Interplanetary origin of multiple‐dip geomagnetic storms

Abstract

In this paper, we have systematically investigated the interplanetary drivers of major dips during intense (Dst ≤ −100 nT) geomagnetic storms in 1996–2006. A major dip is defined as a temporary decrease in Dst index with amplitude larger than 14.5 nT. Multiple dips result in a storm if regions of geoeffective solar wind with strong southward magnetic fields are separated by less geoeffective solar wind. Among these 90 intense storms, we found that only 34% (31 events) showed a classical “one‐dip” profile, while 49% (44 events) had two dips. Another 17% (15 events) had triple or more dips. We found that of a total of 165 major dips associated with the 90 storms, about 45% (74 dips) were caused by interplanetary coronal mass ejections (ICMEs), or ejecta, and 30% (49 dips) were caused by sheaths (SHs) that lie between shocks driven by ICMEs and leading edges of the ICMEs. About 7% (11 dips) were caused by a shock driven by an ICME running into a preceding ICME and intensifying its magnetic field (PICME‐SH). About 11% (18 dips) were due to corotating interaction regions (CIRs) formed by the interaction of high‐speed solar wind from coronal holes with the preceding slower solar wind. Another 7% (12 dips) were caused by various solar wind structures prior the onset of the storm. Among these different types of drivers, the largest storms dips on average were produced by shocks propagating through preceding ICMEs (PICME‐SH). One frequent cause of a two‐dip storm is that the first dip is produced by the upstream sheath and the second dip is produced by the driving ICME. Another common cause of a two‐dip or multiple‐dip storm is the presence of multiple subregions of southward magnetic field within a complex solar wind flow, resulting from two successive, closely spaced ICMEs.