A&A 420, 1107-1115 (2004)
Slow magnetoacoustic-like waves in post-flare loopsA. N. Kryshtal and S. V. Gerasimenko
Department of Cosmic Plasma Physics, Main Astronomical Observatory of the National Academy of Sciences of Ukraine, 03680 Zabolotnogo Str., 27, Kiev 127, Ukraine
(Received 12 February 2003 / Accepted 16 February 2004 )
We investigate the stability to the development of plasma waves in the preflare situation of a loop structure at the chromospheric part of a current circuit of a loop. We investigate the conditions under which low-frequency plasma instabilities can develop, assuming the absence of beam instabilities.The large-scale quasi-static electric field in the loop circuit is assumed to be "subdreicer" and weak. Thus the percentage of "runaway" electrons is very small and their influence on the process of instability development is negligible. The pair Coulomb collisions are described by a BGK-model integral. We consider the situation when the plasma at the surface layer of a loop has a spatial gradient of density. In accordance with Heyvaerts-Priest-Rust theory, such a preflare situation would typically exist when the amplitude of the weak electric field in the circuit of an "old" loop in an active region begins to increase when "new" magnetic flux emerges from under the photosphere. We have found that two types of waves are generated in such a plasma due to the growth of instabilities: the "kinetic Alfven-like" waves and new type of waves, in the range of magnetoacoustic ones. The instability of these latter waves has a clear threshold and it can be considered as an "indicator" of the development of a preflare situation in an active region.
Key words: plasmas -- Sun: flares -- Sun: chromosphere
Offprint request: A. N. Kryshtal, email@example.com
© ESO 2004