Very long baseline interferometry of solar microwave radiation

The solar 2.297 GHz radiation has been observed and investigated by very long baseline interferometry (VLBI). The radio observatories of Medicina, Note, Onsala, and Weilheim were involved yielding baselines between 360 km and 3800 km and a nominal resolution of 0.09 '' 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 hours during 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 bursts. A total of 59 solar radio bursts were observed at the VLBI frequency, of which 26 events were analyzed, including narrowband millisecond spikes, type III bursts, patches, pulsations, and diffuse broadband (gyrosynchrotron) emission. Neither during bursts nor in quiet times significant fringes were detected. All sources were well resolved including the narrowband spikes. We interpret the result in terms of relatively large radio sources and/or by scattering to apparent source sizes larger than the lowest resolution and by the lack of 'speckles'. The results are consistent with scattering of the radio emission in the corona. The upper and lower limits of the source size of spikes are discussed. For the apparent source size, l(a), we find 65 km < l(a) < 16 000 km, and for the original source size before scattering l less than or similar to 200 km.