Altitude dependence of middleand low-latitude thermospheric disturbance winds measured by WINDII

Thermospheric neutral winds exhibit strong altitudinal and latitudinal variation during geomagnetically quiet and active times. We use daytime middle and low-latitude neutral winds measured by the Wind Imaging Interferometer (WINDII) instrument on board the Upper Atmosphere Research Satellite (UARS) over the 90–275 km height range to study the altitude and season dependent climatology of disturbance winds (i.e., with quiet time patterns removed) in magnetic coordinates. The daytime perturbations winds are generally equatorward and westward and decrease toward the magnetic equator. Both the zonal and meridional components decrease sharply below 120 km and are essentially insignificant below 100 km. The seasonal dependence of the disturbance winds is strongest in the early morning sector. The zonal disturbance winds are predominantly westward with largest magnitudes near 150 km at late afternoon upper midlatitudes (magnetic latitudes 50°–60°) and near 120 km in the early morning winter between 25° and 45°. At upper midlatitudes in the early morning sector, the zonal perturbation winds are eastward above ∼120 km in the summer hemisphere and between ∼120 and 180 km in the winter hemisphere. Eastward perturbations are also observed near the magnetic equator with largest values near noon at 250 km. The meridional perturbation winds are mostly equatorward and nearly height-independent above ∼150 km, with the largest magnitudes occurring in the early morning winter hemisphere. In the late afternoon sector the upper midlatitude meridional disturbance winds are poleward, and there is a small winter-to-summer flow at lower latitudes. In general, the storm-time dependence of the disturbance winds is quite complex. The perturbation winds begin to develop after 0–9 hours and tend to saturate 12–24 hours after storm onset. The development does not change much with altitude, and the seasonal dependence is generally important only at storm times greater than 6–12 hours