Statistical Analysis of Solar Cycle Distribution of Moderate Storms

In this paper, the statistical analysis of solar cycle distribution of moderate geomagnetic storms (-100nT< Dst ≤-50nT) is investigated based on observed data. The results show that the yearly number of moderate storms have multi-peaks in each cycle with the highest peak appeared in the descending phase of the cycle. This means that the highest peak of moderate storms have a time lag with respect to the yearly number of sunspot numbers. The time lag is about 2～3 year. More than 70% moderate storms appeared in the descending phase of a solar cycle. During the solar maximum, there are less moderate storms occurred than that occurred in other period of a solar cycle. Monthly mean moderate storm numbers and smoothed monthly mean sunspot numbers is also investigated in this paper. The results show that the time of moderate storms peaks lag behind that of the time of sunspots peaks, the time of peaks of both moderate storms and sunspots is different, and the time differences are different in different solar cycles, ranging from a few years to just a few months.