The relationship of thermospheric composition changes and ionospheric total electron content during geomagnetic storms.

Geomagnetic storms drive major changes in the composition of the thermosphere. The FUV sensitive Earth Camera of the Visible Imaging System (VIS) on the Polar spacecraft has observed large decreases of > 50% in the O/N₂ column density ratio in the thermosphere after typical geomagnetic storms. These O/N₂ ratio decreases are observed to reach low to mid-latitudes. The reduction in the O/N₂ column density ratio is due mainly to increases in the molecular species that have welled-up from the lower levels of the atmosphere due to auroral heating. The thermospheric composition changes are associated with simultaneous decreases in the charge density of the ionosphere as evidenced by decreases in the Total Electron Content (TEC) observed with the GPS network. The geomagnetic-storm driven increase in molecular densities at typical ionospheric heights rapidly charge exchange with the ambient ionized atoms and subsequently dissociatively recombine with the ionospheric electrons leading to a reduction in the total charge density. The relationship between thermospheric composition and TEC will be presented for the April 17-21, 2002 and May 29-30, 2003 geomagnetic storm events.