Flare geoefficiency in relation to photospheric magnetic field orientation of active regions

The objective of this study was to investigate the geoefficiency of a set of flares which had occurred in solar cycles Nos 19 and 20, and were followed by the most severe geomagnetic storms with sudden commencements. Flares with simultaneous occurrence of type II radio bursts, observed in the years 1970-1976, were taken as comparison data. More frequent occurrence of geoeffective flares was found at the location where direction of the photospheric magnetic field was southward. Furthermore, flares occurring at the site with the dividing line between polarities oriented approximately perpendicular to the solar meridian and also flares at the site with polarity against the normal bipolar distribution of photospheric magnetic field according to Hale's law, display a tendency toward higher geoefficiency. Location of flares in the central zone at the solar disc is more favourable with regard to their effect on disturbing the geomagnetic field. (author)