Since the F region ion-drag force is proportional to the difference between the ion and neutral velocities, Rishbeth's hypothesis implies that a change in the ion velocity should be accompanied by a corresponding change in the F region neutral velocity in a way that maintains an adequate ion-neutral velocity difference. Rishbeth hypothesized in his theory of the F layer dynamo that the ion-drag force on the F region wind and the associated meridional electric current density of the F region dynamo automatically adjust to balance the horizontal pressure-gradient force on the air associated with the day-to-night pressure differences in the upper thermosphere. The ion-drag force is associated with electric currents flowing within and between the F and E regions, transferring momentum from the rarefied air in the F region to the much denser air in the E region. Because the neutral winds have such a strong influence on the low-latitude evening ionosphere, it is important to understand the processes that determine these winds and their variability.Ī complication of understanding the nightside winds is that not only do the winds drive the plasma convection through neutral-ion collisions but also the convection feeds back on the winds through the ion-drag effect caused by these same collisions. The neutral wind shear also facilitates the generation of plasma instabilities.
The vertical drift affects the amplitude of the equatorial ionization anomaly (EIA) and is associated with plasma instabilities that can lead to deleterious radio wave scintillations. The longitude variation of the zonal plasma convection affects the vertical plasma drift through the condition of a curl-free electric field. Although a few observations of neutral winds above 160 km in the low-latitude evening thermosphere fail to confirm the modeled wind shear, the consistency of modeled and observed plasma convection shears suggests that the modeled neutral wind shear is likely to be a common feature of the low-latitude evening winds. One notable phenomenon in model simulations of the low-latitude evening ionosphere is a shear in the wind from westward at low altitudes (below roughly 180 km) to eastward at high altitudes (above roughly 250 km), and this is associated with a shear of plasma convection from westward on low-apex field lines to eastward on higher-apex field lines, with the transition around roughly 300 km. Eastward or westward zonal winds tend to drag the plasma along, subject to the “frozen-in” constraint that all plasma particles on a geomagnetic field line essentially move together in the low-collision F region and to the constraint that electric current in the coupled E and F regions is divergence free. Plasma convection in the nighttime low-latitude ionosphere is strongly influenced by thermospheric winds through the F layer dynamo effect. 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. 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. Viscosity is an important additional force at non-EIA latitudes and in the bottomside and topside EIA ionosphere. The pressure-gradient and ion-drag forces in the central F region approximately balance for field lines that pass through the EIA.
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. 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. Forces are calculated using the Thermosphere-Ionosphere-Electrodynamics General Circulation Model coupled with the Global Ionosphere-Plasmasphere model. These winds drive the evening F region dynamo that affects the equatorial ionization anomaly (EIA) and the generation of plasma irregularities. We examine the forces that determine zonal wind structure in the low-latitude evening thermosphere and its relation with ion-neutral coupling.
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