Upper Thermospheric Winds: Forcing, Variability, and Effects

This chapter provides a brief review of recent advances in observing and understanding upper thermospheric winds in the altitude range of the ionospheric F 2 -region, and describes the morphology, drivers, and variability of the winds under different geophysical conditions. We first provide a general introduction to the forcing processes that govern the state and variability of upper thermospheric winds. We describe these processes and their relative roles in the neutral momentum balance and their variations with geophysical conditions. These processes include pressure gradient, ion drag, the Coriolis force, the viscous drag force, and momentum advection. All these processes contribute to the overall momentum balance, global circulation, and local wind structures that make the upper thermospheric winds very dynamic, complicated, and not in geostrophic balance. We then discuss the wind behavior during geomagnetically active periods and the various physical processes that determine this behavior. In the next section, we review the processes by which the winds affect other neutral parameters, such as temperature and composition, and the ionosphere through direct transportation and the generation of dynamo electric fields. We then provide a brief review of the solar cycle, season, and hemispheric dependence of the winds and the possible physical processes that determine this dependence. The effects of upward propagating lower atmospheric waves on upper thermospheric winds are then described. We also discuss upper thermospheric wind measurement techniques and observational results. In the last section, we discuss the critical needs in future efforts, including both first principles numerical model development and simulations and wind measurements, to better our understanding of the dynamic behavior of upper thermospheric winds, both globally and locally, and its variability through ion-neutral coupling in the upper thermosphere and ionosphere system.