LARGE ENHANCEMENTS IN THE O N-2 RATIO IN THE EVENING SECTOR OF THE WINTER HEMISPHERE DURING GEOMAGNETIC STORMS/

In this paper, we have looked for enhancements of the O/N-2 ratio in d ata measured by the Dynamics Explorer 2 (DE 2) satellite in the middle latitudes of the winter hemisphere, based on a prediction that was ma de by the National Center for Atmospheric Research thermosphere/tonosp here general circulation model (NCAR-TIGCM) that such increases occur. The NCAR-TIGCM predicts that these enhancements should be seen throug hout the low latitude region and in many middle latitude locations, bu t that the enhancements in O/N-2 are particularly strong in the middle -latitude, evening-to-midnight sector of the winter hemisphere. When t his prediction was used to look for these effects in DE 2 NACS (neutra l atmosphere composition spectrometer) data, large enhancements in the O/N-2 ratio (similar to 50 to 90%) were seen. These enhancements were observed during the main phase of a storm that occurred on November 2 4, 1982, and were seen in the same region of the winter hemisphere pre dicted by the NCAR-TIGCM. They are partially the result of the depleti on of N-2 and, as electron loss is dependent on dissociative recombina tion at F-2 altitudes, they have implications for electron densities i n this area. Parcel trajectories, which have been followed through the NCAR-TIGCM history file for this event, show that large O/N-2 enhance ments occur in this limited region in the winter hemisphere for two re asons. First, these parcels of air are decelerated by the antisunward edge of the ion convection pattern; individual parcels converge and su bsidence occurs. Thus molecular-nitrogen-poor air is brought from high er to lower heights. Because neutral parcels that are found a little p oleward of the equatorial edge of the eveningside convection pattern a re swept inward toward the center of the auroral oval, the enhancement s occur only in a very limited range of latitudes. Second, nitrogen-po or air is transported from regions close to the magnetic pole in the w inter hemisphere. During geomagnetic storms, enhanced meridional winds are driven by the increased pressure-gradient force that is associate d with intensified Joule heating in the auroral oval. These pressure-d riven winds decrease rapidly on the dayside beyond the auroral oval wh ere the parcels originate, limiting the region into which the parcels can be transported. Thus these two processes drive values of O/N-2 in a limited region of the winter hemisphere, and reinforce only in the e vening sector, causing large changes in this region.