DUAL-PEAK SOLAR-CYCLE DISTRIBUTION OF INTENSE GEOMAGNETIC STORMS

The solar cycle distribution of intense geomagnetic storms has been studied for cycles 20 and 21 (1965-1985), using values of the Dst index < - 100 nT, and for cycles 12-19 (1879-1964), using values of the aa index > 100 nT. It is claimed that a dominant dual-peak (DP) distribution exists in the solar cycle variability of these storms, with one peak occurring at the late ascending phase of the cycle or at solar maximum and another at the early descending phase of the cycle. From the 10 cycles studied, the average separation of the peaks from solar maximum is about 8 months ahead for the first peak and about 25 months after for the second one. For the cycles studied, the ratio of the average number of storms occurring at the peaks as compared with that occurring at the valley of the DP distribution was found to be statistically significant to better than a 95% confidence level. Although the present study is restricted to intense storms, their distribution including moderate events for the interval 1965-1985, in terms of Dst values < -50 nT, also shows evidence of a dual-peak distribution. Thus, in contrast with previous work on the solar cycle distribution of geomagnetic storms, which have emphasized the idea that they tend to peak at the descending phase of the cycle, the present work provides evidence for another important enhancement of this distribution at the late ascending phase of the cycle, extending sometimes to solar maximum. The origin of the dual-peak distribution of intense storms is associated to a similar dual-peak distribution obtained for large-amplitude (< -10 nT) and long-duration (> 3 h) values of the negative Bz component of the interplanetary magnetic field (IMF), computed for the interval 1970-1981. This class of Bz fields was shown to be the cause of all intense storms studied for the interval August 1978/2-December 1979 (Gonzalez and Tsurutani, 1987, Planet. Space Sci. 35, 1101). © 1990.