Abstract In this paper, we present a unique decimetric radio burst that oscillates not only in time but also in frequency. Its duration, exceeding 80 minutes, is significantly longer than any radio oscillations previously reported in the literature. The oscillation period of the burst was about 3 minutes, suggesting a connection with sunspot oscillations observed in EUV and radio wavelengths. We detected the same periodicity in a 3000 MHz radio source located in a loop anchored to sunspots and spanning the entire active region, where flares occurred during the burst. Based on these observations, we propose that the frequency-oscillating burst is generated in the loop, serving as a reservoir of accelerated electrons, by the electron-cyclotron mechanism. Waves originating in the sunspot penetrate into the loop containing the source of the frequency-oscillating burst, and the resonant period of this loop appears to be close to that of the incoming waves. As a result, the radio emission of the burst oscillated for about 80 minutes with a 3-minute period. The magnetic-field strength in the burst source was estimated to be about 250 G.
Karlický et al. (Sun,) studied this question.