On the coupling between the lower and the upper thermosphere during the First Lower Thermosphere Coupling Study

Abstract

Simultaneous observations of the neutral winds and temperatures and electron densities in the upper and lower thermosphere are presented. The measurements were made at Arecibo and Millstone Hill during the first Lower Thermosphere Coupling Study (LTCS 1) campaign in September 1987. The observations show a great deal of day‐to‐day variability possibly due to the geomagnetic activity which occurred during the period; storm effects are discernible in the data at low altitudes (near 120 km) even at low latitudes (near 18°N). Theoretical simulations of the period were attempted by the National Center for Atmospheric Research thermosphere‐ionosphere general circulation model. A novel development was the use in the model of lower boundary conditions representative of the upward propagating semidiurnal tides based on the LTCS 1 mesosphere‐lower thermosphere observations. The upward propagating diurnal 1,1 mode was also included in the model. Simulations of the low and mid latitudes did not substantially improve with the use of lower boundary conditions derived from the measurements. The model was also unable to reproduce the observations in both the lower and upper thermosphere. This may be partially due to the effects of geomagnetic activity on the atmospheric fields, since the observational period included two intervals of geomagnetic disturbances. In the lower thermosphere, resolution of the discrepancies between the data and model predictions may require the extension of the model to lower altitudes. At present, the model predictions in the lower thermosphere are strongly dependent on the lower boundary conditions. The model predicts that diurnal waves are not negligible in analysis of lower thermosphere data; indeed, near 100 km at the model location of Arecibo, the diurnal wave amplitudes of the meridional winds were predicted to be greater than the semidiurnal. Investigation of the model predictions awaits the development of techniques to obtain observations of at least 24 hours duration in the lower thermosphere and the extension of the theoretical models to lower altitudes.