Fiber bursts and the coronal magnetic field during solar flares

G. Rausche

H. Aurass, G. Mann, A. Hofmann

Fiber bursts have an intermediate frequency drift rate between type II and type III bursts. They appear in some complex solar radio bursts as a continuum fine structure between 200 MHz and 2 GHz. The question if fiber bursts supply a reliable information about the coronal magnetic field strength in active region post flare loops is not clear until now.

As a fiber burst model Benz and Mann (1999) promote the propagation of whistler wave pakets along coronal field lines within a magnetic trap filled with energetic electrons. If whistler wave pakets become radio visible by fiber burst emission one should find the region of the magnetic field which guided the whistler propagation from radio images of the fiber burst sources using an active region density model.

For a selected event (1997 April 07) we derived this information from AIP spectra and Nancay Radio Heliograph imaging data. Next, we compared the fiber burst source positions with a (static) estimate of the coronal magnetic field above AR 8027. This was obtained by a potential field extrapolation of SOHO-MDI field data.

By combining all data sets we find those field ranges which are identified by the fibers. We find a reasonable evolution of the fiber burst - field line range relation if we compare the results with SOHO-EIT and YOHKOH-SXT imaging data about the eruption. We present estimates of the coronal magnetic field strength from the fiber burst data and compare them with the field extrapolation respecting a possibly varying active region density distribution. Despite the static field approach the presented analysis is promising.