Major geomagnetic storms ( Dst 5100 nT ) generated by corotating interaction regions

Seventy-nine major geomagnetic storms (minimum Dst 5 -100 nT) observed in 1996 to 2004 were the focus of a “Living with a Star” Coordinated Data-Analysis Workshop (CDAW) in March, 2005. In 9 cases, the storm driver appears to have been purely a corotating interaction region (CIR) without any contribution from coronal mass ejection-related material (interplanetary coronal mass ejections, ICMEs). These storms were generated by structures within CIRs located both before and/or after the stream interface that included persistently southward magnetic fields for intervals of several hours. We compare their geomagnetic effects with those of 159 CIRs observed during 1996 2005. The major storms form the extreme tail of a continuous distribution of CIR geoeffectiveness which peaks a t Dst -40 n T but is subject t o a prominent seasonal variation of 40 n T which is ordered by the spring and fall equinoxes and the solar wind magnetic field direction towards or away from the Sun. The O’Brien and McPherron [2000] equations, which estimate Dst by integrating the incident solar wind electric field and incorporating a ring current loss term, largely account for the variation in storm size. They tend to underestimate the size of the larger CIR-associated storms by Dst N 20 nT. This suggests that injection into the ring current may be more efficient than expected in such storms. Four of the nine major storms in 1996 2004 occurred during a period of less than three solar rotations in September November, 2002, also the time of maximum mean IMF and solar magnetic field intensity during the current solar cycle. The maximum CIR-storm strength found in our sample of events, plus additional 23 probable CIRassociated Dst 5 -100 n T storms in 1972 1995, is (Dst = -161 nT). This is consistent with the maximum storm strength (Dst -180 nT) expected from the O’Brien and McPherron equations for the typical range of solar wind electric fields associated with CIRs. This suggests that CIRs alone are unlikely to generate geomagnetic storms that exceed these levels. to identify the interplanetary drivers of these storms, and where possible, their solar counterparts. Consistent with previous studies [e%., Gosling et al., 1991; Tsumtani and Gonzalez, 1997; lkhardson et al., 2001; Zhang et al., 20031 the majority of these storms were found to be driven by interplanetary coronal mass ejections (ICMEs) and/or the related upstream sheaths, Or multiple structures Of these types. The remaining events generally involved a corotating interaction region (CIR) formed ahead of a high-speed stream emanating from a coronal hole. In some 4 cases, the CIR interacted with a preceding ICME, and compression of southward magnetic fields in the ICME intensified the geoeffectiveness. Similar events have been reported by Zhao [1992], Cane and Richardson [1997], Fenrich and Luhmann [1998] and Crooker [2000]. In another 9 cases, the CIR alone was responsible for driving the storm, with little or no evidence of ICMElike structures being involved, and no dausible association with earthward-directed corond mass