Alfvenic Dynamics and Fine Structuring of Discrete Auroral Arcs: Swarm and e-POP Observations

The electrodynamics associated with dual discrete arc aurora with antiparallel flow along the arcs were observed nearly simultaneously by the enhanced Polar Outflow Probe (e-POP) and the Swarm A and C spacecraft. Auroral imaging from e-POP reveals 1-10 km structuring of the arcs, which move and evolve on second timescales and confound the traditional single-spacecraft field-aligned current algorithms. High-cadence magnetic data from e-POP show 1-10 Hz, inferred Alfvenic, perturbations coincident with and at the same scale size as the observed dynamic auroral fine structures. High-cadence electric and magnetic field data from Swarm A reveal nonstationary electrodynamics involving reflected and interfering Alfven waves and modulation consistent with trapping in the ionospheric Alfven resonator (IAR). These observations suggest a role for Alfven waves, perhaps also the IAR, in discrete arc dynamics on 0.2-10 s timescales and similar to 1-10 km spatial scales and reinforce the importance of considering Alfven waves in magnetosphere-ionosphere coupling.