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Abstract Following substorm auroral onset, the active aurora region usually expands poleward toward the poleward auroral boundary. Such poleward expansion is often associated with a bulge region that expands westward and forms the westward travelling surge. In this study, we show all‐sky imager and Poker Flat Advanced Modular Incoherent Scatter Radar observations of two surge events to investigate the relationship between the surge and ionospheric flows that likely have polar cap origin. For both events, we observe auroral streamers, with an adjacent flow channel consisting of decreased density and low electron temperature plasma flowing equatorward. This flow channel appears to impinge and lead/feed surge formation, and to stay connected to the surge as it moves westward. Also, for both events, streamer observations indicate that, following initial surge development, similar flows led to explosive surge enhancements. The observation that the streamers are connected to the auroral polar boundary and that the flow channels consisted of low density, low electron temperature plasma suggests the possibility that the impinging plasma came from the polar cap. For both events, the altitude variations of F region plasma within the surges are related with aurora emission and the poleward/equatorward flow, and the surges develop strong auroral streamers that initiate along the poleward auroral boundary when contacted with the flow. These results suggest that the flow of polar cap origin, which maps to underlying processes in the magnetotail, may play a crucial role in auroral surges by feeding low entropy plasma into surge initiation and development, and also playing an important role in the dynamics within a surge.
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This content will become publicly available on December 1, 2025
Investigating the Spatiotemporal Development of Substorm Expansion Phase Aurora: Successive Onsets or Poleward Boundary Intensifications?
Abstract Following the auroral substorm onset, the active aurora undergoes expansion, which can vary in spatial and temporal extent. The spatiotemporal development of the expansion phase active aurora is controlled by new auroral intensifications that often follow the initial onset. Using seven examples, we investigate the nature of these new auroral intensifications and address a question: are they new auroral onsets, that is, “successive onsets” or poleward‐boundary intensifications (PBIs) and ensuing auroral streamers? We observed events that included both types of auroral features—successive onsets and PBIs—and their combinations. For multiple‐onset substorms, successive onsets may occur eastward, westward, and poleward of the initial onset, resulting in a diverse range of expansion phase spatial extent and durations. Single‐onset substorms show only one auroral onset, but their spatiotemporal development can resemble that of multiple‐onset substorms. However, the additional activations are mainly PBIs and subsequent streamers. In some cases, PBIs undergo explosion, leading to a rapid poleward and azimuthal expansion of the aurora, resembling the auroral substorm onset. A prolonged sequence of PBIs and its longitudinal extension can contribute significantly to the spatiotemporal development of substorms expansion phase. Results suggest that post‐onset flow channels drive the spatiotemporal development of the substorm expansion phase by (a) triggering successive onsets and (b) inducing bursts of PBIs and their prolonged sequence. We speculate that post‐onset flow channels likely originate from the polar cap, but more evaluation is required. Our findings highlight the significance of examining imager data before solely relying on magnetometers to identify substorm onsets.
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- PAR ID:
- 10612850
- Publisher / Repository:
- Journal of Geophysical Research
- Date Published:
- Journal Name:
- Journal of Geophysical Research: Space Physics
- Volume:
- 129
- Issue:
- 12
- ISSN:
- 2169-9380
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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