Large decreases in ionospheric total electron content as a result of thermospheric composition changes during geomagnetic storms

The geomagnetic storms of April 17-21,2002 and May 29-30,2003 caused large decreases in the O/N2 column density ratio in the thermosphere. For these storms, ON2 column density decreases of greater than 50% were observed to extend to mid-to-low latitudes with the FUV sensitive Earth Camera of the Visible Imaging System (VIS) on the Polar spacecraft. Simultaneously in these same regions, the ground-based GPS network observed approximately 80% reductions in the Total Electron Content (TEC) of the ionosphere. The reduction in the Om2 column density ratio is due mainly to increases in the molecular species that have welled-up into the thermosphere from the lower levels of the atmosphere due to auroral heating. 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 transition boundaries between high and low regions of O/N2 as well as TEC can be tracked in the images and the thermospheric winds can be determined from the motion of the boundaries. The motion of these boundaries during the development of the geomagnetic storm will be discussed.