Lower Atmosphere/Thermosphere/Ionosphere Coupling Due to Tides

Abstract

The study of middle atmosphere tides has progressed rapidly due to advances in both observations and theory. Mesospheric tidal measurements during ATMAP (Atmospheric Tides Middle Atmosphere Program) in particular have provided the means to begin addressing the detailed coupling between the lower and upper atmosphere due to tidal waves. The Lower Thermosphere Coupling Study (LTCS) is extending the domain of interest to the thermosphere. Some results of the first LTCS campaign during September 1987, encompassing observations from 14 instruments, are described. These include the derivation of an empirical model of the data set, a method to extrapolate mesosphere predictions and/or data sets into the 100-130km altitude region, and a means to derive boundary conditions for global thermospheric models which simulate the effects of upward propagating tides on the model solutions. The theoretical models exhibit encouraging agreement with the data, particularly for mid-and high latitudes. Surprisingly, worst agreement occurs for observations made at Arecibo, a low latitude site. This has been tentatively attributed to the neglect of electrodynamics in the model. The observations also suggest the presence of a strong (2, 3) mode during the LTCS-1 campaign, which appears unusual for an equinox period. Recent works which investigate the question of tidal set-up times and the effects of transients are briefly discussed. These topics are particularly relevant to discussion of LTCS-1 observations since the interval included two isolated periods of geomagnetic activity.