A Numerical Study of the High Latitudinal Ion-Neutral Coupling Time Scale Under Disturbed Conditions

When solar wind and interplanetary magnetic field (IMF) disturb, thermospheric winds change accordingly. Among the momentum forces driving high-latitude thermospheric winds, ion drag is supposed to greatly affect wind variations through ion-neutral coupling when abrupt and strong changes in ion drifts occur. However, due to the great inertia of thermospheric winds it needs a certain period of time for the wind changes to be prominent both in speed and direction. How long the neutral winds take to change from one steady state to another through the ion-neutral coupling process is currently still a controversial issue. In this paper, we examine the high latitudinal ion-neutral coupling time scale based on the Thermosphere Ionosphere Electrodynamics General Circulation Model simulations, which can determine whether wind variations are dominantly driven by ion drag by analyzing the relative contribution of each momentum force. It is found that the spatial variation of ion-neutral coupling time scale is primarily determined by local electron density, but also varies with neutral density and ion-neutral collision frequency. Simulations during periods of medium solar activity at ∼250 km altitude show that the ion drag-dominated region is generally located at the dayside convection inverse boundary and the coupling time scale (e-folding time) is ∼1 hr when IMF B_y is the dominant component of the IMF and changes direction. Meanwhile, the southward component of IMF B_z enlarges the ion drag-dominated region. When IMF B_z is southward with a large magnitude, ion drag-dominated region is primarily located in the nightside auroral oval with ∼2 hr coupling time scale.