A Numerical Study on the Responses of Thermosphere Density to the IMF B y ${B}_{y}$ Condition at High Latitudes

When the Interplanetary magnetic field (IMF) $B_y$ condition disturbs, the ionospheric convection pattern is tilted, subsequently altering thermospheric dynamics. While both observations and simulations have shown that the thermospheric mass density exhibits significantly different patterns under positive and negative IMF $B_y$ conditions, the underlying mechanisms driving the density variations at different altitudes are not well-established. In this study, we investigate the physical mechanisms responsible for the density variations based on the Thermosphere Ionosphere Electrodynamics General Circulation Model simulations. The model simulations show strong consistency with observations and indicate that density variations have universal time dependence. Meanwhile, the density variations driven by IMF $B_y$ disturbance exhibit different mechanisms in the lower and upper thermosphere. In particular, at around 200 km, the effect of non-local thermal variations dominates density changes, where the altitude-integrated effect of neutral temperature changes at lower altitudes primarily drives mass density variations at higher altitudes. At around 400 km, the composition effect associated with local vertical wind becomes the dominant driver in changing the mass density.