Seasonal variation of nocturnal temperature and sodium density in the mesopause region observed by a resonance scatter lidar over Uji, Japan

摘要

Temperature and sodium (Na) density profiles in the mesosphere and lower thermosphere region were obtained by a Na temperature lidar observation at Uji, Japan (35°N, 136°E) for 136 nights (more than 1200 h) from October 2007 to January 2009. The seasonal and height variations of Na density and temperature in a Northern Hemisphere midlatitude Asian sector are reported for the first time and compared with results obtained in the United States over Colorado (CO) and New Mexico (NM), Monthly composite nocturnal variation indicated significant amplitude of semidiumal tide in winter and a different tidal mode in summer. An annual variation was dominant in the Na density seasonal variation, with a maximum in November and a minimum in May. An additional second peak was seen at high altitudes (～94 km) in June–July. This second peak was probably generated by the sporadic Na (Na_s) layer frequently observed over Japan. Temperature above 98 km showed an annual variation with a summer maximum. Temperature at 83-98 km showed semiannual variation with peaks in September and April. This temperature variation was similar to that in NM, at the same latitude (35°N), but different from that in CO at 40°N, suggesting latitudinal difference. The summer temperature at 87 km agreed well among the three sites, but the winter temperature at Uji was significantly lower than at the other sites. This difference in winter could be due to an interannual variation and/or a longitudinal dependency possibly caused by nonmigrating tides.