Statistical comparison of interplanetary conditions causing intense geomagnetic storms (Dst ≤-100 nT)

It is well known that intense southward magnetic field and convection electric field (V x B) in the interplanetary medium are key parameters that control the magnitude of geomagnetic storms. By investigating the interplanetary conditions of 82 intense geomagnetic storms from 1998 to 2006, we have compared many different criteria of interplanetary conditions for the occurrence of the intense geomagnetic storms (Dst <= -100 nT). In order to examine if the magnetosphere always favors such interplanetary conditions for the occurrence of large geomagnetic storms, we applied these conditions to all the interplanetary data during the same period. For this study, we consider three types of interplanetary conditions as follows: B-z conditions, E-y conditions, and their combination. As a result, we present contingency tables between the number of events satisfying the condition and the number of observed geomagnetic storms. Then we obtain their statistical parameters for evaluation such as probability of detection yes, false alarm ratio, bias, and critical success index. From a comparison of these statistical parameters, we suggest that three conditions are promising candidates to trigger an intense storm: B-z <= -10 nT for > 3 h, E-y = 5 mV/m for > 2 h, and B-z = -15 nT or E-y >= 5 mV/m for > 2 h. Also, we found that more than half of the "miss" events, when an intense storm occurs that was not expected, are associated with sheath field structures or corotating interacting regions. Our conditions can be used for not only the real-time forecast of geomagnetic storms but also the survey of interplanetary data to identify candidate events for producing intense geomagnetic storms.