energies of fast electrons, and it should be measurable using precise photometric color index determination, if only the fractional density of fast electrons is not too small.