A Simple Model of the Transient Response of the Thermosphere to Impulsive Forcing

Abstract

Substantial variations in density, temperature and winds in the high-latitude thermosphere during geomagnetic substorms have been recorded using a wide variety of observational techniques. The disturbed state of the thermosphere may be brought about by a sudden addition of heat or momentum. The heating may be due either to Joule heating or energetic particle precipitation, and the momentum source is due to the drag that neutrals feel when colliding with ions. We have studied dynamical adjustment of the disturbed high-latitude thermosphere toward geostrophic equilibrium using an idealized high-resolution time-dependent scale model that includes the effects of the earth's rotation. The model describes the response of a compressible stratified atmosphere to horizontally and vertically extended sources. Our results show that the effect of rotation depends strongly upon the type and vertical scale of the forcing. In the ion-drag case, the pressure gradients that are required to maintain steady balanced motion develop slowly if the forcing is in a shallow layer, and much of the energy of the initial flow goes into inertial-gravity waves; but if the forcing is in a deep layer, pressure gradients develop quickly and most of the initial energy comes into geostrophic balance in a time much less than the inertial time scale. In the heating case there is little residual motion in the source region unless the forcing is quite shallow. For deep forcing the initial energy is transported away by inertial-gravity waves.