Elucidation of the physics of the gravity wave-TID relationship with the aid of theoretical simulations

The physics of the relationship between atmospheric gravity waves (AGWs) and traveling ionospheric disturbances (TIDs) is thoroughly investigated with emphasis on large-scale AGW/TIDs at F region heights at middle and high latitudes (provided field-perpendicular drifts are small). In support, simulations using a realistic AGW model (''Clark-based AGW model'') in combination with a realistic ionospheric model (''Graz Ionospheric Flux Tube Simulation (GIFTS) model'') were performed. All fundamental AGW/TID quantities are treated consistently, i.e., perturbations in neutral densities, wind, and temperature as well as disturbances in electron density, ion drift, and ion and electron temperature. The AGW-induced ionospheric response is inspected for all TID quantities, based on their governing conservation equations. The results are discussed by means of detailed and approximative formulae as well as instructive figures which provide a firm quantitative understanding significantly beyond the current state of knowledge. Especially the physics of the electron temperature disturbance (T-e-TID), up to now not yet quantitatively inspected, is thoroughly explored. A major finding is that the disturbance in specific terms of the electron energy equation is an order of magnitude more pronounced than the net disturbance determining the strength of the T-e-TID. Furthermore, simulation results illustrating the natural variability of the AGW/TID quantities (1) due to varying AGW properties and (2) due to changing thermosphere/ionosphere background conditions are discussed. Features observed include the following: AGW period and magnetic field line-induced south-north asymmetries are major causes of variability. Change from high/moderate to low solar activity enhances amplitudes of most AGW/TID quantities, electron density and temperature being likely exceptions; nighttime conditions tend to lower amplitudes versus daytime. The insight gained is valuable from a basic research point of view and also for suitable AGW/TID descriptions for thermosphere/ionosphere weather modeling.