Radio occultation measurements of forced atmospheric waves on Mars

Mars Global Surveyor performed a series of radio occultation experiments in December 1998, resulting in 36 profiles of the neutral atmosphere in late northern spring (L-s = 74.1 degrees -77.3 degrees), The measurements are confined in latitude (64.6 degrees -67.2 degreesN) and local time (0321-0418), but their distribution in longitude is fairly uniform. We used least squares spectral analysis to characterize the zonal structure of the atmosphere and constructed longitude-height cross sections of both temperature and geopotential. Zonal variations of temperature exceed 12 K near the surface but are much smaller (2-3 K) at higher altitudes. Zonal variations of geopotential are similar to 200 m throughout the vertical range of the measurements. These patterns of temperature and geopotential appear to be stationary relative to the surface with little day-to-day variation within the 7-sol span of the measurements. We relied heavily on Mars general circulation models (GCMs) for guidance in understanding these data. Stationary planetary waves are responsible for some aspects of the temperature and geopotential fields, particularly at pressures exceeding 100-200 Pa. On the basis of strong similarities between a GCM simulation and the observations, we conclude that the disturbance takes the form of a planetary wave train excited by Alba Patera. The data also include the signature of non-Sun-synchronous thermal tides, which produce a pattern that appears to be stationary when sampled at fixed local time. Comparison between a GCM simulation and the measured geopotential field provides evidence for the presence of the resonantly enhanced, diurnal, wave-1 Kelvin mode.