Microwave Signatures in the Pre-impulsive Phase of a Solar Flare

Abstract:

In this study we focus on the microwave signatures at the pre-impulsive phase of solar flares, by examining simultaneous radio-EUV imaging data obtained with Nobeyama Radioheliogragh (NORH) and Atmospheric Imaging Assembly / Solar Dynamics Observatory (AIA/SDO) for the M7.7 flare on 2012 July 19. The corresponding thermal bremsstrahlung emission was deduced on the basis of the temperature and column density values given by the SDO Differential Emission Messure (DEM) method. Then we obtained the non-thermal gyrosynchrotron emission profile by removing the thermal contribution from the NORH data at 17/34 GHz. We find that there was a dimming region at the cusp of the flare loop, corresponding to a maximum of hard X-ray emission observed by RHESSI, possibly caused by the presence of a magnetic null at the loop cusp. In addition, we also discovered several local maximum of microwave emission (termed as microwave islands) in the corona, possibly emitted by energetic electrons accelerated and confined within magnetically-closed structures (e.g., magnetic islands formed by magnetic reconnections). This is the first time such microwave features was reported.