Tidal oscillations in the thermosphere: a theoretical investigation of their sources

Measurements of tidal oscillations in the mid to high latitude thermosphere reveal a dependency of diurnal and semidiurnal wind amplitudes on geomagne tic activity. This cannot be explained on the basis of the classical assump tion that tides in the lower thermosphere originate primarily from below th e mesopause. We use an updated version of the Coupled Thermosphere-Ionosphe re-Plasmasphere model (CTIP) to numerically simulate the thermospheric wind oscillations, distinguishing between those propagating upwards through the mesopause and those generated in situ. These simulations suggest that in s itu diurnal and semidiurnal oscillations generated at mid to high atitudes by ion-neutral interactions such as ion drag and Joule heating are comparab le in magnitude and, towards higher latitudes, stronger than the upwards pr opagating tides with which they interact through both destructive and const ructive interference. Due to their geomagnetic origin, the in-situ oscillat ions strongly depend on K-p and thus cause an overall K-p dependency in the observed diurnal and semidiurnal winds. We predict the occurrence of measu rable in-situ tides also for mid-latitude sites with higher geomagnetic lat itude, such as Millstone Hill, during geomagnetically disturbed conditions. (C) 2001 Elsevier Science Ltd. All rights reserved.