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The results of a direct comparison of solar microwave recordings at the Metsahovi Radio Research station, Finland, and H-alpha flares at the Hvar Observatory, Yugoslavia, during the Solar Maximum Year compaigns of 1979-1981 and 1983 are reported. At the Metsahovi station, a 14-meter radio telescope is used and at the Hvar Observatory a solar double-telescope is used. It is found that the correlation between H-alpha flares and microwave gradual rise and fall events (GRF), with a time scale of tens of minutes, is high. At Metsahovi, 23 GRF events are measured and in 22 cases (96 percent) a simultaneous flare is observed at Hvar. On the other hand, impulsive microwave bursts with a time scale of tens of seconds have low correlation with H-alpha flares. It is shown that the main contribution to the microwave GRF bursts comes from thermal radiation and that the correlation of flares and microwave events is low at long wavelengths and very high at shorter wavelengths. 6 references.

Solar microwave maps (at 22 and 37 GHz) performed at Metsahovi (Finland) in the period 1979–1982 have revealed about a dozen radiation sources, microwave limb sources (MW LS), located up to 50 000 km above the optical limb of the Sun (in normal conditions the sources for microwave emission are locate 2 000 – 4 000 km above the photosphere). It could be established that MW LS showed up after the occurrence of two ribbon flares situated close to the limb of the visible hemisphere or that they have been connected to flares occuring behind the limb, which was documented by the appearance of Hα post flare loop systems. The life time of the MW LS was typically about 3 hours and they have been correlated to long decay events (LDE) in X-rays. In one case the active region where the MW LS appeared was also tracked at Metsahovi and a typical gradual rise and fall event was recorded. It is proposed that the MW LS are of thermal origin and that they originate from condensed flaring loops appearing at successive higher altitudes.