The radio signature of twisted magnetic ropes and reconnection site in the low corona

Abstract

A solar radio burst on 25 August 1999 with fine structures (FS) at 4.5 - 7.5 GHz is studied in this paper. The FS started about one minute prior to the main burst. The maximum emission took place at 4 - 5 GHz for the FS, and at 10 - 11 GHz for the main burst, respectively. The time profiles at 4.5 - 7.5 GHz coincide very well with those of hard X-rays ( from 25 keV to > 300 keV) in both the main burst and the FS, which shows that the same population of accelerated electrons is responsible for both the microwave and hard X-ray bursts. The source of FS is 20 arc sec away from the main source close to a compact dipolar magnetic field, which is confirmed by different time and polarization profiles in the FS and main sources. It is interesting that the FS at 4.5 - 7.5 GHz are associated with a series of twisted magnetic loops or ropes, which may be modulated by Alfven waves with a period of 1 s and a spatial wavelength of 10(3) km in respect to the typical Alfven velocity of 10(3) km s(-1) in corona. These magnetic ropes may be rooted in the dipole site, which extended into the corona during the event and retracted after the event. Therefore, the FS in this event may show an important signature or precursor for energy release. The magnetic reconnection may be triggered by the interaction of the magnetic ropes at the height corresponding to 5 - 6 GHz, followed by cascaded energy release close to the foot-point of the magnetic ropes.