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

Abstract

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.

Are solar maximum fan streamers a consequence of twisting sheet structures?

“…2b with the NRGF and an edge-enhancement routine. The threads seen in the edge-enhanced image are very similar to the threads described by Koutchmy & Nikoghossian (2002). Figure 2 shows clearly that the base of these rays, or threads, extends down to the solar surface, as seen in the EIT field of view.…”

“…These rays are close to linear (but not necessarily radial) at heights above ∼3 R , but are often bent at lower heights. It is not entirely clear whether these rays are the same features described by Koutchmy & Nikoghossian (2002) as coronal linear threads, since the eclipse white light images given in that work are processed in a different manner to the images of Fig. 1.…”

“…These structures are highly filamentary, and contain bright rays which, in contrast to helmet streamers, seem to diverge super-radially. Fan streamers have been seen in eclipse observations for many decades (see references in Koutchmy & Nikoghossian 2002). Observations of equatorial poleward-diverging rays during solar maximum have been described by Eselevich & Eselevich (2006) as an unexpected phenomenon.…”

“…Observations of equatorial poleward-diverging rays during solar maximum have been described by Eselevich & Eselevich (2006) as an unexpected phenomenon. Coronal linear threads were studied in detail by Koutchmy & Nikoghossian (2002) from eclipse and other observations, who show a collection of bright, diverging threads rising from flaring active regions. In this Letter, we provide observational evidence to support the conjecture that fan streamers are a consequence of twisting high density sheet-like structures.…”

The August 11th, 1999 CME

“…We did not include models apparently producing a CME as the result of a large explosion at the solar surface like a flare. This last class of events is better analyzed as X-ray jets or even W-L linear rays (Koutchmy & Nikoghossian 2002).…”