Solar cycle distribution of major geomagnetic storms

We examine the solar cycle distribution of major geomagnetic storms ( Dst ≤ −100 nT), including intense storms at the level of −200 nT< Dst ≤ −100 nT, great storms at −300 nT< Dst ≤ −200 nT, and super storms at Dst ≤ −300 nT, which occurred during the period of 1957–2006, based on Dst indices and smoothed monthly sunspot numbers. Statistics show that the majority (82%) of the geomagnetic storms at the level of Dst ≤ −100 nT that occurred in the study period were intense geomagnetic storms, with 12.4% ranked as great storms and 5.6% as super storms. It is interesting to note that about 27% of the geomagnetic storms that occurred at all three intensity levels appeared in the ascending phase of a solar cycle, and about 73% in the descending one. Statistics also show that 76.9% of the intense storms, 79.6% of the great storms and 90.9% of the super storms occurred during the two years before a solar cycle reached its peak, or in the three years after it. The correlation between the size of a solar cycle and the percentage of major storms that occurred, during the period from two years prior to maximum to three years after it, is investigated. Finally, the properties of the multi-peak distribution for major geomagnetic storms in each solar cycle is investigated.