Dependence of the high-latitude lower thermospheric momentum forcing on the interplanetary magnetic field

We analyze the forces acting on the high-latitude lower thermospheric wind system below 170 km for Southern Hemisphere summer conditions, as a function of the interplanetary magnetic field (IMF) direction, on the basis of numerical simulations. The pattern and magnitude of the forces and their relative contributions to the wind system vary strongly with respect to the direction of the IMF. At higher altitudes, above 130 km, for negative B y , strong anticyclonic winds are accelerated primarily by rotational Pedersen ion drag and are maintained by an approximate balance among the divergent/convergent Coriolis, horizontal advection, and relatively weak pressure-gradient accelerations. For positive B y , the pressure-gradient acceleration is increased, while the inertial forces are reduced. For negative B z , in comparison with negative and positive B y , the winds and forces extend to lower latitudes. The patterns of the accelerations for positive B z are similar to those for negative B z , but the magnitudes tend to be significantly smaller. At lower altitudes, below 120 km, the horizontal advection acceleration is less important but still contributes significantly to the maintenance of the neutral circulation in the polar cap region for positive B y . The difference of winds and forces above 130 km for negative and positive B y , with respect to winds and forces for zero IMF, show a simple structure with a strong anticyclonic or cyclonic vortex near the pole, respectively, centered differently for the two B y directions. The difference of winds and forces for negative and positive B z are more complex than those for negative and positive B y and extend to lower latitudes. Below 120 km, the difference of winds and forces for negative and positive B y are much stronger near the pole than for negative and positive B z , indicating that the IMF B y component tends to dominate effects on the neutral winds in the polar cap at low thermospheric altitudes. For all IMF conditions, at higher altitudes, the rotational ion-drag acceleration makes the dominant contribution to the neutral velocity tendency. This feature is most pronounced when the IMF B z is negative.