A Simulation Study on the Relationship Between Field-Aligned and Field-Perpendicular Plasma Velocities in the Ionospheric F Region

This study addresses a long-standing scientific puzzle regarding ionospheric F₂ region dynamics. Incoherent scatter radar observations of F₂ region plasma drifts showed a strong anticorrelation between temporal variations of field-aligned upward plasma velocity (V_i‖) and field-perpendicular poleward plasma drift (V_i ⊥ N) over time scales from a few hours to a day at middle latitudes. The underlying physical processes remain a highly controversial issue, despite a number of speculations and qualitative inspections. Previous studies lacked especially 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 variations using a self-consistent Thermosphere-Ionosphere-Electrodynamics General Circulation Model. It is found that the anticorrelation between V_i‖ and V_i ⊥ N has strong altitudinal and latitudinal dependences. The anticorrelation between the diurnal variations of V_i‖ and V_i ⊥ N is associated with the neutral wind dynamo. Poleward meridional winds result in downward V_i‖ and poleward V_i ⊥ N, and vice versa. The anticorrelation between short-term temporal disturbances of V_i‖ and V_i ⊥ N is mainly caused by ion drag, in response to high-latitude convection electric field forcing. This forcing penetrates to lower latitudes and affects poleward plasma drifts V_i ⊥ N, which drags poleward meridional winds and modulates downward V_i‖. As the enhanced convection electric fields subside, the anticorrelation is mainly associated with disturbance meridional wind dynamo. The storm time high-latitude energy and momentum inputs change global meridional winds which modify zonal electric fields to induce V_i ⊥ N changes. Furthermore, ambipolar diffusion plays a significant role in modulating the relationship between V_i‖ and V_i ⊥ N.