Mesosphere‐ thermosphere tidal coupling during the September 21–25, 1987, LTCS 1 Campaign

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Abstract
The thermosphere (≥ 90 km) is dynamically coupled to the mesosphere and lower atmospheric regions by gravity waves, tides, and planetary waves which propagate upward from below and often undergo dissipation in the 80 to 150‐km region. This paper focuses on the mesosphere‐thermosphere coupling due to the solar semidiurnal atmospheric tide during the first Lower Thermosphere Coupling Study (LTCS) campaign interval of September 21–25, 1987. During this period, determinations of semidiurnal eastward (u) and northward (υ) velocity components and temperature (δT) are available within the 80 to 150‐km region from a total of 13 incoherent scatter, meteor, and spaced antenna radars. A new method is developed and reported here for assimilating such data sets into a self‐consistent global empirical description of the tidal dynamics between 80 and 150 km. Using a set of numerically generated ‘Hough mode extension’ (HME) functions for u, v, and δT corresponding to the (2,2), (2,3), (2,4), and (2,5) semidiurnal tidal modes, a least squares fit to the radar data is performed. The fit results in a single (complex) normalizing factor for each HME from which globally continuous and internally consistent semidiurnal oscillations in u, v, and δT (as well as vertical velocity and density) can be derived. The quality of fit to these data from various height regions including both northern and southern hemispheres establishes the internal consistency between these measurements as well as the applicability of the method to future data sets. The resulting empirical model reproduces the data significantly better than recent theoretical models, suggesting use of the method for deriving tidal boundary conditions for thermospheric general circulation models.