Multispacecraft and ground‐based observations of substorm timing and activations: Two case studies

Abstract

Two case studies are performed to investigate substorm timing and activations based on Double Star TC1, Cluster, Polar, IMAGE, LANL geostationary satellites and ground‐based geomagnetic field measurements. In both events, an earthward flow associated with plasma sheet thinning is measured by Cluster 8–10 min ahead of the auroral breakup. A couple of minutes after the breakup, either TC1 at ∼X‐10 RE first detects plasma sheet expansion and then the LANL satellites near the midnight measure energetic electron injections at geostationary orbit or the LANL satellites first measure the electron injections and then TC1 detects the plasma sheet expansion. More than about 20 min later, Cluster at X∼16 RE and Polar (at higher latitude) successively observe plasma sheet expansion. The open magnetic flux of the polar cap, Ψ, is found to continually increase during the early substorm phase and then to rapidly fall when the IMF turns northward. When Ψ reaches its minimum value, bright and broad auroral activities start to decrease. Tailward progression of the magnetic dipolarization and a poleward expansion of auroral bulges are shown to closely map to one another. These results suggest that substorm activations start in the midtail before ground onset and then move earthward, which leads to an expansion onset in the near‐Earth tail around X∼ ‐(8–9) RE. After onset, the activations progress both earthward and tailward. Substorm onset is possibly related to plasma sheet reconnection of close field lines, while tail lobe reconnection of open field lines release more energy to support the full expansion of the substorm. In a fully developed expansion phase, an initial dipolarization in the near‐Earth may eventually evolve to enable disruption of the cross‐tail current over a wide region of the magnetotail.