A simulation study on the relationship between the field-aligned and field-perpendicular plasma velocities in the ionospheric F-region

Incoherent scatter radar observations of the ionospheric F2 region plasma drifts since the 1970s have indicated a strong anti-correlation between local time variations of field-aligned upward plasma velocity (V_(i||)) and field-perpendicular poleward plasma velocity (V_(i⊥N)) at middle latitudes. The underlying physical processes of the fundamental ionospheric dynamics remains a long-standing controversial issue, despite of a number of speculations and qualitative inspections up to the 1990s. Previous studies lack especially of quantitative analysis that could lead to decisive conclusions. In this study, we provide a comprehensive modeling study to explore the physical processes relating V_(i||) with V_(i⊥N) using a self-consistent Thermosphere-Ionosphere-Electrodynamics General Circulation Model. This study provides a geomagnetic storm scenario where local dynamic changes in V_(i||) and V_(i⊥N) are under strong external forcing. It is found that the anti-correlation between V_(i||) and V_(i⊥N) is mainly caused by ion drag, in responding to high latitude convection electric field changes. The plasma drifts of V_(i⊥N) drag the poleward horizontal (meridional) winds, which then modulate the downward V_(i||). As the enhanced convection electric fields are subsided, the anti-correlation between V_(i||) and V_(i⊥N) during most times of the night is mainly associated with the F-region wind dynamo. The F-region meridional winds generate zonal electric fields to induce the changes of V_(i⊥N). While both V_(i⊥N) and meridional winds can influence field-aligned ambipolar diffusion velocity (by redistributing plasma density), which subsequently contributes to V_(i||), the ambipolar diffusion plays only a minor role in causing the anti-correlation between V_(i||) and V_(i⊥N) .