Observations of thermal tides in the middle atmosphere of Mars by the SPICAM instrument

We use vertical profiles of Martian atmospheric density, pressure, and temperature from the Mars Express SPICAM UV spectrometer to study thermal tides in the poorly studied middle atmosphere region at 70-120 km. Here we show that nonmigrating tides cause zonal pressure variations of tens of percent and zonal temperature variations on the order of 10 K in these observations. Wave-2 and wave-3 components are dominant, consistent with previous work at lower and higher altitudes and with theoretical predictions. Normalized pressure amplitudes tend to increase with altitude for the cases and altitudes studied here. Phases of the pressure variations vary little with altitude, indicating long vertical wavelengths for the underlying tidal modes. We derive theoretical relationships between zonal variations in temperature and in pressure and find that they are generally satisfied. Failure of these relationships can be used to infer the presence of multiple tidal modes contributing to a single observed wave component. The wave-2 component is dominated by the diurnal Kelvin wave 1 (DK1) above about 80 km but contains multiple tidal modes below this altitude. In one unusual instance, 40 degrees S-30 degrees S, Ls = 150 degrees-180 degrees, and local time of 22-24 h, the usually strong wave-2 component is extremely weak. The wave-3 component is always dominated by a single tidal mode, which for tropical and extratropical latitudes we identify as the diurnal Kelvin wave 2 (DK2).