Dynamic effects of restoring footpoint symmetry on closed magnetic field-lines: Asymmetric Birkeland currents
Thursday, 26 May 2016
Jone Peter Reistad1, Nikolai Ostgaard1, Paul Tenfjord1, Karl Laundal1, Kristian Snekvik1, Stein Haaland1,2, Adrian Grocott3, Harald U Frey4, Kjellmar Oksavik1,5 and Steve E. Milan1,6, (1)University of Bergen, Birkeland Centre for Space Science, Bergen, Norway, (2)Max-Planck Institute, Goettingen, Germany, (3)Lancaster University, Department of Physics, Lancaster, United Kingdom, (4)University of California Berkeley, Berkeley, CA, United States, (5)University Centre in Svalbard, Longyearbyen, Norway, (6)University of Leicester, Physics and Astronomy, Leicester, United Kingdom
Abstract:
We present an event where simultaneous global imaging of the aurora from both hemispheres reveals a large longitudinal shift of the nightside auroral features of about 3 h, being the largest shift reported on from conjugate auroral imaging. This is interpreted as evidence of highly twisted field-lines. We attribute this large shift to the combined effect of a persistent positive y-component of the interplanetary magnetic field before and during the event, and the positive dipole tilt angle (summer in Northern Hemisphere). At the same time, the Super Dual Auroral Radar Network (SuperDARN) observes the nightside Harang region in both hemispheres. In addition to confirm the large longitudinal shift between the two hemispheres, the radar data indicate a faster flow towards the dayside in the dusk cell in the Southern Hemisphere compared to its conjugate region. We interpret this as a signature of untwisting of the closed magnetic field-lines, a process that acts to restore magnetic footpoint symmetry. The event is analysed with emphasis on the field-aligned Birkeland Currents (BC) associated with this rectification process as previously suggested. 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 this process on BCs, namely stronger currents in the region of the untwisting flows. Consistent results are also obtained for other combinations of IMF By and dipole tilt. 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. This is known as the untwisting process, representing a mechanism for asymmetric BCs, relevant for understanding hemispheric differences in the M-I coupling.
