Universal time variation of high‐latitude thermospheric disturbance wind in response to a substorm

Abstract

AbstractThe temporal and spatial variation in thermospheric winds are studied at 400 km altitude in response to substorms that start at different universal time (UT), using a global ionosphere and thermosphere model. The substorm‐induced disturbance winds at high latitudes are mainly in the poleward, westward, and upward directions in the dusk sector and in the equatorward, westward, and upward directions in the nighttime. The daytime perturbation is due to ion drag, driven by variations in interplanetary magnetic field Bz, whereas the nighttime perturbation is due to both the Bz and hemispheric power input. The nightside disturbed winds respond somewhat later than the dayside owing to low background ion density. Ion drag is the dominant driving force in the daytime for both meridional and zonal disturbed winds, whereas Joule heating is the dominant factor for the vertical winds. The nighttime meridional and zonal winds are driven by a combination of ion drag, Joule heating, and heating of the auroral belt, whereas the vertical winds are mainly caused by auroral belt heating. The viscous force acts to resist the ion drag, whereas the Coriolis force is negligible. The disturbed winds exhibit large variations with UT during equinox and local winter conditions. With more solar illumination, stronger disturbed winds can be generated. Weak or even opposite variations with UT in the disturbed winds are found in local summer, which is due to a smaller UT variation of the daytime ion density and a larger contribution from auroral heating than from ion drag.