  Using a three-dimensional nonhydrostatic general circulation model, we
investigate the response of the thermosphere-ionosphere system to the 5-6
August 2011 major geomagnetic storm. The model is driven by measured storm-time
input data of the Interplanetary Magnetic Field (IMF), solar activity, and
auroral activity. Simulations for quiet steady conditions over the same period
are performed as well in order to assess the response of the neutral and plasma
parameters to the storm. During the storm, the high-latitude mean ion flows are
enhanced by up to 150-180%. Largest ion flows are found in the main phase of
the storm. Overall, the global mean neutral temperature increases by up to 15%,
while the maximum thermal response is higher in the winter Southern Hemisphere
at high-latitudes than the summer Northern Hemisphere: 40% vs. 20%increase in
high-latitude mean temperature, respectively. The global mean Joule heating
increases by more than a factor of three. There are distinct hemispheric
differences in the magnitude and morphology of the horizontal ion flows and
thermospheric flows during the different phases of the storm. The largest
hemispheric difference in the thermospheric circulation is found during the
main and recovery phases of the storm, demonstrating appreciable geographical
variations. The advective forcing is found to contribute to the modeled
hemispheric differences.
