Dynamic effects of restoring footpoint symmetry on closed magnetic field lines

TL;DR

This study reports on a conjugate auroral event revealing large footpoint asymmetries and the dynamic process of restoring symmetry in closed magnetic field lines, supported by multi-instrument observations and analysis of Birkeland currents.

Contribution

It provides new observational evidence of the dynamic effects of footpoint symmetry restoration on closed magnetic field lines using multi-instrument data.

Findings

Large conjugate auroral shift of about 3 hours observed.

Faster ionospheric convection in the dusk cell indicating symmetry restoration.

Distinct Birkeland current patterns associated with the restoring process.

Abstract

Here we present an event where simultaneous global imaging of the aurora from both hemispheres reveals a large longitudinal shift of the nightside aurora of about 3 h, being the largest relative shift reported on from conjugate auroral imaging. This is interpreted as evidence of closed field lines having very asymmetric footpoints associated with the persistent positive $\textit{y}$ component of the interplanetary magnetic field before and during the event. At the same time, the Super Dual Auroral Radar Network observes the ionospheric nightside convection throat region in both hemispheres. The radar data indicate faster convection toward the dayside in the dusk cell in the Southern Hemisphere compared to its conjugate region. We interpret this as a signature of a process acting to restore symmetry of the displaced closed magnetic field lines resulting in flux tubes moving faster along the banana cell than the conjugate orange cell. The event is analyzed with emphasis on Birkeland currents (BC) associated with this restoring process, as recently described by Tenfjord et al. (2015). Using data from the Active Magnetosphere and Planetary Electrodynamics Response Experiment (AMPERE) during the same conditions as the presented event, the large-scale BC pattern associated with the event is presented. It shows the expected influence of the process of restoring symmetry on BCs. We therefore suggest that these observations should be recognized as being a result of the dynamic effects of restoring footpoint symmetry on closed field lines in the nightside.