Flow Velocity and Field-Aligned Current Associated With Field Line Resonance: SuperDARN Measurerments

This study presents observations of the two-dimensional velocities and field-aligned currents of magnetospheric field line resonances. Knowledge of the two-dimensional plasma flows is essential for determining the field-aligned current structure of the field line resonance and for understanding the role that field line resonances play in the generation of some auroral arc forms. In this work the full velocity vectors are determined from a high-resolution fit to the Super Dual Auroral Radar Network (SuperDARN) line-of-sight data. Two discrete field line resonance events are presented, one at 4.0 mHz (4- to 5-min period) and the other at 0.6 mHz (30-min period). Band-pass filtering the north-south and east-west components of the total convection velocity vectors reveals the two-dimensional velocity field specific to an individual field line resonance. The field line resonance velocities exhibit periodic vorticity, which correspond to periodic poleward moving bands of enhanced field-aligned current. Assuming a constant ionospheric Pedersen conductance of 10 S, peak field-aligned current values are estimated to be 2-4 mu A/m(2). The field-aligned currents alternate between upward and downward directed and peak in magnitude similar to 0.5-1 degrees poleward and/or equatorward of the peak in field line resonance flow velocity. A Swarm satellite overpass occurs directly over the 0.6-mHz field line resonance. The SuperDARN field-aligned current profile along the Swarm trajectory is found to be similar in magnitude and structure to that determined from Swarm-A and Swarm-C in situ measurements.