Theoretical effects of tides and auroral activity on the low latitude ionosphere

The effects of tidal and auroral activity on the low-latitude ionosphere are investigated using simulations from the National Center for Atmospheric Research thermosphere-ionosphere-electrodynamic general circulation model (TIEGCM). Calculations were performed for March solar cycle minimum conditions. The model predicts that the tidal waves propagating vertically from the lower atmosphere into the thermosphere affect n(max) and h(max) on the order of 10-20% during the daytime and as much as 40% during the night, depending on latitude. The lower atmosphere waves introduce semidiurnal oscillations into h(max), causing it to be alternately higher and lower than in simulations without the waves, due to perturbations in the vertical velocities. The theoretical results indicate that n(max) and h(max) may also be perturbed by geomagnetic activity even at low latitudes. Typical perturbations to n(max) from the auroral energy and momentum sources were on the order of 10%. Consequently, the TIEGCM simulations suggest that the peak electron density in the F region is dependent on the relative contributions from perturbations due to auroral sources and the vertically propagating tides from the lower atmosphere. Since both these sources are quite variable, the perturbations induced by the tidal and auroral forced waves may contribute to the observed daily variability in the peak electron density and its height. (C) 1997 Elsevier Science Ltd.