A Modeling Study of the Responses of Mesosphere and Lower Thermosphere Winds to Geomagnetic Storms at Middle Latitudes

Thermosphere Ionosphere Mesosphere Electrodynamics General Circulation Model (TIMEGCM) simulations are diagnostically analyzed to investigate the causes of mesosphere and lower thermosphere (MLT) wind changes at middle latitudes during the 17 April 2002 storm. In the early phase of the storm, middle-latitude upper thermospheric wind changes are greater and occur earlier than MLT wind changes. The horizontal wind changes cause downward vertical wind changes, which are transmitted to the MLT region. Adiabatic heating and heat advection associated with downward vertical winds cause MLT temperature increases. The pressure gradient produced by these temperature changes and the Coriolis force then drive strong equatorward meridional wind changes at night, which expand toward lower latitudes. Momentum advection is minor. As the storm evolves, the enhanced MLT temperatures produce upward vertical winds. These upward winds then lead to a decreased temperature, which alters the MLT horizontal wind pattern and causes poleward wind disturbances at higher latitudes. Plain Language Summary In a recent work, we found that in the mesosphere and lower thermosphere (MLT) region at middle latitudes, adiabatic heating/cooling and vertical heat advection, both associated with vertical wind changes, are the dominant processes that determine the temperature responses to storms. However, the cause of MLT vertical wind changes during storms has not been elucidated. Thus, there is a compelling need to understand how and why the wind changes during storms in the MLT region. Here we address this question by exploring theoretically the processes that determine the MLT wind response to storms at middle latitudes. During the early phase of the storm, the middle-latitude upper thermospheric wind changes are greater and occur earlier than those in the MLT region. The horizontal wind changes cause vertical wind changes, which are transmitted to the MLT region. The pressure gradient produced by the temperature changes associated with vertical wind changes and the Coriolis force are the dominant processes that drive storm time MLT wind changes at middle latitudes. Momentum advection is minor. As the storms evolve, the enhanced temperatures produce upward vertical winds. The upward vertical winds then lead to a depleted temperature, which consequently alters the MLT horizontal wind pattern.