Localized activation of the distant tail neutral line just prior to substorm onsets

We have found flow burst features in the nightside ionosphere that are thought to be the ionospheric signature of distant tail reconnection. These are observed to form just prior to substorm onsets. Simultaneous observations by the Goose Bay-Stokkseyri dual HF radars and DMSP satellites provide the data. Our conclusions are based on equatorward flow bursts on the nightside during two isolated substorms that followed a long period of magnetospheric inactivity associated with a northward interplanetary magnetic field. Both flow bursts start ∼60 min after the growth phase onset and last ∼10–20 min until the expansion phase onset, migrating equatorward with time. Simultaneous DMSP observations of precipitating particles show that the flow burst occurs at the polar cap boundary, suggesting that the equatorward migration corresponds to the expansion of the polar cap during the growth phase. For one event, the reconnection electric field at 400 km altitude was 14 mV/m and its longitudinal scale was 290 km, which is equivalent to a reconnection voltage of 4.1 kV. For the other event, these values were 11 mV/m (reconnection electric field), 380 km (longitudinal scale), and 4.0 kV (reconnection voltage). In addition to the reconnection signatures, we discuss implications for substorm dynamics during the final stage of the substorm growth phase. The morphology indicates that the distant tail neutral line is activated ∼1 hour after the growth phase onset and at the same time the nightside separatrix starts to move equatorward much faster than during the preceding early and middle growth phases. The 1-hour time lag would correspond to the timescale on which slow rarefaction waves from both northern and southern tail lobes converge in the equatorial magnetotail. The fast-moving separatrix on the nightside implies a rapid change of magnetotail configuration resulting from nonlinear enhancement and/or earthward movement of the cross-tail current for the last 10–20 min prior to the expansion phase onset.

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