Geomagnetic storms over the last solar cycle: A superposed epoch analysis

Presented here is a discussion of the results of a superposed epoch analysis of geomagnetic storms over the last solar cycle. Storms, identified by means of their characteristic SYM-H evolution, are separated by size into weak (-150 < SYM-H <= -80) nT, moderate (-300 < SYM-H <= -150) nT, and intense (SYM-H <= -300) nT categories. Where possible, the corresponding solar wind (SW) onset mechanisms were located by means of 1 min ACE OMNI data. Intense storms were observed to be driven solely by coronal mass ejections (CMEs); moderate storms were dominated by CME onset, while only weak storms were driven by both CMEs and corotating interaction regions (CIRs) at a ratio of similar to 2:1, respectively. As might be expected, more intense storms resulted from the largest SW enhancements. Individual storm phase durations for different storm sizes were investigated, revealing that the duration of the main phase increases with storm size to a critical point, then decreases for more intense storms, contrary to the findings of a previous study by Yokoyama and Kamide (1997). Various SW-magnetosphere coupling functions were investigated for this data set in an attempt to estimate storm size from SW conditions.