Wind and temperature effects on thermosphere mass density response to the November 2004 geomagnetic storm

A unique conjunction of the Thermosphere Ionosphere Mesosphere Energetics and Dynamics (TIMED) and the Challenging Minisatellite Payload (CHAMP) satellites provided simultaneous columnar neutral composition, Sigma O/N-2, and thermosphere density observations, enabling a novel study of thermospheric response to the 7-9 November 2004 geomagnetic storm. Both Sigma O/N-2 and mass density showed profound response to this severe geomagnetic storm, but their latitudinal and temporal structures differed markedly. In particular, high-latitude depletion and low-latitude enhancement in Sigma O/N-2 were observed throughout the storm period, especially during the main phase. In contrast, neutral density at 400 km altitude increased from pole to pole shortly after the storm, with strongest enhancement of order 200%-400% during the main phase. Comparisons of observed thermosphere response with simulations from the National Center for Atmospheric Research Thermosphere-Ionosphere-Electrodynamics General Circulation Model (TIEGCM) were carried out to interpret the observed contrasting characteristics of thermosphere composition and mass density in response to this geomagnetic storm. The TIEGCM simulations show that the contrasting characteristics occur not only in Sigma O/N-2 and mass density at a constant altitude at 400 km, but also in O/N-2 and mass density on a constant-pressure surface. At an altitude of 400 km (CHAMP altitude), storm-time mass densities significantly increase due to an increase in scale height throughout the vertical column between the heat source and satellite altitude. For a given increase in scale height, the more scale height increments separating the heat source from the satellite altitude, the greater is the mass density response. It is shown that scale height change is caused partly by storm-time neutral temperature enhancements due to heating and partly by changes in mean molecular weight due to winds. These findings indicate that wind effects can cause significant deviations from a mass density pattern resulting solely from neutral temperature changes by altering the mean molecular weight, particularly at high latitudes.