ACCELERATION, HEATING, AND COMPOSITIONAL MIXING OF THE THERMOSPHERE DUE TO UPWARD PROPAGATING TIDES

The National Center for Atmospheric Research thermosphere-ionosphere general circulation model (TIGCM) is utilized to evaluate the role of upward propagating diurnal and semidiurnal tides in determining the zonal mean states of the thermosphere and ionosphere above 100 km. Differences between various zonal mean fields from TIGCM simulations are examined with and without tidal forcing at the lower boundary. The following effects are observed: (1) A westward jet of the order of 10-30 m s-1 in the equatorial lower thermosphere and weaker eastward flanking mid-latitude jets are induced by eddy and molecular dissipation of the tidal motions. (2) Salient characteristics of the equatorial jet are independent of tidal phase. (3) Increases in the zonal mean N2 and O2 number densities of the order of 10-30% we produced above 110 km and between +/- 30-degrees latitude, mostly by virtue of the net heating due to tidal dissipation; these are accompanied by electron density depletions of the order of 10-15% above about 160 km due to the enhanced chemical loss. (4) A low-latitude (less-than-or-equal-to 30-degrees) depletion (approximately 30-50%) of atomic oxygen occurs due to the increase in effective recombination rate within a tidally displaced air parcel. The latter may explain the low-latitude depression in the OI (5577 angstrom) airglow intensity from satellite-based observations. In addition, variations in the zonal mean densities of NO, N(4S), and N(2D) of the order of 30-60% are induced by reactions with the tidally modified distributions of O, O2, and temperature. The occurrence frequency of these effects is influenced by the amplitude of diurnal propagating tide assumed at the lower boundary of the TIGCM, which according to radar observations is exceeded about 20-40% of the time, depending on season.