Ion‐neutral coupling effects on low‐latitude thermospheric evening winds

Abstract We examine the forces that determine zonal wind structure in the low‐latitude evening thermosphere and its relation with ion‐neutral coupling. These winds drive the evening F region dynamo that affects the equatorial ionization anomaly (EIA) and the generation of plasma irregularities. Forces are calculated using the Thermosphere‐Ionosphere‐Electrodynamics General Circulation Model coupled with the Global Ionosphere‐Plasmasphere model. At 19 LT, the horizontal pressure gradient dominates the net acceleration of neutral winds below ∼220 km, while it tends to be offset by ion drag and viscosity higher up. The eastward pressure‐gradient acceleration above 200 km increases approximately linearly with height and tends to be similar for different latitudes and different levels of solar activity. The pressure‐gradient and ion‐drag forces in the central F region approximately balance for field lines that pass through the EIA. Viscosity is an important additional force at non‐EIA latitudes and in the bottomside and topside EIA ionosphere. An increase in E region drag on plasma convection due to increased nighttime ionization causes both the ion and neutral velocities in the F region to decrease, while the velocity difference tends to be maintained. The presence of a low‐latitude evening time vertical shear in the zonal wind is associated primarily with a strong eastward pressure‐gradient acceleration at high altitude that reverses the daytime westward wind and a weak low‐altitude pressure‐gradient acceleration of either eastward or westward direction that fails to reverse the low‐altitude westward wind present in the afternoon.