Vertical gradients in the zonal wind observed in the equatorial F-region under postsunset conditions

In the early evening sector of the F region near the geomagnetic equator, an eastward pressure gradient as the sun sets reorients the neutral flow toward the east, typically occurring within one hour of local sunset. Very few vertically-resolved measurements of this effect exist. We present recent in-situ chemical tracer results from the EVEX campaign, as well as results from the earlier Guara campaign, that show strong vertical shear in the zonal wind during sunset hours in the F region, up to a 150 m/s westward shift over 60 km altitude. Eastward F-region neutral winds near the geomagnetic equator drive vertical Pedersen currents at sunset that, in turn, drive the prereversal enhancement (PRE) of the eastward electric field in the equatorial F-region that is thought to be a primary driver of equatorial spread-F. Studies of the neutral winds relating to the PRE have been primarily focused on the winds observed from ground-based interferometry and from satellite accelerometer data, techniques which generally lack vertical resolution. We show that eastward winds at one altitude are not necessarily accompanied by eastward winds at higher altitudes, i.e., that the forces that drive the neutral wind are not constant with altitude at sunset. At sunset, solar heating varies significantly with altitude, decreasing at lower altitudes first, which would create a thermal pressure gradient with a similar vertical profile to that observed in the neutral winds. We discuss the magnitude of this effect as well as other factors that could contribute to the observed vertical gradients. We then apply these effects to typical ionospheric conditions at the time of the experiments and examine the resulting neutral forcing in relation to the observed wind profiles