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We investigate the solar flare of 20 October 2002. The flare was accompaniedby quasi-periodic pulsations (QPP) of both thermal and nonthermal hard X-rayemissions (HXR) observed by RHESSI in the 3-50 keV energy range. Analysis ofthe HXR time profiles in different energy channels made with the Lombperiodogram indicates two statistically significant time periods of about 16and 36 seconds. The 36-second QPP were observed only in the nonthermal HXRemission in the impulsive phase of the flare. The 16-second QPP were morepronounced in the thermal HXR emission and were observed both in the impulsiveand in the decay phases of the flare. Imaging analysis of the flare region, thedetermined time periods of the QPP and the estimated physical parameters ofmagnetic loops in the flare region allow us to interpret the observations asfollows. 1) In the impulsive phase energy was released and electrons wereaccelerated by successive acts with the average time period of about 36 secondsin different parts of two spatially separated, but interacting loop systems ofthe flare region. 2) The 36-second periodicity of energy release could becaused by the action of fast MHD oscillations in the loops connecting theseflaring sites. 3) During the first explosive acts of energy release the MHDoscillations (most probably the sausage mode) with time period of 16 secondswere excited in one system of the flare loops. 4) These oscillations weremaintained by the subsequent explosive acts of energy release in the impulsivephase and were completely damped in the decay phase of the flare.

Observations of quasi-periodic solar X-ray emission as a result of MHD oscillations in a system of multiple flare loops