Statistics of geomagnetic storms and ionospheric storms at low and mid latitudes in two solar cycles

The statistics of occurrence of the geomagnetic storms, and ionospheric storms at Kokubunji (35.7 degrees N, 139.5 degrees E; 26.8 degrees N magnetic latitude) in Japan and Boulder (40.0 degrees N, 254.7 degrees E; 47.4 degrees N) in America are presented using the Dst and peak electron density (Nmax) data in 1985-2005 covering two solar cycles (22-23) when 584 geomagnetic storms (Dst <= -50 nT) occurred. In addition to the known solar cycle and seasonal dependence of the storms, the statistics reveal some new aspects. (1) The geomagnetic storms show a preference for main phase (MP) onset at around UT midnight especially for major storms (Dst <= -100 nT), over 100% excess MP onsets at UT midnight compared to a uniform distribution. (2) The number of positive ionospheric storms at Kokubunji (about 250) is more than double that at Boulder, and (3) the occurrence of the positive storms at both stations shows a preference for the morning-noon onset of the geomagnetic storms as expected from a physical mechanism of the positive storms. (4) The occurrence of negative ionospheric storms at both stations follows the solar cycle phases (most frequent at solar maximum) better than the occurrence of positive storms, which agrees with the mechanism of the negative storms.