CRITERIA FOR THE ASSOCIATION OF SOLAR FLARES WITH GEOMAGNETIC DISTURBANCES

The statistical relation between solar flares of importance [Formula: see text] and the onset of geomagnetic disturbances was studied for 1949–61. Disturbances were defined as an increase in ap[Formula: see text], occurring within 12 hours or less, and these disturbances include both gradual and sudden commencement events. Associated radio noise bursts at frequencies below 300 Mc were used to select 240 major-burst flares, and the effects of flare importance, time of solar epoch, and associated polar-cap-absorption (PCA) events were considered in turn. A detailed examination was made of the effect of flare heliographic location. There is a strong positive correlation between the occurrence of major-burst flares and geomagnetic disturbances 1 to 3 days afterwards. The degree of correlation is approximately doubled when the major-burst flare is associated with a PCA event. The most probable time delay between flares and disturbances is 1.5 to 2.0 days, and this timing is relatively constant over a wide range of conditions.The dominance of northern storm-flares (Bell 1961) was confirmed for 1949–61. Detailed analysis shows that the effect is strongest for heliographic latitudes above 15 °N. In addition to this effect, there was a consistent western bias of the storm-flares in both the northern and southern hemispheres, for flares associated with either gradual or sudden commencement (SC) events, with maximum concentration occurring at 10 °W. to 15 °W. Major-burst flares associated with PCA events also show this western bias; the data suggest that there is a systematic increase in east–west asymmetry in going from major-burst storm-flares to major-burst PCA flares to PCA flares accompanied by ground-level cosmic-ray increases. The corresponding longitudes of maximum concentration of the associated flares are about 10 °– 15 °W., 45 °W., and 75 °W.