Observations from lidars and satellites have shown that large neutral temperature increases and decreases occur in the middle and low latitudes of the mesosphere and lower thermosphere region during geomagnetic storms. Here we undertake first-principles simulations of mesosphere and lower thermosphere temperature responses to storms using the Thermosphere Ionosphere Mesosphere Electrodynamics General Circulation Model to elucidate the nature and causes of these changes. Temperature variations were not uniform; instead, nighttime temperatures changed earlier than daytime temperatures, and temperatures changed earlier at high latitudes than at low ones. Furthermore, temperatures increased in some places/times and decreased in others. As the simulation behaves similar to observations, it provides an opportunity to understand physical processes that drive the observed changes. Our analysis has shown that they were produced mainly by adiabatic heating/cooling that was associated with vertical winds resulting from general circulation changes, with additional contributions from vertical heat advection. Plain Language Summary Both ground-and space-based observations have showed that storm time temperature have strong variations in the mesosphere and lower thermosphere (MLT) region. However, the possible physical mechanisms causing these storm time thermal responses have not been fully understood. Therefore, in this paper, we use the Thermosphere Ionosphere Mesosphere Electrodynamics General Circulation Model to investigate the possible mechanisms for the observed storm time temperature changes in the MLT region. As the simulation behaves similar to observations, it provides an opportunity to understand physical processes that drive the observed changes. By analyzing Thermosphere Ionosphere Mesosphere Electrodynamics General Circulation Model results, we found storm time adiabatic heating/cooling and vertical heat advection, both associated with changes in vertical winds, are the MLT-dominant heating processes in the MLT region at middle latitudes. Horizontal heat advection and radiative cooling also contribute to temperature changes, but they occur later than the other major heating terms. At middle and low latitudes, MLT changes are first seen in the vertical winds, followed by temperature changes; horizontal winds change later. The changes of vertical winds in the MLT region are associated with those occurring at higher altitudes. There is no direct contribution of auroral Joule heating and particle heating to the storm time MLT temperature changes 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.