STATISTICAL NATURE OF GEOMAGNETIC STORMS

On the basis of geomagnetic activity indices and solar wind parameters , a superposed epoch analysis has been conducted for more than 300 geo magnetic storms. The intensity of magnetic storms is found to depend o n the duration of the main phase; larger storms have longer timescales . For intense storms, however, not only the duration of energy injecti on into the ring current but also the strength of injection is importa nt in determining their size. It is confirmed that the southward compo nent of the interplanetary magnetic field plays a crucial role both in triggering the storm main phase and in determining the magnitude of m agnetic storms. It is also found that the time profile of the energy i njection rate during the main phase tends to have two peaks. This is p articularly the case for intense magnetic storms, where the second pea k is more intense than the first. Implications of our findings are dis cussed in terms of the existing concept of geomagnetic storms and rece nt observations of ring current particles and of interplanetary distur bances.