SEASONAL DEPENDENCE OF MESOSPHERIC GRAVITY-WAVES (LESS-THAN-100KM) ATPEACH MOUNTAIN OBSERVATORY, MICHIGAN

We present results from a 14-month study of all-sky camera observation s of the Hydroxyl (OH) nightglow made at the Peach Mountain Observator y, Michigan (42.3 degrees N; 83.7 degrees W). Spatial variations in th e observed OH airglow images have been used to assess gravity-wave (GW ) occurrence frequency at similar to 85 km altitude as a function of s eason. A Strong seasonal dependence of mesospheric GW activity is obse rved, with peak activity in the summer months and much reduced activit y during the winter months. Gravity waves (as defined by observed cohe rent variations in relative OH brightnesses of >similar to 7.5) were f ound to be present on about 70% of the clear-sky nights during the sum mer months. During the spring, fall, and winter months, however, the o bserved GW occurrence frequency was very low (<10%). Most of the GWs w ere observed to propagate towards the eastward hemisphere. We suggest that the tropospherically-generated GWs are anisotropic (eastward) thu s passing through to the mesosphere only in the summer and being filte red out by the intervening neutral winds during other seasons. It is a lso possible that the GWs are able to reach higher altitudes without b reaking because of their smaller amplitudes at lower altitudes during the summer season relative to the winter season.