The Relation of N‐S Auroral Streamers to Auroral Expansion.

We investigate the relation of fast flows at the inner edge of the plasma sheet to the onset of auroral expansion. Recent work suggests that nearly all expansions are an instability triggered by an auroral streamer from far out in the magnetotail. We investigate an 8‐hr interval of activity on 14 March 2008 using ground magnetometer and all‐sky camera data to determine the onset times of six substorm expansions. We compare these times with Time History of Events and Macroscale Interactions during Substorms observations of plasma flow and magnetic field. We show that every expansion followed the arrival of a fast flow and dipolarization event at the inner edge of the plasma sheet. To relate the aurora to flows, we develop procedures for removing fixed lights, the moving Moon and its reflection, and contamination due to scattered moonlight. We scan movies of enhanced images for auroral streamers. Three onsets were tentatively associated with streamers. For two, the apparent source was very close to the growth phase arc mapping close to Earth. For one, an onset occurred in the recovery phase of an earlier substorm after a double oval had formed. For this one, the end of an N‐S streamer stopped about 2° north of the breakup arc. For the remaining three expansions, no streamers were associated with the onsets. Most substorms exhibit N‐S streamers in the recovery phase. These usually cannot be associated with fast flows. Either fast flows in the growth phase do not produce streamers or they make streamers that require significant image enhancement. Plain Language Summary: Auroral streamers are narrow bands of auroral luminosity that originate near the poleward edge of the auroral oval. They generally propagate from north to south often with a large azimuthal component. Researchers think that these streamers impact the outer magnetosphere triggering an instability that appears as a localized expansion of aurora towards the poles. This paper describes the processing of ground auroral images to identify the existence of streamers. We compare their occurrence with ground magnetometer data signaling the onset of auroral expansion and with spacecraft data showing high‐speed flows transporting magnetic flux to the inner edge of the plasma sheet. We find that all six expansions were associated with the arrival of fast flows transporting magnetic flux towards Earth. Only three of these could be associated with a streamer. Two were primarily E‐W from a source close to Earth. The third was a classic N‐S streamer that occurred in the recovery phase of an earlier substorm. At this time, aurora was present in two bands at the edges of the auroral oval. There was no obvious connection of the equatorward end of this streamer to the activated arc. We find that the fast flows causing substorm expansion either do not produce visible aurora or are so weak that special processing of the camera images is required to see the streamers. Key Points: All six auroral expansions in the study interval were preceded by the arrival of a bursty bulk flow at inner edge of plasma sheetOnly three of the fast flows were definitely associated with auroral streamersOnly one expansion occurred when classic streamers originating at the poleward edge of the auroral oval were present [ABSTRACT FROM AUTHOR]