Differences in geomagnetic storms driven by magnetic clouds and ICME sheath regions

Twenty‐eight geomagnetic storms driven by magnetic clouds or by sheath regions ahead of interplanetary coronal mass ejections are examined to address the dependence of the driver properties on the storm evolution and storm‐substorm relationship. A superposed epoch analysis shows that the sheath‐driven storms have stronger auroral activity, stronger magnetotail field stretching, and larger asymmetry in the inner magnetosphere field configuration. We suggest that the strong stretching during the sheath‐driven storms leaves ions drifting Earthward from the plasma sheet on open drift paths, which limits the symmetric ring current growth. This decouples the substorm injections from the ring current enhancement, and can in part explain why there is no direct relationship between the auroral electrojet AL index and the midlatitude ring current Dst index.