The 26 December 2001 Solar Eruptive Event Responsible for GLE63. II.   Multi-Loop Structure of Microwave Sources in a Major Long-Duration flare
  Analysis of the observations of the SOL2001-12-26 event related to ground-level-event GLE63, including microwave spectra and images from NoRH at 17 and 34 GHz, SSRT at 5.7 GHz, and TRACE in 1600 \AA\ has led to the following results. A flare ribbon overlapped with the sunspot umbra, which is typical of large particle events. Atypical were: i) long duration of the flare of more than one hour; ii) moderate intensity of a microwave burst, about $10^4$ sfu; iii) low peak frequency of the gyrosynchrotron spectrum, around 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 \AA. Comparison of the 17 GHz and 1600 \AA\ images has confirmed that the microwave sources were associated with multiple flare loops, whose footpoints appeared in 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 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. An apparent simplicity of 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. 
V. V. Grechnev, A. M. Uralov, V. I. Kiselev, A. A. Kochanov (Institute   of Solar-Terrestrial Physics SB RAS)
astro-ph.SR