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Small Scale Microwave Bursts in Long-duration Solar Flares

Solar small scale microwave bursts (SMBs), including microwave dot, spike, and narrow band type III bursts, are characterized with very short timescales, narrow frequency bandwidth, and very high brightness temperatures. Based on observations of the Chinese Solar Broadband Radio Spectrometer at Huairou with superhigh cadence and frequency resolution, this work presents an intensive investigation of SMBs in several flares occurred in active region NOAA 10720 during 2005 Jan 14-21. Especially long-duration flares, SMBs occurred not only in early rising and impulsive phase, but also in the flare decay phase, and even in time of after the flare ending. These SMBs are strong bursts with inferred brightness temperature at least 8.18*10^11 - 1.92*10^13 K, very short lifetime of 5-18 ms, relative frequency bandwidths of 0.7-3.5%, and superhigh frequency drifting rates. Together with their obviously different polarizations from the background emission (the quiet Sun, and the underlying flaring broadband continuum), such SMBs should be individual independent strong coherent bursts which is related to some non-thermal energy releasing and production of energetic particles in small scale source region. These facts show the existence of small scale strong non-thermal energy releasing activities after the flare maxima, which is meaningful to the prediction of space weather. Physical analysis indicates that plasma mechanism may be the most favorable candidate for the formation of SMBs. From plasma mechanism, the velocities and kinetic energy of fast electrons can be deduced, and the region of electron acceleration can also be tracke.