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

Robert Pratt, Boston University, USA
Paul Withers, Boston University, USA
Jeffrey Russo, Boston University, USA
Jean-Loup Bertaux, Service d’Aeronomie du CNRS, France
Franck Montmessin, Service d’Aeronomie du CNRS, France

Aerobraking accelerometer observations have shown that non-migrating thermal tides, such as the diurnal Kelvin wave, have large effects on the structure of the martian atmosphere above 120 km altitude. Here we extend studies of these tides downwards into the middle atmosphere using SPICAM stellar occultation measurements. These tides, which are created by interactions at the surface of Mars, are significant at altitudes as low as 80 km. As previously observed at 130-160 km, the phases of the dominant tidal modes vary little with changing altitude, indicating long vertical wavelengths. The characteristic vertical lengthscale of the rate of change of amplitude with altitude can be used to infer the importance of dissipative processes in the middle atmosphere. Unlike atmospheric measurements from many infra-red instruments, the SPICAM temperature and pressure profiles are referenced to an absolute altitude scale. Analysis of tidal amplitudes and phases in density/pressure data alone is limited in its ability to distinguish between the underlying tidal modes responsible for observed zonal structure. However, the direct comparison of tidal amplitudes and phases in both pressure and temperature data from SPICAM provides valuable tools for determining whether one or several tidal modes contribute to an observed component of the zonal structure.