SEASONAL-VARIATIONS OF THE THERMAL STRUCTURE OF THE MESOPAUSE REGION AT URBANA, IL (40-DEGREES-N, 88-DEGREES-W) AND FT-COLLINS, CO (41-DEGREES-N, 105-DEGREES-W)

Beginning in 1991, Na lidars were used to make routine measurements of the temperature structure between 80 and 105 km altitude during 65 ni ghts at Urbana, IL and 116 nights at Ft. Collins, CO. More than 13,000 temperature profiles representing more than 1100 h of observations we re used to characterize the seasonal temperature variations in the upp er mesosphere and lower thermosphere. The seasonal behavior is quite s imilar at the two sites. The mesopause altitude is near 100 km in wint er during Nov-Feb and near 86 km in summer during May-Jul. The mesopau se temperature varies from 190 K in mid-winter to 175 K in mid-summer. The transitions between winter and summer mesopause structures occur rapidly in Mar-Apr and Aug-Sep. The temperature differences between th e two sites are less than +/-15 K and appear to be the result of atmos pheric planetary, tidal, and gravity wave perturbations.