Observations of polar cap flow channel and plasma sheet flow bursts during substorm expansion

We present the first simultaneous observations of an enhanced polar cap flow impinging on the nightside polar cap boundary (PCB), two flow bursts in the plasma sheet and a conjugate ionospheric flow burst within the auroral oval. The ionospheric measurements on 3 September 2006 were made by the European Incoherent Scatter (EISCAT) radars and the magnetospheric measurements by the four Cluster spacecraft. In the end of a substorm growth phase, EISCAT measured a channel of enhanced equatorward plasma flow within the polar cap, which was about 1° wide in latitude and drifted slowly equatorward. During the substorm expansion phase, the PCB started to contract poleward. The interaction between the equatorward drifting polar cap flow channel and the poleward contracting PCB took 2–3 min. During this time, the F‐region electron temperature was elevated at the PCB, which is interpreted as a possible signature of an auroral poleward boundary intensification (PBI). After that, enhanced equatorward flows were measured inside the auroral oval by EISCAT. During this period, the Cluster satellites measured two fast earthward flow bursts in the plasma sheet, which were associated with dipolarizations of the magnetic field, depletions in plasma density, and return flows. We suggest that the second flow burst in the plasma sheet represents the same flow burst that is seen in the ionosphere by EISCAT and propose that the plasma sheet flow bursts were triggered by the enhanced flow structure on open polar cap field lines. The suggestion is in line with Lyons et al. (2011). Key Points Polar cap flow channel is observed in the nightside ionosphere The polar cap flow channel possibly triggers bursty bulk flows (BBFs) BBFs are associated with tailward flows consistent with return flows