Abstract

We study the evolution of the active region (AR) NOAA 7321 in which appeared a so-called `neutral-line-associated source' (NLS) on the basis of data of the Nobeyama Radioheliograph and Yohkoh/SXT. We provide a physical interpretation of the NLS in terms of a topological magnetic reconnection model in a quadrupole magnetic configuration and discuss its relation to the evolution of the active region. Two kinds of the NLS were observed at 17 GHz. One of them, `rising NLS', was found in the growth stage. The other was `stationary NLS' detected in the maximal stage of the AR. Their presence was associated with substantial expansion of the active region's magnetosphere and accompanied by gradual development of spine-like structures visible in soft X-rays before homologous long-duration arcade flares. We suggest that the rising 17 GHz NLS corresponded to a fragment of a `horizontal' current sheet moving upward. Bright X-ray spines were boundaries of that current sheet. Almost all bursts observed from 26 to 28 October 1992 which accompanied class C and M flares occurred in the rising NLS. Formation of magnetic X-point singularities is believed to be responsible for the low-lying NLS. Reversal of circular polarization due to the effect of radio wave propagation was detected in that NLS on the limb. The initial stage of the microwave burst of the long-duration X9 class flare on 2 November 1992 occurred in this NLS. We also revealed observational manifestations of the presence of `vertical' non-neutral current sheet in the spatial structure of this NLS before the flare.