The 26 December 2001 Solar Eruptive Event Responsible for GLE63. II. Multi-Loop Structure of Microwave Sources in a Major Long-Duration Flare

Abstract

Our analysis of the observations of the SOL2001-12-26 event, which was related to ground-level enhancement of cosmic-ray intensity GLE63, including microwave spectra and images from the Nobeyama Radioheliograph at 17 and 34 GHz, from the Siberian Solar Radio Telescope at 5.7 GHz, and from the Transition Region and Coronal Explorer in 1600 Å, has led to the following results: A flare ribbon overlapped with the sunspot umbra, which is typical of large particle events. Atypical were i) the long duration of the flare, which lasted more than one hour; ii) the moderate intensity of the microwave burst, which was about $10^{4}~\mbox{sfu}$; iii) the low peak frequency of the gyrosynchrotron spectrum, which was about 6 GHz; and its insensitivity to the flux increase by more than one order of magnitude. This was accompanied by a nearly constant ratio of the flux emitted by the volume in the high-frequency part of the spectrum to its elevated low-frequency part determined by the area of the source. With the self-similarity of the spectrum, a similarity was observed between the moving microwave sources and the brightest parts of the flare ribbons in 1600 Å images. We compared the 17 GHz and 1600 Å images and confirm that the microwave sources were associated with multiple flare loops, whose footpoints appeared in the ultraviolet as intermittent bright kernels. To understand the properties of the event, we simulated its microwave emission using a system of several homogeneous gyrosynchrotron sources above the ribbons. The scatter between the spectra and the sizes of the individual sources is determined by the inhomogeneity of the magnetic field within the ribbons. The microwave flux is mainly governed by the magnetic flux passing through the ribbons and the sources. The apparent simplicity of the microwave structures is caused by a poorer spatial resolution and dynamic range of the microwave imaging. The results indicate that microwave manifestations of accelerated electrons correspond to the structures observed in thermal emissions, as well-known models predict.