Solar cycle phase and occurrence of intense geomagnetic storms

Abstract

Solar cycles have an asymmetrical shape with a fast rise and a slow decline, which varies from cycle to cycle. This makes it difficult to study the solar cycle phase dependence of the occurrence of intense solar-terrestrial events such as intense geomagnetic storms. In the previous works, differences in the rise and fall time lengths of each solar cycle were not fully considered. We normalized the asymmetric shape of each solar cycle using the rise and fall time lengths and showed the solar cycle phase dependence of occurrence of the intense geomagnetic storms selected using the Dst index for solar cycles 19−24 and using the aa index for solar cycles 12−24. The previous works noted that the occurrence of geomagnetic disturbances shows double peaks before and after solar maximum. Our results showed that the occurrence of the intense geomagnetic storms selected using the Dst and aa indices does not always show clear double peaks and increased more in the earlier half of the fall time than in the other time periods. The Dst and aa indices have been used to select geomagnetic storms in the previous studies. We pointed out that the solar cycle phase dependence of occurrence of the geomagnetic storms selected using the aa index is slightly different from those selected using the Dst index. We showed the geomagnetic storm on 4 August 1972 as an extreme case, which showed difference between Dst and aa. We also showed that intense geomagnetic storms associated with eruptive flares from the active sunspot groups occurred around the solar minima of cycles 17, 19, 20, and 21.Graphical