Observations of Asymmetries in Ionospheric Return Flow During Different Levels of Geomagnetic Activity

• Published in: Journal of Geophysical Research: Space Physics
• Main Authors: Reistad, JP, Ostgaard, N, Laundal, KM, Ohma, A, Snekvik, K, Tenfjord, P, Grocott, A, Oksavik, K, Milan, SE, Haaland, S
• Format: Article in Journal/Newspaper
• Language: English
• Published: American Geophysical Union (AGU), Wiley 2019
• Subjects: Science & Technology; Physical Sciences; Astronomy & Astrophysics; IMF B-Y; HIGH-LATITUDE CONVECTION; NON-SUBSTORM INTERVALS; MAGNETIC-FIELD; COMPONENT; MAGNETOTAIL; MAGNETOSPHERE; PLASMA; WIND; RECONNECTION; Arctic; Canada; Norway; Omni
• Online Access: https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2017JA025051; http://hdl.handle.net/2381/44976; https://doi.org/10.1029/2017JA025051

It is known that the magnetic field of the Earth's closed magnetosphere can be highly displaced from the quiet‐day configuration when interacting with the interplanetary magnetic field (IMF), an asymmetry largely controlled by the dawn‐dusk component of the IMF. The corresponding ionospheric convection has revealed that footprints in one hemisphere tend to move faster to reduce the displacement, a process we refer to as the restoring of symmetry. Although the influence on the return flow convection from the process of restoring symmetry has been shown to be strongly controlled by the IMF, the influence from internal magnetospheric processes has been less investigated. We use 14 years of line‐of‐sight measurements of the ionospheric plasma convection from the Super Dual Auroral Radar Network to produce high‐latitude convection maps sorted by season, IMF, and geomagnetic activity. We find that the restoring symmetry flows dominate the average convection pattern in the nightside ionosphere during low levels of magnetotail activity. For increasing magnetotail activity, signatures of the restoring symmetry process become less and less pronounced in the global average convection maps. We suggest that tail reconnection acts to reduce the asymmetric state of the closed magnetosphere by removing the asymmetric pressure distribution in the tail set up by the IMF By interaction. During active periods the nightside magnetosphere will therefore reach a more symmetric configuration on a global scale. These results are relevant for better understanding the dynamics of flux tubes in the asymmetric geospace, which is the most common state of the system. SuperDARN (Super Dual Auroral Radar Network) is an international collaboration involving more than 30 low‐power HF radars that are operated and funded by universities and research organizations in Australia, Canada, China, France, Italy, Japan, Norway, South Africa, United Kingdom, and USA. The convection data were retrieved as “gridex files” from Virginia Tech using the DaViTpy software (https://github.com/vtsuperdarn/davitpy). We acknowledge the use of NASA/GSFC's Space Physics Data Facility (http://omniweb. gsfc.nasa.gov) for OMNI data. Kjellmar Oksavik is grateful for being selected as the 2017–2018 Fulbright Arctic Chair, and his sabbatical at Virginia Tech is sponsored by the U.S.‐Norway Fulbright Foundation for Educational Exchange. Financial support has also been provided to the authors by the Research Council of Norway under the contract 223252. A. G. is supported by STFC grant ST/M001059/1 and NERC grant NE/P001556/1. Peer-reviewed Publisher Version