Solar and interplanetary sources of major geomagnetic storms (<i>Dst</i> ≤ −100 nT) during 1996–2005

We present the results of an investigation of the sequence of events from the Sun to the Earth that ultimately led to the 88 major geomagnetic storms (defined by minimum Dst ≤ −100 nT) that occurred during 1996–2005. The results are achieved through cooperative efforts that originated at the Living with a Star (LWS) Coordinated Data‐Analysis Workshop (CDAW) held at George Mason University in March 2005. On the basis of careful examination of the complete array of solar and in situ solar wind observations, we have identified and characterized, for each major geomagnetic storm, the overall solar‐interplanetary (solar‐IP) source type, the time, velocity, and angular width of the source coronal mass ejection (CME), the type and heliographic location of the solar source region, the structure of the transient solar wind flow with the storm‐driving component specified, the arrival time of shock/disturbance, and the start and ending times of the corresponding IP CME (ICME). The storm‐driving component, which possesses a prolonged and enhanced southward magnetic field ( B s ), may be an ICME, the sheath of shocked plasma (SH) upstream of an ICME, a corotating interaction region (CIR), or a combination of these structures. We classify the Solar‐IP sources into three broad types: (1) S‐type, in which the storm is associated with a single ICME and a single CME at the Sun; (2) M‐type, in which the storm is associated with a complex solar wind flow produced by multiple interacting ICMEs arising from multiple halo CMEs launched from the Sun in a short period; (3) C‐type, in which the storm is associated with a CIR formed at the leading edge of a high‐speed stream originating from a solar coronal hole (CH). For the 88 major storms, the S‐type, M‐type, and C‐type events number 53 (60%), 24 (27%), and 11 (13%), respectively. For the 85 events for which the surface source regions could be investigated, 54 (63%) of the storms originated in solar active regions, 11 (13%) in quiet Sun regions associated with quiescent filaments or filament channels, and 11 (13%) were associated with coronal holes. Remarkably, nine (11%) CME‐driven events showed no sign of eruptive features on the surface or in the low corona (e.g., no flare, no coronal dimming, and no loop arcade, etc.), even though all the available solar observations in a suitable time period were carefully examined. Thus while it is generally true that a major geomagnetic storm is more likely to be driven by a frontside fast halo CME associated with a major flare, our study indicates a broad distribution of source properties. The implications of the results for space weather forecasting are briefly discussed.