Perturbing the Navier-Stokes equation in the ionospheric F-region, the disp
ersion relation and the unstable regions in wavenumber space are derived as
a function of two control parameters, the colatitude theta and the slow ti
me evolution for tidal modes tau. Our results satisfy the internal gravity
wave (IGW) dispersion relation and are in good agreement with observational
data. Also, we obtained a nice prediction for the excitation regions of th
ese modes in the parameter space (theta, tau). The distribution of IGW exci
tations, in this parameter space is in very good agreement with observation
al data distributed along the day. An alternative explanation to the disapp
earence of IGW near the equator is given. Our model predicts an interesting
latitude dependence, which needs to be tested by new observational data.