A STATISTICAL STUDY OF THE RELATION BETWEEN SOLAR FLARE AND GEOMAGNETIC DISTURBANCE FOR 1966-1978

This paper provides a set of curves for the variation of various geomagnetic indices,Ap,AE,Dst and Kp initiated by solar flares grouped by their optical importance,duration,and the distance from the central meridian.Fig.1 and 2 show that both the duration and optical importance are good criteria for the geomagnetically-effective flares.Statistically,only the major flares of importance 3 or that of importance 2 but with duration ≥1.5 hours can significantly disturb the geomagnetic field onto the levels of Kp≥4.For these major flares,brightness plays an important role in determing both the intensity and the character of the resulted disturbance.In general,the bright major flares affect the Ap,AE,and Dst indices,onto Kp≥4,while the normal or faint ones result in significant disturbance of the AEindex limited to Kp-4 or 5 only.It can be shown from Fig.3 that there are two kinds of the east-west asymmetry in the distribution of flare's position in response to the geomagnetic disturbance.Statistically,the disturbance in Ap and Dst indices intensified onto Kp≥6 are related mainly to the flares situated between₃₀°Eand 60°W,whereas the occurrence frequency of the disturbance with intermediate intensity of Kp=4 or 5 increases when the flare goes from the east to the west of the central meridian,peaking at 60°-90°W.The disturbance of AE index has bath the above asymmetries.Also,there are some indications of the dependence of the intensity of geomagnetic disturbance upon the sun-earth propagation velocity of the flare ejecta,which may be sped up when the flare goes from the east to the west of the central meridian.These results reflect the complex nature of the interplanetary disturbance produced by solar flares,which are under study and will be published elsewhere.