  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.
