Statistical study of substorm timing sequence

Abstract

Observations of timing sequences of substorms expected in various onset mechanisms are examined by using a space‐time diagram, which correlates observed space signatures and auroral signatures on the ground during substorm onset. Results from a statistical study of 11 substorms show that signatures in the midtail (x ∼ 15–25 RE) typically occur before the ground signatures and those in the near‐Earth tail (x ∼ 10 RE) and that signatures in the midtail region observed prior to the substorm onset often occur at a time which was shorter than that expected from MHD wave propagation between the different regions. This suggests that the disturbance onsets in different active regions do not seem to have a simple causal relationship between them as described by the reconnection or current disruption models of substorms. The activation of perturbed fields and plasma flows in space including the signatures of reconnection and current disruption may occur in multiple localized regions throughout the stressed tail current sheet. The activation seems to be continuously observed well after the substorm onset. These results to some extent are consistent with suggested global Alfvénic interaction considerations, in which the substorm onset is the result of Alfvénic interaction in the global current systems.