Observational Evidence for Energy Accumulation and Dissipation in Coronal Magnetic Loops

Abstract

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.