Magnetospheric convection and upper atmospheric dynamics

Abstract

Ionospheric plasma, driven into motion by the magnetospheric convection electric field, interacts with the neutral upper atmosphere via collisions. These collisions provide both a momentum source (ion drag) and an energy source (Joule heating) for the neutral atmosphere. A three-dimensional numerical model has been used to investigate the dynamical effects of this interaction on the neutral upper atmosphere. The effects of Joule heating, of the redistribution of heat input by global horizontal winds, and of adiabatic heating and cooling associated with vertical motions all depend on the neutral wind velocity. Thus the coupled set of neutral hydrodynamic and ion mobility equations is solved self-consistently in order to determine the global neutral density, temperature, and wind fields simultaneously. These calculations indicate that notable effects on the characteristics of the neutral upper atmosphere are caused by this interaction. The Joule heating rate, although small at low latitudes, increases strongly with latitude Λ up to Λ ∼70° and becomes comparable to that due to EUV absorption there. Joule heating also provides a heat source at night, whereas EUV heating provides none. Thermospheric temperatures and densities rise in response to the additional heating caused by ion-neutral collisions. The thermospheric wind fields are affected in two ways by magnetospheric convection. Joule heating causes a general equatorward flow which opposes the daytime poleward and the nighttime equatorward flow caused by EUV heating. Ion drag, on the other hand, causes an increase in the neutral poleward motion during the day and an equatorward motion at night. Thus the combined effects of ion drag and Joule heating lead to a substantial increase in the nighttime neutral meridional velocity; the effect during the day depends on the relative size of the effects of ion drag and Joule heating. These results emphasize the importance of the interaction among the magnetosphere, the ionosphere, and the neutral upper atmosphere.