A radio study of the evolution of spatial structure of an active region and flare productivity

We present the results of a radio study of the evolution of an active region through its flare productivity. The radio study was carried out with data obtained by the Nobeyama Radio Heliograph at 17 GHz. We chose the active region AR 7515, which appeared at the east limb on 1993 May 23 and then evolved during its passage across the disk. We followed its evolution until June 2. This region produced many small flares. We consider this region to be a typical active region in the sense that it did not produce any large flares, but a large number of weak flares. We investigate the optical and magnetic development of the region and show how this affects the locations of the flaring activity. We discuss a number of events in detail in order to investigate the roles of nonthermal and thermal radio emission in the flares. The nonthermal gyrosynchrotron emission generally occurs in regions of strong magnetic fields, is generally circularly polarized, and often varies rapidly in time. On the other hand, gradual radio components tend to be thermal and only weakly polarized, if at all. An interesting aspect of evolution of the flares in this region is that many of the flares in the early phase of the evolution show strong but brief nonthermal radio emission in the impulsive phase followed by gradual thermal emission, whereas in the last 3 days more gradual events without a strong spike of radio emission in the impulsive phase tend to be seen. Correspondingly, the flare images suggest that the radio sources are more compact during the early phases and more extended in the last half of the period covered. The most dominant component of the preflare region is often not the component that undergoes immediate flaring. Sometimes a number of components in the preflare region participate in the flare process together. We speculate that these component sources are unresolved compact bipolar loops that flare in sequence. Loop-loop interactions occurring at many different sites at the same time seems to be a less plausible explanation of these events.