Favourable Magnetic Field Configurations for Generation of Flare-Associated Meter-Wave Type III Radio Bursts

Magnetic field structures of Hα flares associated with meter-wave type III bursts during periods of low solar activity in 1975 – 1977 and 1985 – 1987 were investigated. In a statistical analysis it was confirmed that the association rate depends less on flare importance than on brightness. For subflares (95% of the sample), the location of the Hα flare in the bipolar pattern turned out to be crucial for the association rate. It is almost one order of magnitude larger for flares occurring at the border of the active regions, compared to flares located inside the general bipolar pattern. For selected typical examples of flares, extrapolations of the measured magnetic fields were performed. By matching Hα filtergrams and calculated 3-D structures it was found that the positions at the border where the flares associated with type III bursts occurred were close to open field lines extending into the corona. In most investigated cases intrusions of parasitic polarity were found in the vicinity of the flare locations. The extrapolations showed that subflares located inside the bipolar pattern but have not been associated with type III bursts were covered by dense arcades of magnetic loops.