Comprehensive ground‐based and in situ observations of substorm expansion phase onset

In this paper, we present comprehensive ground‐based and space‐based in situ geosynchronous observations of a substorm expansion phase onset on 1 October 2005. The Double Star TC‐2 and GOES‐12 spacecraft were both located within the substorm current wedge during the substorm expansion phase onset, which occurred over the Canadian sector. We find that an onset of ULF waves in space was observed after onset on the ground by extending the AWESOME timing algorithm into space. Furthermore, a population of low‐energy field‐aligned electrons was detected by the TC‐2 PEACE instrument contemporaneous with the ULF waves in space. These electrons appear to be associated with an enhancement of field‐aligned Poynting flux into the ionosphere which is large enough to power visible auroral displays. The observations are most consistent with a near‐Earth initiation of substorm expansion phase onset, such as the Near‐Geosynchronous Onset (NGO) substorm scenario. A lack of data from further downtail, however, means other mechanisms cannot be ruled out.