We present observations during two substorms using simultaneous Time History of Events and Macroscale Interactions During Substorms satellites and all‐sky imagers to determine plasma sheet dynamics associated with substorm auroral onset beads. The multi‐satellite observations showed that the cross‐tail current decreased and the field‐aligned currents increased at the substorm auroral onset, indicating that the satellites detected an initiation of the currents being deflected to the ionosphere. For duskward‐propagating beads, the electric field was tailward, and ions were accumulated closer to the Earth than electrons. The mapped bead propagation speed was close to energetic ion drift speed. The
In the present study we examine three substorm events, Events 1–3, focusing on the spatio‐temporal development of auroral electrojets (AEJs) before auroral breakup. In Events 1 and 2, auroral breakup was preceded by the equatorward motion of an auroral form, and the ground magnetic field changed northward and southward in the west and east of the expected equatorward flow, respectively. Provided that these magnetic disturbances were caused by local ionospheric Hall currents, this feature suggests that the equatorward flow turned both eastward and westward as it reached the equatorward part of the auroral oval. The auroral breakup took place at the eastward‐turning and westward‐turning branches in Events 1 and 2, respectively, and after the auroral breakup, the westward AEJ enhanced only on the same side of the flow demarcation meridian. The zonal flow divergence is considered as an ionospheric manifestation of the braking of an earthward flow burst in the near‐Earth plasma sheet and subsequent dawnward and duskward turning. Therefore, in Events 1 and 2, the auroral breakup presumably mapped to the dawnward and duskward flow branches, respectively. Moreover, for Event 3, we do not find any pre‐onset auroral or magnetic features that can be associated with an equatorward flow. These findings suggest that the braking of a pre‐onset earthward flow burst itself is not the direct cause of substorm onset, and therefore, the wedge current system that forms at substorm onset is distinct from the one that is considered to form as a consequence of the flow braking.
more » « less- NSF-PAR ID:
- 10373518
- Publisher / Repository:
- DOI PREFIX: 10.1029
- Date Published:
- Journal Name:
- Journal of Geophysical Research: Space Physics
- Volume:
- 127
- Issue:
- 6
- ISSN:
- 2169-9380
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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