Fiber bursts as 3D coronal magnetic field probe in postflare loops

Abstract

Fiber bursts appear in some complex solar radio bursts as a continuum fine structure in the frequency range of 1503000 MHz. We present and test a new method to use fiber bursts as a probe of the magnetic field strength and the 3D field structure in postflare loops. Thereby we assume that fiber bursts are driven by whistler waves ascending in the postflare loops which act as magnetic traps for nonthermal flare electrons. For a selected event (1997 April 07) we derive from dynamic radio spectra (Potsdam) and Nançay Radio Heliograph imaging data of fiber bursts the coronal magnetic field strength within the fiber burst source. We compare the fiber burst source positions and field strength estimates with the extrapolated potential magnetic field above the flaring active region NOAA 8027 using SOHO-MDI photospheric field data. The field strength from fiber bursts are within a factor of 0.6 to 1.4 of the field strength of the selected subset of potential field lines and give preference to a 3.5 times Newkirk (1961, ApJ, 133, 983) coronal density model within the evolving postflare loops. We find independent proof of the physical significance of considering selected potential field lines as postflare loop field information regarding topology and field strength over a time interval of one hour after the impulsive flare phase. We conclude that radio decimeter and meter wave spectra and radio imaging at two representative frequencies are sufficient for a reliable estimate of the (otherwise not measurable) coronal magnetic field strength in postflare loops. This can be an important field sounding method using the forthcoming FASR (Frequency Agile Solar Radiotelescope) instrument.