Very Long Baseline Interferometry of Solar Micorwave Radiation

Abstract

The solar 2.297 GHz radiation has been observed and investigated by very long baseline interferometry (VLBI). The radio observatories of Medicina, Noto, Onsala, and Weilheim were involved yielding baselines between 360 km and 3800 km and a nominal resolution of 0.009" to 0.008" or 70 to 6 km on the Sun. This solar VLBI network operated successfully with at least one useful baseline for 167 hoursduring five campaigns at the maximum of the most recent activity cycle in 1989 and 1990. the Phoenix spectrometer at Zurich was used to detect and classify the radio burst. A total of 59 solar radio bursts were observed at hte VLBI frequency, of which 26 events were analyzed, including narrowband millisecond spikes, type III bursts, patches, pulsations, and diffuse broadband (gyrosynchroton) emission. Neither during bursts nor in quiet times significant fringeswere detected. All sources were well resolved including the narrowband spikes. We interpret the result in terms of relatively of large radio sources and/or by scattering to apparent sources sizes larger that the lowerst resolution and by the lack of "speckles". The results are consistent with scattering of the radio emission in the coroma. The upper and lower limits of the source size of spikes are discussed. For the apparent sour size la, we find 65<la< 16000km, and for the original source size before scattering l<200 km.