著者

吉田 明夫 外谷 健 吉田 真希子

出版者

Tokyo Geographical Society

雑誌

地学雑誌 (ISSN:0022135X)

巻号頁・発行日

vol.113, no.1, pp.107-124, 2004-02-25 (Released:2009-11-12)

参考文献数

39

被引用文献数

1 1

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We investigated solar-cycle and semiannual variations of magnetic storms based on data observed at the Kakioka Magnetic Observatory since 1924. First, we show that magnetic storms with a peak value of ≥ 100 nT are considered to have been almost completely recorded for both types of storm, i.e., those with sudden commencement (Ssc) and those with gradual commencement (Sg). Using data for storms larger than 100 nT, we found that the variation of numbers of Ssc correlates well with that of the sunspot Wolf numbers, but that of Sg does not. The number of storms decreases linearly with the peak value in the semi-logarithmic plot. The inclination is steeper for Sg storms than for Ssc storms. The semiannual variation, that is, more storms are observed in spring and fall than in summer and winter, is clearly seen for storms with a peak value ≥ 100 nT for Sg, but it is recognized only for larger storms with a peak value ≥. 150 nT for Ssc. Concordant with the difference between Ssc and Sg in the semiannual variation, the decrease in the number of large storms in the semi-logarithmic plot is less in spring and fall than that in summer and winter for Ssc. Although such a difference is not apparently seen for Sg, the ratio of the number of storms with a peak value ≥ 150 nT to that of storms with a peak value between 100 nT and 150 nT is larger for spring and fall. We propose the hypothesis that the efficiency of magnetic reconnection depends on the three dimensional direction of the magnetic field in the solar wind relative to the geomagnetic field, and on the north-south symmetry of the geomagnetosphere when it is seen from the stream of the solar wind. We think these “equinoctial effects” are the main causes of the semiannual variation in the number of large magnetic storms. As for the Sudden Commencement (SC), the semiannual variation is not seen, but an annual variation with the number of SC being larger in summer is observed. The reason may be that the shock wave in the solar wind hits against the northern part of the geomagnetosphere in summer, and the southern part in winter.