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

Superposed epoch analyses of 549 storms are performed to make a comparison of solar wind features of geomagnetic storm events at solar minimum (July 1974 to June 1977; July 1984 to June 1987; July 1994 to June 1997) and solar maximum (January 1979 to December 1981; January 1989 to December 1991; July 1999 to June 2002). In this study, geomagnetic storms are defined by the pressure-corrected Dst (Dst*) and classified into moderate storms (-100 nT<=-50 nT) and intense storms (Dst*<=-100 nT). The average values of interplanetary magnetic field (IMF), solar wind plasma, NOAA/POES hemispheric power, Kp, and Dst* are analyzed and compared among the different storm categories. During the main phase of storms in each category, the average solar wind plasma parameters and IMF components are disturbed and compressed by a relative high-speed plasma stream. It is shown that the peak of the average solar wind density leads the minimum Dst* (the zero epoch time) by 4.3-7.0 hours, which is longer than the peak time difference (0.3-1.0 hour) between the average IMF B-s and Dst*(min). For intense storms at solar minimum, the average IMF B-y is greatly disturbed during both the main phase and the recovery phase. In addition, the average solar wind density is enhanced up to 28 cm(-3), but the average solar wind bulk flow in this storm category is lower than those in all other categories. A significant finding is that the average interplanetary causes of intense storms at solar minimum are found to be against the well-known empirical criteria (B-s >= 10 nT or VBs >= 5.0 mV/m for >= 3 hours), having a long interval of average B-s=similar to 10 nT with dual peaks separated by similar to 4.0 hours. The interplanetary and solar origins of storms in the different storm categories are also discussed.