Thermospheric zonal temperature gradients observed at low latitudes

Fabry-Perot interferometer (FPI) measurements of thermospheric temperatures from the Doppler widths of the OI 630 nm nightglow emission line have been carried out at Cachoeira Paulista (23◦ S, 45◦ W, 16◦ S dip latitude), Brazil. The east-west components of the thermospheric temperatures obtained on 73 nights during the period from 1988 to 1992, primarily under quiet geomagnetic conditions, were analyzed and are presented in this paper. It was observed that on 67% of these nights, the temperatures in both the east and west sectors presented similar values and nocturnal variations. However, during 33% of the nights, the observed temperatures in the west sector were usually higher than those observed in the east sector, with zonal temperature gradients in the range of 100 K to 600 K, over about an 800 km horizontal distance. Also, in some cases, the observed temperatures in the east and west sectors show different nocturnal variations. One of the possible sources considered for the observed zonal temperature gradients is the influence of gravity wave dissipation effects due to waves that propagate from lower altitudes to thermospheric heights. The observed zonal temperature gradients could also be produced by orographic gravity waves originated away, over the Andes Cordillera in the Pacific Sector, or by dissipation of orographic gravity waves generated over the Mantiqueira Mountains in the Atlantic sector by tropospheric disturbances (fronts and/or subtropical jet streams).