Recurrent substorm activity during the passage of a corotating interaction region

Recent observations of magnetospheric dynamics driven by interaction with both high-speed solar wind streams (HSSs) and interplanetary coronal mass ejections (ICMEs) have shown periodic substorms to be common under strong driving, but with different periods of ∼ 4 and 2–3 h, respectively. However, it is unclear what causes these substorms and what determines their periodicity. Observations during the passage of a corotating interaction region preceding a high-speed stream are presented here and the observed enhancements in the AE index are shown to be a quasi-periodic sequence of substorms. The occurrence times of these substorms and the variation of the AL index are shown to be consistent with a simple loading–unloading model without the need for external triggers. A possible explanation is given for the lengthening of the inter-substorm period during HSS relative to that of sawtooth events during ICMEs.