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The mesospheric polar vortex (MPV) plays a critical role in coupling the atmosphere-ionosphere system, so its accurate simulation is imperative for robust predictions of the thermosphere and ionosphere. While the stratospheric polar vortex is widely understood and characterized, the mesospheric polar vortex is much less well-known and observed, a short-coming that must be addressed to improve predictability of the ionosphere. The winter MPV facilitates top-down coupling via the communication of high energy particle precipitation effects from the thermosphere down to the stratosphere, though the details of this mechanism are poorly understood. Coupling from the bottom-up involves gravity waves (GWs), planetary waves (PWs), and tidal interactions that are distinctly different and important during weak vs. strong vortex states, and yet remain poorly understood as well. Moreover, generation and modulation of GWs by the large wind shears at the vortex edge contribute to the generation of traveling atmospheric disturbances and traveling ionospheric disturbances. Unfortunately, representation of the MPV is generally not accurate in state-of-the-art general circulation models, even when compared to the limited observational data available. Models substantially underestimate eastward momentum at the top of the MPV, which limits the ability to predict upward effects in the thermosphere. The zonal wind bias responsible for this missing momentum in models has been attributed to deficiencies in the treatment of GWs and to an inaccurate representation of the high-latitude dynamics. In the coming decade, simulations of the MPV must be improved. 

Abstract The migrating diurnal tide (DW1) is one of the dominant wave motions in the mesosphere and lower thermosphere. It plays a crucial role in neutral atmosphere and ionosphere coupling. The DW1 can vary over a range of time scales from days to years. While the long‐term variability of the DW1 is mainly attributed to the source and background atmosphere variability, the driving mechanism of short‐term DW1 variability is still openly debated. Herein the daily structure of the DW1 is extracted from observations using a novel multi‐satellite estimation technique and compared with model simulations (NOGAPS‐ALPHA and WACCM‐X). Both the observations and the models show that the day‐to‐day variability of the DW1 is a persistent and ubiquitous feature. The standard deviation peak of DW1 amplitudes, which is used to measure the maximum variability, is generally aligned with the DW1 amplitude peak. This result indicates that the day‐to‐day variability of the DW1 reflects global‐scale changes rather than local excitation of diurnal oscillation. The spatial lag‐correlation analysis of the diurnal (1,1) and (1,2) Hough modes suggests that the day‐to‐day variability of the diurnal (1,1) Hough mode is likely driven by variability in the lower atmosphere and the source of day‐to‐day variability of the (1,2) mode is uncertain. The significant correlation of the DW1 day‐to‐day variability between the NOGAPS‐ALPHA and the multi‐satellite estimation techniques also indicates that the model is capable of reproducing the DW1 structure on a daily basis.