FLAT MICROWAVE-SPECTRA SEEN AT X-CLASS FLARES

We report peculiar spectral activity of four large microwave bursts as obtained from the Solar Arrays at theo Owens Valley Radio Observatory during Observations of X-class flares on 1990 May 24 and 1991 March 7 , 8, and 22. Main observational points that we newly uncovered are: (1 ) flat flux spectra over 1-18 GHz in large amounts of flux ranging fro m 10(2) to 10(4) s.f.u. at the maximum phase, (2) a common evolutionar y pattern in which the spectral region of dominant flux shifts from hi gh frequencies at the initial rise to low frequencies at the decaying phase, and (3) unusual time profiles that are impulsive at high freque ncies but more extended at lower frequencies. In an attempt to elucida te these new properties, we carry out the model calculations of microw ave spectra under assumptions of gyrosynchrotron mechanism and a dipol e field configuration to reproduce the observational characteristics. Our results are summarized as follows. First, a flat microwave spectru m reaching up to 10(2)-10(4) s.f.u. may occur in a case where a magnet ic loop is extended to an angular size of approximately (0.7-7.0) x 10 (-7) sterad and contains a huge number (N(E > 10 keV) approximately 10 (36)-10(38)) of nonthermal electrons with power-law index delta approx imately 3-3.5 over the entire volume. Second, the observed spectral ac tivity could adequately be accounted for by the shrinking of the regio n of nonthermal electrons to the loop top and by the softening of the power-law spectrum of electrons in a time scale ranging 3-45 min depen ding on the event. Third, the extended microwave activity at lower fre quencies is probably due to electrons trapped in the loop top where ma gnetic fields are low. Finally, we clarify the physical distinction be tween these large, extended microwave bursts and the gradual/post-micr owave bursts often seen in weak events, both of which are characterize d by long-period activity and broadband spectra.