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

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.