Abstract
It is shown that high intensity ( AE > 1,000 nT), long duration ( T > 2d) continuous auroral activity (HILDCAA) events are caused by outward (from the sun) propagating interplanetary Alfvén wave trains. The Alfvén waves are often (but not always) detected several days after major interplanetary events, such as shocks and solar wind density enhancements. Presumably magnetic reconnection between the southward components of the Alfvén wave magnetic fields and magnetospheric fields is the mechanism for transfer of solar wind energy to the magnetosphere. If the stringent requirements for HILDCAA events are relaxed, there are many more AE events of this type. A brief inspection indicates that these are also related to interplanetary Alfvénic fluctuations. We therefore suggest that most auroral activity may be caused by reconnection associated with Alfvén waves in the interplanetary medium.
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