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 F-2 region dynamics. Incoherent scatter radar observations of F-2 region plasma drifts showed a strong anticorrelation between temporal variations of field-aligned upward plasma velocity (V-i parallel to) and field-perpendicular poleward plasma drift (V-i perpendicular to 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 parallel to with V-i perpendicular to N variations using a self-consistent Thermosphere-Ionosphere-Electrodynamics General Circulation Model. It is found that the anticorrelation between V-i parallel to and V-i perpendicular to N has strong altitudinal and latitudinal dependences. The anticorrelation between the diurnal variations of V-i parallel to and V-i perpendicular to N is associated with the neutral wind dynamo. Poleward meridional winds result in downward V-i parallel to and poleward V-i perpendicular to N, and vice versa. The anticorrelation between short-term temporal disturbances of V-i parallel to and V-i perpendicular to 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 perpendicular to N, which drags poleward meridional winds and modulates downward V-i parallel to. 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 perpendicular to N changes. Furthermore, ambipolar diffusion plays a significant role in modulating the relationship between V-i parallel to and V-i perpendicular to N.