SIMULATIONS OF THE BEHAVIOR OF THE POLAR THERMOSPHERE AND IONOSPHERE FOR NORTHWARD IMF

The dynamics and structure of the polar thermosphere and ionosphere wi thin the polar regions are strongly influenced by the magnetospheric e lectric field. The convection of ionospheric plasma imposed by this el ectric field generates a large-scale thermospheric circulation which t ends to follow the pattern of the ionospheric circulation itself. The magnetospheric electric field pattern is strongly influenced by the ma gnitude and direction of the interplanetary magnetic field (IMF), and by the dynamic pressure of the solar wind. Previous numerical simulati ons of the thermospheric response to magnetospheric activity have used available models of auroral precipitation and magnetospheric electric fields appropriate for a southward-directed IMF. In this study, the U CL/Sheffield coupled thermosphere/ionosphere model has been used, incl uding convection electric field models for a northward IMF configurati on. During periods of persistent strong northward IMF B(Z), regions of sunward thermospheric winds (up to 200 m s-1) may occur deep within t he polar cap, reversing the generally anti-sunward polar cap winds dri ven by low-latitude solar EUV heating and enhanced by geomagnetic forc ing under all conditions of southward IMF B(Z). The development of sun ward polar cap winds requires persistent northward IMF and enhanced so lar wind dynamic pressure for at least 2-4 h, and the magnitude of the northward IMF component should exceed approximately 5 nT. Sunward win ds will occur preferentially on the dawn (dusk) side of the polar cap for IMF B(Y) negative (positive) in the northern hemisphere (reverse i n the southern hemisphere). The magnitude of sunward polar cap winds w ill be significantly modulated by UT and season. reflecting E- and F-r egion plasma densities. For example, in northern mid-winter, sunward p olar cap winds will tend to be a factor of two stronger around 1800 UT , when the geomagnetic polar cusp is sunlit. then at 0600 UT, when the entire polar cap is in darkness.