Evidence for potential and inductive convection during intense geomagnetic events using normalized superposed epoch analysis

The relative contribution of storm-time ring current development by convection driven by either potential or inductive electric fields has remained an unresolved question in geospace research. Studies have been published supporting each side of this debate, including views that ring current buildup is entirely one or the other. This study presents new insights into the relative roles of these storm main phase processes. We perform a superposed epoch study of 97 intense (Dst(Min)<-100 nT) and 91 moderate (-50 nT>Dst(Min)>-100 nT) storms using OMNI solar wind and ground-based data. Instead of using a single reference time for the superpositioning of the events, we choose four reference times and expand or contract each phase of every event to the average length of this phase, creating a normalized timeline for the superposed epoch analysis. Using the bootstrap method, we statistically demonstrate that timeline normalization results in better reproduction of average storm dynamics than conventional methods. Examination of the Dst reveals an inflection point in the intense storm group consistent with two-step main phase development, which is supported by results for the southward interplanetary magnetic field and various ground-based magnetic indices. This two-step main-phase process is not seen in the moderate storm timeline and data sets. It is determined that the first step of Dst development is due to potential convective drift, during which an initial ring current is formed. The negative feedback of this hot ion population begins to limit further ring current growth. The second step of the main phase, however, is found to be a more even mix of potential and inductive convection. It is hypothesized that this is necessary to achieve intense storm Dst levels because the substorm dipolarizations are effective at breaking through the negative feedback barrier of the existing inner magnetospheric hot ion pressure peak. Citation: Katus, R. M., M. W. Liemohn, D. L. Gallagher, A. Ridley, and S. Zou (2013), Evidence for potential and inductive convection during intense geomagnetic events using normalized superposed epoch analysis, J. Geophys. Res. Space Physics, 118, 181-191, doi:10.1029/2012JA017915.