Flow bursts, braking, and Pi2 pulsations

Abstract

We examine six near‐Earth dipolarization events during which rapid flows were observed by Geotail at distances between 8 and 15 RE in the magnetotail. Each flow event was associated with local dipolarization, auroral arc brightening, and Pi2 pulsations (periods of 40–150 s). Variations in earthward flow velocity delayed by 60–90 s match the Pi2 waveforms on the ground at low latitudes on the flank. We conclude that low‐latitude Pi2 pulsations are directly driven by compressional pulses associated with braking of oscillatory earthward flows. In addition, we identify a new type of nightside Pi2, which is related to the oscillatory braking current that modulates the current in the substorm current wedge. For one event we estimate the magnitude of the braking current from the amplitude of ground perturbations and find I∼2–3×104 A. An independent calculation of the inertial current using the flow measurements yields 6×104 A. We separate Pi2 into three distinct physical types: low‐latitude directly driven, nightside transient response, and nightside inertial current. We propose a phenomenological model linking flow bursts, the substorm current wedge, and the three types of Pi2 pulsations.