[1] 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 À100 nT) that occurred during 1996-2005. The results are achieved through cooperative efforts that originated at the Living with a Star (LWS) Coordinated DataAnalysis 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.

Abstract. To this day, the prediction of space weather effects near the Earth suffers from a fundamental problem: The radial propagation speed of "halo" CMEs (i.e. CMEs pointed along the Sun-Earth-line that are known to be the main drivers of space weather disturbances) towards the Earth cannot be measured directly because of the unfavorable geometry. From inspecting many limb CMEs observed by the LASCO coronagraphs on SOHO we found that there is usually a good correlation between the radial speed and the lateral expansion speed V exp of CME clouds. This latter quantity can also be determined for earthward-pointed halo CMEs. Thus, V exp may serve as a proxy for the otherwise inaccessible radial speed of halo CMEs. We studied this connection using data from both ends: solar data and interplanetary data obtained near the Earth, for a period from January 1997 to 15 April 2001. The data were primarily provided by the LASCO coronagraphs, plus additional information from the EIT instrument on SOHO. Solar wind data from the plasma instruments on the SOHO, ACE and Wind spacecraft were used to identify the arrivals of ICME signatures. Here, we use "ICME" as a generic term for all CME effects in interplanetary space, thus comprising not only ejecta themselves but also shocks as well. Among 181 front side or limb full or partial halo CMEs recorded by LASCO, on the one hand, and 187 ICME events registered near the Earth, on the other hand, we found 91 cases where CMEs were uniquely associated with ICME signatures in front of the Earth. Eighty ICMEs were associated with a shock, and for 75 of them both the halo expansion speed V exp and the travel time T tr of the shock could be determined. The function T tr =203-20.77* ln (V exp ) fits the data best. This empirical formula can be used for predicting further ICME arrivals, with a 95% error margin of about one day. Note, though, that in 15% of comparable cases, a full or partial halo CME does not cause any ICME signature at Earth at all; every fourth partial halo CME and every sixth limb halo CME does not hit the Earth (false Correspondence to: R. Schwenn (schwenn@linmpi.mpg.de) alarms). Furthermore, every fifth transient shock or ICME or isolated geomagnetic storm is not caused by an identifiable partial or full halo CME on the front side (missing alarms).

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 À100 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.

Abstract. The main drivers of strong geomagnetic activity at the Earth are interplanetary manifestations of coronal mass ejections. A magnetic storm can be caused by compressed sheath fields before the CME, by the CME ejecta or by the combination of these two structures. The most geoeffective subset of CMEs are magnetic clouds. When observed near 1 AU magnetic clouds are characterized by monotonous rotation of magnetic field direction through a large angle, high magnetic field magnitude, low temperature and low plasma beta. We have investigated the magnetic structure and the geomagnetic consequences of magnetic clouds identified from WIND and ACE data for the years 1997-2003. The geomagnetic response of a certain magnetic cloud depends greatly on its magnetic structure and orientation of sheath fields. We have investigated drivers of intense magnetic storms (Dst ¡ -100 nT) during the interval of 1997-2002, i.e. rising, maximum and early declining phases of solar cycle 23. Sheath regions and post-shock streams caused nearly half of all intense storms. Importance of sheath regions as storm drivers even increased as the level of the storm increased. In 2003 two most intense geomagnetic storms of the solar cycle 23 took place. Both of these were driven by southward fields embedded in a magnetic cloud that had axis highly inclined to the ecliptic plane. Though sheath regions alone efficiently drive intense Dst storms (¡ -100 nT) the largest storms (Dst ¡ -300 nT) require exceptionally long-time and intense southward magnetic fields that presumably only magnetic clouds can provide. High solar wind dynamic pressure seems to be important in generating extremely intense Dst storms. As an example we show solar wind condition during Nov 19-20, 2003 magnetic cloud that caused the largest storm of the solar cycle 23.Magnetic clouds have smoothly changing magnetic field direction combined with low solar wind dynamic pressure. Sheath regions typically have rapidly varying magnetic field direction and high dynamic pressure. Thus, these two solar wind drivers put magnetosphere under different type of driving. We also studied the responses of the Dst index that aims to measure the strength of the equatorial ring current and the Kp index that records more global and higher latitude activity than Dst to different storm drivers. We found that in general sheath regions generate higher Kp activity when compared to the level of the the Dst disturbance than magnetic clouds. In some cases rapidly fluctuating magnetic field in the sheath region caused very strong highlatitude activity (Kp 8-9) though the Dst index was significantly less enhanced. This suggest that magnetospheric current systems have different responses to different solar wind drivers.