Seasonal Variation of Magnetic Storms
Yoshida, A., Toya, T. & Yoshida, M.
Abstract
We investigated the seasonal variation of magnetic storms based on the data at the Kakioka Magnetic Observatory since 1924. First we show that magnetic storms with the range of 100 nT or larger are considered to have been almost completely recorded for both types of storms, i.e., those with a sudden commencement (Ssc) and those with a gradual commencement (Sg). The number of storms decreases linearly with the range in a semi-logarithmic plot. The inclination of the decrease is steeper for Sg than for Ssc. The seasonal variation, i.e., more storms are observed in the spring and fall seasons than in the summer and winter seasons, is clearly seen for Sg storms with the range of 100 nT or larger. For Ssc storms the seasonal variation is recognized for those with the range of 150 nT or larger, but it is not clear for those with a smaller range. The inclination of the decrease of storm numbers with the range is gentler in the spring and fall seasons than in the summer and winter seasons for Ssc. Though such a difference is not apparently seen for Sg, the ratio of the number of storms with the range of 150 nT or larger to that of storms with the range between 100 nT and 150 nT is larger for the spring and fall seasons. On the basis of the idea that the seasonal variation in the occurrence of large storms is attributed to the equinoctial effect, we propose a hypothesis that the magnetic reconnection process is most efficient when (1) IMF is anti-parallel to the geomagnetic field, and (2) the magnetosphere-solar wind system has a north-south symmetry.
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