Impact of the semidiurnal lunar tide on the midlatitude thermospheric wind and ionosphere during sudden stratosphere warmings

Variability of the midlatitude ionosphere and thermosphere during the 2009 and 2013 sudden stratosphere warmings (SSWs) is investigated in the present study using a combination of Constellation Observing System for Meteorology, Ionosphere, and Climate (COSMIC) observations and thermosphere-ionosphere-mesosphere electrodynamics general circulation model (TIME-GCM) simulations. Both the COSMIC observations and TIME-GCM simulations reveal perturbations in the F region peak height (hmF₂) at Southern Hemisphere midlatitudes during SSW time periods. The perturbations are ∼20-30 km, which corresponds to 10-20% variability of the background mean hmF₂. The TIME-GCM simulations and COSMIC observations of the hmF₂ variability are in overall good agreement, and the simulations can thus be used to understand the physical processes responsible for the hmF₂ variability. Through comparison of simulations with and without the migrating semidiurnal lunar tide (M₂), we conclude that the midlatitude hmF₂ variability is primarily driven by the propagation of the M₂ into the thermosphere where it modulates the field-aligned neutral winds, which in turn raise and lower the F region peak height. Though there are subtle differences, the consistency of the behavior between the 2009 and 2013 SSWs suggests that variability in the Southern Hemisphere midlatitude ionosphere and thermosphere is a consistent feature of the SSW impact on the upper atmosphere.