An analysis of the high‐latitude thermospheric wind pattern calculated by a thermospheric general circulation model: 1. Momentum forcing

Abstract

Previous studies have shown that the wind, temperature, and composition structures calculated by the National Center for Atmospheric Research (NCAR) thermospheric general circulation model (TGCM) were in reasonable agreement with measurements made by the Dynamics Explorer 2 satellite over the southern hemisphere polar cap during October 1981. The winds at F region heights follow but generally lag behind the two‐cell ion‐drift pattern of magnetospheric convection. A diagnostic package for the TGCM has been developed to calculate the magnitude and direction of the various terms in the hydrodynamic and thermodynamic equations within the model. The package has been used to decompose the thermospheric momentum equations at each hour of universal time for a 24 hour “steady state” run of the TGCM. Displaced geomagnetic and geographic poles are considered, and a 60‐kV cross‐tail potential is assumed for the magnetospheric convection model. The individual momentum forcing terms for constant‐pressure levels z = 1 (300 km) and z = −4 (120 km) are presented to illustrate the balance of forces acting on the neutral gas at F and E region altitudes over the southern hemisphere polar cap. The results show that at F region altitudes the largest forces in the “steady state” are due to ion drag induced by sunward‐drifting ions on the dawnside and duskside of the auroral oval/polar cap boundary. There is a universal time dependence of thermospheric momentum forcing due to the diurnal oscillation of the convection pattern with respect to the solar terminator. At F region altitudes the basic balance of forces is between ion drag and pressure, whereas at E region altitudes Coriolis and advection forces also have significant contributions. The lower thermosphere cold, low‐pressure cyclonic circulation and the warm, high‐pressure anticyclonic circulation calculated by the TGCM on the dawnside and duskside of the polar cap, respectively, show a near gradient flow balance. The ion‐drag, Coriolis, and pressure forces are in approximate balance on both sides of the polar cap with the advection force enhancing and diminishing the strength of the dawn and dusk vortices, respectively.