Structure and evolution of solar radio bursts at 26.4 MHz

Abstract

Two dimensional source brightness distributions at 26.4 MHz for solar bursts of spectral type II, III, IV, and V are derived from observations with a multiple-baseline, time-sharing interferometer system. It was designed explicitly to study the large angle (∼40′ halo) component of low frequency solar bursts first reported by Weiss and Sheridan (1962). Thirty-two bursts occurring in the interval of June–August, 1975, were fit with a circular gaussian ‘core’ and an elliptical gaussian ‘halo’ component. Half-power halo diameters (E-W×N-S) averaged 30′×28′ for type III bursts and 42′×27′, 28′×37′, 30′×25′ for type V, II and IV bursts respectively. Typical core sizes fell in the range of 10′±4′ giving ∼ 3∶1 halo to core size ratio. All burst types were found to have some large angle structure: the specific intensity was ∼10% compared to the core but the total power in each component was comparable. Two processes for producing the core-halo structure of type III bursts are compared: scattering and refraction of a point source and refraction from many sources over an extended region. It is concluded that the core can be explained by either model but the halo is more consistent with emission from an extended source region of ∼40° in longitude.