Abstract
AbstractHere 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 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.
Highlights
When the interplanetary magnetic field (IMF) interacts with the Earth’s magnetic field, a dawn-dusk component in the IMF will affect the two hemispheres differently in a number of ways
This is interpreted as evidence of closed field lines having very asymmetric footpoints associated with the persistent positive y component of the interplanetary magnetic field before and during the event
Displaced Footpoints and Asymmetric Convection From the direct comparison presented in Figure 7 we conclude that the westward ionospheric convection on closed field lines close to the nightside convection throat region in the dusk cell is stronger in the Southern Hemisphere compared to its conjugate region
Summary
When the interplanetary magnetic field (IMF) interacts with the Earth’s magnetic field, a dawn-dusk component in the IMF will affect the two hemispheres differently in a number of ways. Since the footpoints of the field line are displaced in longitude on the nightside, the Southern Hemispheric end needs to move faster in the westward direction in the dusk cell to catch up with the Northern Hemisphere as the field line convects toward the dayside Seen from the side, Figure 2b2 illustrates that this situation represents Alfvèn wavefronts carrying a pair of BCs that are launched primarily into the Southern Hemisphere These waves act to restore footpoint symmetry along the equatorward part of the banana cell as the stress is being released. The emphasis will be on investigating how the observations fit our understanding of the large-scale electrodynamics in terms of the restoring footpoint symmetry process
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