Impact of Midnight Temperature Maximum on the Nighttime Equatorial Vertical Drifts

During the recent solar minimum, a large amount of C/NOFS satellite measurements have revealed an upward drift that occurs during the post-midnight period (~2-3 LT). The upward drift is also frequently shown in radar measurements. The phenomenon has not yet been explained and the physics are still unknown. Our simulations have successfully reproduced the unusual upward drift using the coupled Whole Atmosphere Model and Global Ionosphere Plasmasphere Model (WAM/GIP). Simulations also reveal strong seasonal and longitudinal dependence of the upward drift. Preliminary results indicate that the upward drift during post-midnight period may be associated with the dynamics accompanying the midnight temperature maximum (MTM) that occurs in the thermosphere. The MTM reverses the typical zonal and meridional winds in the nighttime and the changes of winds are likely to have further impact on the nighttime electrodynamic processes. We will present the morphology and characteristics of the post-midnight upward drift shown in the simulations and propose theories to explain the causal mechanisms.