Radio burst from converging separatrices

Context. In a previous paper on the X17 flare October 28, 2003, we found, among other radio flare sources, one displaced at approximate to 0.2 R-circle dot north of the flaring AR 10486, away from the main H alpha and hard X-ray flare emission. The source came to our attention due to its timing, its spatial displacement from the flaring active region, and due to the behavior of the radio spectral fine structure sources which are embedded in the continuum emission. We speculated that the source is situated at a separator of coronal magnetic flux systems. Aims. Here we analyze the topology of the potential coronal magnetic field extrapolated from SOHO-MDI data near the source site of this meter-decimeter radio continuum. Methods. Using the Source Method for magnetic field modeling, and the displacement gradient of the field lines as a connectivity measure, we give a description of the critical field structure. We compare it with the Nancay Radio Heliograph source positions of the continuum which have been classified by the spectral data of Astrophysical Institute Potsdam. Results. We find that the radio source occurs near the contact of three separatrix surfaces between magnetic flux cells. There are other separatrix surfaces in the field which are not distinguished by a strong radio source during the analyzed flare. Conclusions. This is the first evidence for the occurence of a strong coronal radio burst continuum source at such coronal magnetic field structures.