Inner magnetospheric onset preceding reconnection and tail dynamics during substorms: Can substorms initiate in two different regions?

Abstract

AbstractWe present a detailed ground‐based and in situ examination of two substorms on 9 April 2011 providing one of the most detailed observations of a series of activations to date. Using auroral observations we demonstrate that the initial signature of substorm onset is a localized brightening on closed field lines. In both cases auroral onset precedes any geosynchronous indications of substorm onset. Despite excellent in situ coverage of the near‐midnight plasma sheet there is no evidence of tail flows or topological changes which might indicate the initiation of reconnection and related disturbances prior to auroral onset. Similarly, no auroral streamers are observed prior to either onset. For the second substorm, following auroral onset and dipolarization of the geosynchronous field there is evidence of a rapid thinning of the plasma sheet followed by a secondary auroral activation and dipolarization of the magnetotail outside of geosynchronous orbit. This morphological change suggests that near‐Earth neutral line reconnection is eventually triggered in the tail, but only following the earlier auroral onset. These observations demonstrate that expansion phase onset is not always initiated by reconnection in the tail. Finally, our observations suggest that activations at the near‐Earth neutral line, or related to plasma instabilities and near‐Earth onset, can develop independently during expansion phase onset. In this paradigm, free energy stored during the growth phase can be stored in the stretched tail and the inner magnetosphere where the dipole field begins to stretch. During expansion phase onset either region could become unstable, independent of the other, leading to substorm initiation and onset.