The mesopause oxygen rotational temperature imager, MORTI, was operated at Bear Lake Observatory (41.9¿ N, 111.4¿ W) during the period November, 1991 to May, 1993. Fluctuations of long period in both emission rate and temperature of the O2 Atmospheric (0--1) nightglow layer are evident that seem related to the diurnal and semidiurnal tides. The data show a dominant semidiurnal tidal mode in January, but a dominant diurnal component at the spring equinox. Other long-period fluctuations also appear for which a linear tidal explanation does not seem applicable. ¿ American Geophysical Union 1995.