Oscillations of the Microwave Emission of Solar Active Region 12673 before Flares

Abstract

We present a study of quasi-periodic pulsations in the microwave emission from solar active region (AR) NOAA 12673, which produced the strongest flares of the 24th solar activity cycle. The data from daily observations of the Sun at the Nobeyama Radioheliograph at a frequency of 17 GHz were used. The microwave emission of the AR was analyzed for September 3–4, 2017, i.e., before the first M-class flare, which occurred on September 4. Long-period pulsations in the microwave emission with periods of approximately 100 min appeared at least 7 h before the first M-class flare, which was followed by strong flare activity in NOAA 12673. The effect is similar to the previously discovered preflare oscillatory phenomena with different periods manifested in different ranges.