A statistical comparison of solar wind sources of intense and moderate geomagnetic storms at solar maximum and minimum

Superposed epoch analyses of 153 storms are performed to make a comparison of solar wind typical features of geomagnetic storm events at solar minimum (January, 1995 - June, 1997) and solar maximum (July, 1999 - June, 2002). In this study, geomagnetic storms are defined by the pressure corrected Dst (Dst^*) and classified into intense storms and moderate storms. After the seasonal distribution of storms in each category is shown,the average values of interplanetary magnetic fields (IMF), solar wind plasma, NOAA/POES Hemispheric Power, Kp and Dst^* are analyzed and compared among different storm categories. Occurrence peak shiftings between intense storms and moderate storms are found in the storm seasonal distributions. It is shown that the peak of solar wind density leads minimum Dst* by 5.2-7.3 hours, which is longer than the peak time difference (0.5-1.0 hour) between IMF B_z and Dst^*_min. During intense storms, it is found that much higher solar wind density and lower solar wind bulk flow speed at solar minimum than at solar maximum. The results obtained also show that the average interplanetary cause of intense storms is IMF B_s ¡Y 10 nT lasting for over 3 hours with longer duration and less fluctuations at solar maximum than at solar minimum. As for moderate storms, the primary cause is IMF B_s ¡Y 5 nT lasting for over 2 hours with almost no average value difference among solar wind parameters at solar maximum and minimum, except for higher solar wind temperature during moderate storms at solar minimum than at solar maximum. The interplanetary and solar origins of storms in different categories are also discussed.