Pi1B pulsations and Alfvenic aurora at substorm onset

Previous observations have shown that Alfvenic aurora occurs routinely near the polar cap boundary of the auroral zone during the expansion phase of substorms. Recent results, however, show that Alfvenic aurora also occurs at substorm onset, in conjunction with the initial brightening, both in time and location. The observations, acquired by IMAGE and FAST satellites, suggest that Alfven waves are generated in conjunction with substorm onset (i.e., at the onset region in the tail) and that these waves propagate earthward from the tail. Other studies have shown that Pi1B pulsations, waves that are observed on the ground at substorm onset, can also be observed at geosynchronous orbit as compressional waves, suggesting that these waves propagate from (or beyond) geosynchronous orbit to the ionosphere at substorm onset. Recent results using Cluster data show that Pi1B pulsations are strongly correlated with fast flows at onset. The similarity between these independent observations suggests that Pi1B pulsations may provide the wave energy for Alfvenic aurora at onset and, in fact, may play an important role in the onset process, delivering energy perturbations from the tail to the auroral ionosphere. In this study, data from specific events are presented to show that the data do, in fact, support the conjecture that these phenomena are related.