  We analyse quasi-periodic pulsations (QPP) detected in the microwave and
decimeter radio emission of the SOL2017-09-05T07:04 solar flare, using
simultaneous observations by the Siberian Radioheliograph 48 (SRH-48, 4-8 GHz)
and Mingantu Spectral Radioheliograph (MUSER-I, 0.4-2 GHz). The microwave
emission was broadband with a typical gyrosynchrotron spectrum, while a
quasi-periodic enhancement of the decimetric emission appeared in a narrow
spectral band (500-700 MHz), consistent with the coherent plasma emission
mechanism. The periodicity that we found in microwaves is about 30 s, coming
from a compact loop-like source with a typical height of about 31 Mm. The
decimetric emission demonstrated a periodicity about 6 s. We suggested a
qualitative scenario linking the QPPs observed in both incoherent and coherent
spectral bands and their generation mechanisms. The properties of the QPPs
found in the microwave signal are typical for perturbations of the flare loop
by the standing sausage mode of a fast magnetohydrodynamic (MHD) wave. Our
analysis indicated that this sausage-oscillating flare loop was the primary
source of oscillations in the discussed event. The suggested scenario is that a
fundamental sausage harmonic is the dominant cause for the observed QPPs in the
microwave emission. The initiation of oscillations in the decimetric emission
is caused by the third sausage harmonic via periodic and nonlinear triggering
of the acceleration processes in the current sheets, formed at the interface
between the sausage-oscillating flare loop and the external coronal loop that
extended to higher altitudes. Our results demonstrate the possible role of MHD
wave processes in the release and transport of energy during solar flares,
linking coherent and incoherent radio emission mechanisms.
