Coronal mass ejections and large geomagnetic storms

Previous work indicates that coronal mass ejection (CME) events in the solar wind at 1 AU can be identified by the presence of a flux of counterstreaming solar wind halo electrons (above about 80 eV). Using this technique to identify CMEs in 1 AU plasma data, we find that most large geomagnetic storms during the interval surrounding the last solar maximum (Aug. 1978 – Oct. 1982) were associated with Earth‐passage of interplanetary disturbances in which the Earth encountered both a shock and the CME driving the shock. However, only about one CME in six encountered by Earth was effective in causing a large geomagnetic storm. Slow CMEs which did not interact strongly with the ambient wind ahead were particularly ineffective in a geomagnetic sense.