Pi1B pulsations as a possible driver of Alfvénic aurora at substorm onset

Ground-based observations have shown Pi1B magnetic pulsations are associated with substorm onset. These pulsations can also be observed at geosynchronous orbit, suggesting that they propagate from (or beyond) geosynchronous orbit to the ionosphere at substorm onset. Independently, investigations have shown that the initial brightening of an arc at subtorm onset is Alfvénic in nature (i.e., that the aurora during the initial brightening is wave-driven). These results raise the question of whether Pi1B pulsations might drive Alfvénic aurora at substorm onset. In this paper, data from a single event are presented that show Pi1B pulsations observed simultaneously at geosynchronous orbit, by FAST just above the ionosphere and by various ground stations. The event is observed by FAST within a few minutes of the onset of Pi1B pulsations, with an electron signature of the onset arc that is quite characteristic of Alfvénic aurora, showing that at least a portion of the initial brightening is wave-driven and associating this with the presence of Pi1B pulsations. The implication of this work is that Pi1B pulsations propagate to the ionosphere from beyond geosynchronous orbit and provide the wave power to drive Alfvénic aurora at substorm onset, at least for this single, isolated event. The luminosity associated with the Alfvénic aurora, however, may only provide a small contribution to the total brightness.