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Dynamic spectra of low-frequency (LF) modulation of microwave emission during solar flares are obtained. Observation data for five radio bursts at frequency 37 GHz recorded by a 14-m Metsaahovi observatory (Finland) radiotelescope in the period from 1990 to 1993 were used. Frequency modulation of the radio emission intensity with the average period 296±37 (1σ) s, which is close to the period of well-known photospheric oscillations, was observed. Possible mechanisms of photospheric oscillation influence on the regions of radio burst generation are discussed.

Using the Wigner-Ville transform, we study the dynamic spectra of low-frequency modulation of the intensity of microwave emission from several solar flares and detect the microwave bursts modulated by chirp signals (i.e., signals whose frequencies ω=ω0± kt, where k is a constant and t is the time) with a positive chirp rate. Such a modulation corresponds to the process of energy accumulation in the corresponding coronal magnetic loop (CML). We also detect chirp modulation with a negative chirp rate corresponding to powerful dissipation of electric current in the CML during a solar flare. The chirp modulation of the intensity of microwave emission from a CML arises from the excitation of eigenoscillations of the loop as an equivalent electric circuit. In this case, the modulation frequency is proportional to the loop electric current and varies with the variation of the latter. The electric-current values found for a few events using the dynamic spectra of chirp signals lie in the range 1011-1012A, and the rate of energy increase (decrease) is estimated to be 1.4·1026-3·1029 erg/sec for the characteristic time scale τ≃ 103-4·104 sec. The events studied give evidence for the possibility of realizing the “circuit” model of solar flares under solar-corona conditions.

We analyze low-frequency intensity fluctuations of the microwave emission from solar flares at frequencies 22 and 37 GHz. The three microwave bursts of durations of about 1 h, observed at the Metsahovi Radio Observatory (Finland) with the time resolution of 0.1 and 0.05 s, are studied. To obtain spectral-temporal characteristics of the low-frequency fluctuations, we apply the Wigner-Ville method, i.e., the time-lag Fourier transform of the “local” autocorrelation function of an analytical signal. As a result, we obtain for the first time the dynamical spectra of the low-frequency fluctuations, which are identified as MHD eigenoscillations of coronal magnetic loops. The features of the dynamical spectra testify that several types of low-frequency pulsations are excited in coronal magnetic loops during solar flares: 1) Fast and slow magnetosonic oscillations with periods of 1-1.5 s and 200-280 s, respectively. Fast magnetosonic oscillations appear as pulse trains of duration 100-200 s and have the positive frequency drift dν/ dt≈ 0.125 Hz/min and the frequency splitting δν≈ 0.05 Hz; 2) The eigenoscillations of a coronal magnetic loop as an equivalent electric circuit. The period of these oscillations is about 1 s during the initial stage of a microwave burst and increases gradually up to 4 s during the decay stage of the radio emission; and 3) Intensity modulation of the microwave radiation by a periodic pulse sequence with a period of about 1 s at the burst onset and about 2 s at its end. The parameters of the dynamical spectra and identification of the MHD pulsations allow us to obtain information on the loop parameters, such as the ratio of the loop radius to its length (r/L≈ 0.1), the ratio of the gas pressure to the magnetic-field pressure inside the loop (β ≈ 3· 10-3), the ratio of plasma densities outside and inside the loop, and the electric current in the coronal loop (I≈ 1.5 · 1012 A).