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

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 14years 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 B-y 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. Plain Language Summary In this study we use observations of plasma drift from the Earth's ionosphere to study the symmetry of the Earth's magnetosphere on a large scale. On this global scale we say that the magnetic field is asymmetric when the field lines connecting the two hemispheres are displaced from their usual location. This can happen when the magnetosphere interact with the interplanetary magnetic field, especially when the latter has a significant magnitude in the east-west direction. The major discovery of this study is that geomagnetic activity related to processes within the magnetosphere (tail reconnection) also seem to influence the degree of global asymmetry in the system. We find that the magnetosphere can become very asymmetric during periods of low geomagnetic activity, while it is more symmetric during times with higher activity. These results give us a better understanding of the processes leading to an asymmetric magnetosphere, which is needed to better understand the complex near-Earth space system that is becoming increasingly important for our society.