Long‐lived geomagnetic storms and coronal mass ejections

Abstract

Coronal mass ejections (CMEs) are major solar events that are known to cause large geomagnetic storms (Dst< −100 nT). Isolated geomagnetic storms typically have a main phase of 3–12 hours and a recovery phase of around 1 day. However, there are some storms with main and recovery phases exceeding ∼3 days. We trace the origin of these long‐lived geomagnetic storms (LLGMS) to frontside halo CMEs. We studied 37 LLGMS events withDst< −100 nT and the associated CMEs which occurred during 1998–2002. It is found that LLGMS events are caused by (1) successive CMEs, accounting for ∼64.9% (24 of 37); (2) single CMEs, accounting for ∼21.6% (8 of 37); and (3) high‐speed streams (HSS) in corotating interaction regions (CIRs) with no related CME, accounting for ∼13.5% (5 of 37). The long duration of the LLGMS events was found to be due to successive CMEs and HSS events; the high intensity of the LLGMS events was related to the interaction of CMEs with other CMEs and HSS events. We find that the duration of LLGMS is well correlated to the number of participating CMEs (correlation coefficientr= 0.78). We also find that the intensity of LLGMS has a good correlation with the degree of interaction (the number of CMEs interacting with a HSS event or with themselves) (r= 0.67). The role of preconditioning in LLGMS events, where theDstdevelopment occurred in multiple steps in the main and recovery phases, has been investigated. It is found that preconditioning does not affect the main phase of the LLGMS events, while it plays an important role during the recovery phase of the LLGMS events.