GEOMAGNETIC STORM EFFECTS IN THE LOW-LATITUDE TO MIDDLE-LATITUDE UPPER THERMOSPHERE

In this paper, we use data from the Dynamics Explorer 2 (DE 2) satelli te and a theoretical simulation made by using the National Center for Atmospheric Research thermosphere/ionosphere general circulation model (NCAR-TIGCM) to study storm-induced changes in the structure of the u pper thermosphere in the low- to middle-latitude (20 degrees-40 degree s N) region of the winter hemisphere. Our principal results are as fol lows: (1) The winds associated with the diurnal tide weaken during geo magnetic storms, causing primarily zonally oriented changes in the eve ning sector, few changes in the middle of the afternoon, a combination of zonal and meridional changes in the late morning region, and mainl y meridional changes early in the morning. (2) Decreases in the magnit udes of the horizontal winds associated with the diurnal tide lead to a net downward tendency in the vertical winds blowing through a consta nt pressure surface. (3) Because of these changes in the vertical wind , there is an increase in compressional heating (or a decrease in cool ing through expansion), and thus temperatures in the low- to middle-la titudes of the winter hemisphere increase. (4) Densities of all neutra l species increase on a constant height surface, but the pattern of ch anges in the O/N-2 ratio is not well ordered on these surfaces. (5) Th e pattern of changes in the O/N-2 ratio is better ordered on constant pressure surfaces. The increases in this ratio on constant pressure su rfaces in the low- to middle-latitude, winter hemisphere are caused by a more downward tendency in the vertical winds that blow through the constant pressure surfaces. Nitrogen-poor air is then advected downwar d through the pressure surface, increasing the O/N-2 ratio. (6) The da ytime geographical distribution of the modeled increases in the O/N-2 ratio on a constant pressure surface in the low- to middle-latitudes o f the winter hemisphere correspond very closely with those of increase s in the modeled electron densities at the F-2 peak.