Substorm expansion onset caused by intense localized convection driven jointly by plasma-sheet reconnection and dipolarization

The explosive substorm expansion onset is shown, by means of the M-I coupling model, to be caused by an intense localized convection imposed on the nightside ionosphere. It is proposed that the intense localized convection is driven jointly by the plasma-sheet reconnection beyond ∼20 Re and dipolarization within ∼10 Re. Plasma-sheet reconnection beyond ∼20 Re enhances dipolarization within ∼10 Re, and vice versa in a positive-feedback. The positive-feedback is initiated by the explosive thinning at the end of the growth phase. Thinning in the near-Eath plasma sheet proceeds gradually during the growth phase, except near the end it turns explosive. The gradual thinning is caused by enhanced dayside magnetopause reconnection. The explosive thinning is attributed to the nightside magnetopause reconnection enhanced along the dawn-dusk flanks of the magnetosphere.