Vertical variations in thermospheric O/N2 and the relationship between O and N2 perturbations during a geomagnetic storm

The ratio of O to N-2 number densities (O/N-2) at different altitudes is an important parameter in describing thermospheric neutral composition changes and their effects on the ionosphere during geomagnetic storms. However, storm-induced vertical variations in O/N-2 and its dependence on the O and N-2 perturbations are still not fully understood. Here, the Thermosphere/Ionosphere Electrodynamics General Circulation Model simulations were used to investigate the responses of thermospheric composition at different pressure levels to the super geomagnetic storm occurred on November 20 and 21 in 2003. Our analysis shows that the behaviors of O/N-2 perturbations on different pressure levels are similar above similar to 180 km altitude. In the middle and low thermosphere of below similar to 300 km, the storm-time O/N-2 decrease is mainly caused by a large reduction of O number density. However, N-2 enhancement plays a vital role in O/N-2 decreases in the upper thermosphere. The O/N-2 enhancement is mainly attributed to the N-2 decreases at all pressure levels. The changes of O and N-2 number densities at a constant pressure level can be explained by the perturbations of their mass mixing ratio (mmr) and total mass density (rho). The regions of the O/N-2 decrease are characterized by the O mmr decrease and N-2 mmr enhancement, whereas the regions of the O/N-2 increase are characterized by the O mmr increase and N-2 mmr decrease. The rho value that shows the decrease globally at most pressure levels during the storm either enhance or reduce the O and N-2 perturbations.