The Effects of Composition on Thermosphere Mass Density Response to Geomagnetic Activity

The ability to determine the thermosphere mass density response to geomagnetic disturbances is of critical importance in understanding how energy deposited in the thermosphere affects satellite drag. The response is dependent on the state of the thermosphere prior to geomagnetic activity. The recent extreme solar minimum of 2008 results in the preconditioned thermosphere to be a cold and contracted multiconstituent gas. The associated reduced constituent scale heights led to more concentrated transitions of heavy species to light species with altitude. This dissertation focuses on investigating the effects of composition on the thermosphere mass density change during geomagnetic activity, with particular emphasis on conditions of the recent extreme solar minimum. A study of the mass density response to geomagnetic activity demonstrated complex behavior in the region near the oxygen to helium transition near 450 km. This study was expanded to explore the altitude variation of mass density response throughout the thermosphere and identified the helium / oxygen transition to have the greatest impact. Further analysis related the mass density peturbation with changes in the density scale height. It was found that the molecular weight scale height perturbation near the helium / oxygen transition region contributed significantly to the mass density response. The significant role of helium on mass density response warranted the extraction of helium number densities from the CHAMP and GRACE satellite measurements. A comparison between the derived helium concentration from satellite and the NRL-MSISE00 estimate indicated more helium was present in the winter polar regions than represented by MSIS. Physical models of the thermosphere in general do not include helium. A helium module was implemented into the Thermosphere Ionosphere Electrodynamics General Circulation Model (TIEGCM). A comparison between TIEGCM modeling with helium and without helium illustrated the need to include helium in order to reproduce the observations. The mechanism for the winter helium bulge formation was also revisited in this work. It was found that vertical advection of helium dominates its formation while hemispheric horizontal transport, once considered the primary process, is of secondary importance