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

Abstract

[1] 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°S-30°S, Ls = 150°-180°, 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).

Longitudinal structures in Mars' upper atmosphere as observed by MAVEN/NGIMS

“…Comparing the density with temperature, their variation patterns are very similar but appear to be out of phase, specifically for wave 2 and wave 3 components. This indicates that the temperature variation is due to the adiabatic cooling/warming caused by tides as discussed by Withers et al [].…”

“…Withers et al [] have found that both wave 2 and wave 3 can dominate the longitudinal structures at 70–120 km altitudes, and the wave amplitudes change dramatically with altitude. Our observations are for higher altitudes, but similar features are seen.…”

“…This temperature variation has a phase difference from the density at a fixed altitude (shown in Figure ), and this figure (Figure ) shows the amplitude of the density wave changes with altitude. As explained by Withers et al [], these are consistent with the signatures of dissipating tidal waves in this altitude region.…”

“…Atmospheric tides, originating from the interaction of solar radiation with Mars' topography and the atmosphere, can create significant changes in the upper atmosphere density and dynamics [e.g., Keating et al, ; Forbes and Hagan , ; Forbes et al, ; Wilson, ; Bougher et al , ; Moudden and Forbes , , ; Withers et al , ; England et al, ]. Tides are planetary‐scale oscillations with periods that are harmonics of the solar day.…”

“…Due to Mars' topography, these variations are much larger than the thermospheric density perturbations on Earth peaking at values around only 5% [e.g., Oberheide and Forbes, ]. Observations of atmospheric tidal effects on Mars' upper atmosphere have also been made using techniques including atmospheric refractivity of radio waves [ Bougher et al, ; Cahoy et al , ], atmospheric absorption of stellar ultraviolet light [ Forget et al, ; Withers et al, ], and multichannel observations of infrared emissions [ Lee et al, ]. The impact of such waves on atmospheric composition is expected to be significant but has only been recently observed by Mars Atmosphere and Volatile Evolution Mission (MAVEN) [ Jakosky et al, ] as reported by England et al [].…”