The Role of the Upper Atmosphere for Dawn-Dusk Differences in the Coupled Magnetosphere-Ionosphere-Thermosphere System

The interaction between the solar wind with its inherent interplanetary magnetic field (IMF) and the geomagnetic field sets up a large-scale plasma circulation in the Earth's magnetosphere and the magnetically tightly connected ionosphere. The ionospheric ion drift at polar latitudes accelerates the neutral gas as a nondivergent momentum source primarily in force balance with pressure gradients, which are mainly directed from afternoon to midnight. The high-latitude neutral wind circulation is essentially modified by apparent forces due to Earth's rotation (Coriolis and centrifugal forces), while advection and viscous forces can significantly contribute. The apparent forces affect the dawn and dusk side asymmetrically, favoring a large dusk-side neutral wind vortex that dominates over the dawn-side vorticity cell. The IMF By component induces additional dawn-dusk differences in both ion drift and neutral wind patterns. Nonuniformities and steep gradients of the ionospheric conductivity can further modify locally the ionospheric convection and neutral wind circulation. Feedback effects of the neutral wind dynamics and its embedded ionosphere modify to a certain degree the large-scale magnetospheric convection, which includes dawn-dusk asymmetries of their behavior.