EFFECTS OF AN ATMOSPHERIC GRAVITY-WAVE ON THE MIDLATITUDE IONOSPHERICF-LAYER

A modeling study of the atmospheric response to a single short burst o f enhanced ion convection at high latitudes, undertaken using the Shef field/University College London/Space Environment Laboratory coupled i onosphere/thermosphere model, has revealed a large-scale atmospheric g ravity wave (AGW) moving equatorward from a source in the dawn sector auroral zone. The wave propagates to midlatitudes, perturbing the iono sphere and creating a traveling ionospheric disturbance. Analysis of t he interaction between the thermosphere and ionosphere during the pass age of the AGW at midlatitudes is undertaken and reveals a complex hei ght-dependent response. At lower altitudes the field-aligned velocity of the ions follows closely the field-aligned wind. Above the F peak, diffusion processes become important and the field-aligned ion velocit y shows fluctuations which exceed those in the wind. Changes in N(m)F2 and h(m)F2, during the interaction, are due to redistribution of plas ma alone with changes in production and loss insignificant. As the F l ayer is lifted by the positive surge in the gravity wave, N(m)F2 decre ases, due to a divergence in the ion flux, itself caused by the combin ation of a divergent neutral wind and an increase in the effects of di ffusion with altitude. The slab thickness also increases. Subsequently , the opposite happens as h(m)F2 falls below its equilibrium value.