Dependence of Magnetic Storm Intensity on Interplanetary Electric Field Variability

Recent studies have shown that magnetic storms are relatively weak when the solar wind dawn-dusk electric field (Ey) is smooth and accordingly there is a lack of substorm expansion phases over intervals as long as several hours during ring current intensification. This suggests that magnetic storm intensity is controlled not only by the strength, but also by the variability of the interplanetary electric field. Abrupt variations in Ey reflect sudden changes, both southward and northward, of the IMF Bz component, which lead to a frequent occurrence of substorm expansion phases. In this study, we report the results of a statistical analysis of geomagnetic events in two categories; those induced by interplanetary magnetic clouds, and those which occurred during intervals of highly variable Ey (such as those found in the sheath region between an interplanetary shock and a magnetic cloud). We use the integrated Epsilon function during the storm main phase as a proxy for the input solar wind energy that is responsible for ring current intensification. Specifically, we compare storm intensities for two events in the categories mentioned above for which there is similar solar wind energy input. It is confirmed that a geomagnetic storm is more intense when there are frequent substorm expansion phases during the main phase growth.