North-South Differences in the Polar Ionosphere and Thermosphere: The Role of Magnetic Field Asymmetry, Seasonality, and Solar Activity

Abstract:

Both satellite and ground-based observations have indicated that there are systematic differences between the polar regions of the northern hemisphere (NH) and southern hemisphere (SH) thermosphere and ionosphere. At an altitude of ~400 km, cross-polar neutral wind and ion drift speeds are generally larger in the NH than in the SH, while the spatial variance of the neutral wind is larger in the SH. The magnitude of these North-South differences depends on the orientation of the interplanetary magnetic field (IMF), and probably also on the season and solar activity level. We use simulations with the Coupled Magnetosphere-Ionosphere-Thermosphere model to explore the causes of the observed North-South differences. At least part of the differences can be explained by the asymmetry of the Earth’s magnetic field. First, the magnetic flux density in the NH polar region is lower than in the SH. Since ion drift speeds scale as E/B, this result in stronger ion drifts in the NH, and also stronger neutral winds due to ion drag. Second, the offset between the invariant magnetic pole and the geographic pole is smaller in the NH than the SH. Model simulations indicate that a smaller offset should also contribute to stronger ion drifts and neutral winds. In addition, it could explain the greater spatial variance in the neutral wind in the SH. The dependencies of inter-hemispheric differences on season and solar activity level are only in part reproduced by the model. Further work is still needed to understand these dependencies fully.