Plasma flow during the brightening of proton aurora in the cusp

On the basis of simultaneous observations from the Super Dual Auroral Radar Network (SuperDARN), the far ultraviolet instrument on the IMAGE spacecraft, and a magnetometer installed on the east coast of Greenland, we present the characteristics of plasma flow during a westward moving proton aurora in the cusp, observed on 28 July 2000. Data with a time resolution of 12 and 25 s from SuperDARN at Þykkvibær, Iceland, show that the flow having a poleward component, which was accompanied by a quick equatorward expansion of the flow region, occurred in the early stages (approximately 1.5 min) of the brightening. Data from the magnetometer from a ground station on the Greenland east coast, which is located a few hundred kilometers west of the radar's field of view, show perturbations that are consistent with equatorward flow overhead, coincident with the expansion. The presence of flow vectors in opposite directions suggests that vortical flow was present during the proton aurora. Evidence supporting the suggested flow pattern has been found in an event recorded by the Greenland west coast magnetometer chain and in a cusp pass of the DE 2 spacecraft in the literature. We attribute the moving proton aurora to the traveling bulge at the polar cap boundary, which is the footprint of a flux transfer event, and imply that the preexisting vortical flow may be intensified when it becomes inflow to the bulge.