LOWER THERMOSPHERE COUPLING STUDY - COMPARISON OF OBSERVATIONS WITH PREDICTIONS OF THE UNIVERSITY-COLLEGE-LONDON-SHEFFIELD THERMOSPHERE-IONOSPHERE MODEL

During the first Lower Thermosphere Coupling Study (LTCS), September 21-25 1987, data were recorded from the incoherent scatter radar sites at EISCAT, Millstone Hill, Sondrestrom, and Arecibo. These experimental facilities measured ionospheric parameters (Ne, Te, Ti, and plasma velocity) in the E and the F regions which have been used to determine the E region neutral wind and infer the neutral temperature in the height range 100 - 150 km. Propagating tides are clearly visible in some of the parameters, and the latitude structure and phase variations with height indicate the presence of at least the (2,2) and (2, 4) global tidal Hough modes. The influence of geomagnetic forcing is also clearly present at high latitudes. The University College London-Sheffield University three-dimensional coupled thermosphere-ionosphere model has been used to simulate this period of observation, by imposing tidal forcing at the lower boundary and magnetospheric forcing at high latitudes, in an attempt to interpret and understand the experimental data. Model simulations are able to predict where the signature of a particular tidal mode is likely to be observed in the respective responses of the temperature and wind structure. The numerical simulations predict the range of observed tidal amplitudes at mid and high latitudes, provided the tidal forcing functions imposed near the lower boundary of the model are larger (400 m geopotential height variation) than those inferred from linear tidal models. At high latitudes, the semi-diurnal ion temperature response is driven by geomagnetic heating, rather than lower atmospheric tides and, as low as 110 km altitude, there is a large difference between ion and neutral temperatures during active conditions. Temperature and zonal wind tidal amplitudes observed at the low-latitude site of Arecibo are consistently larger than those produced by any of the simulations.