Mechanism for producing the U-shaped spectrum of the type-IV solar radio bursts

Abstract A mechanism is proposed to account for the U-shaped spectrum of the type IV solar radio burst during the explosive phase of proton flare. The U-shaped spectrum consists of two parts, namely the part of type IV μ burst and that the type IV dm burst. Each of them is believed to be produced by gyrosynchroton radiation of nonthermal electrons (100keV ≤ ϵ ≤ 2MeV) accelerated simultaneously with protons in explosive phase and gyroresonance absorption of thermal electrons. Numerical calculations are presented for the flux densities of type IV μ burst and type IV dm during 7 August 1972 event, assuming the volume of these burst sources being cylinders. It is shown that the results of numerical calculations are consistent with that of observations. The reversal of polarized waves sense from type IV μ burst source into type IV dm sources is explained by the fact that the direction of the magnetic field line in one source is anti-parallel to that in the other. Besides, we also discuss the relation between the U-shaped spectrum of type IV radio burst and the occurrence of proton event.