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 co nfined in latitude (64.6 degrees -67.2 degreesN) and local time (0321-0418) , but their distribution in longitude is fairly uniform. We used least squa res spectral analysis to characterize the zonal structure of the atmosphere and constructed longitude-height cross sections of both temperature and ge opotential. Zonal variations of temperature exceed 12 K near the surface bu t are much smaller (2-3 K) at higher altitudes. Zonal variations of geopote ntial are similar to 200 m throughout the vertical range of the measurement s. These patterns of temperature and geopotential appear to be stationary r elative to the surface with little day-to-day variation within the 7-sol sp an of the measurements. We relied heavily on Mars general circulation model s (GCMs) for guidance in understanding these data. Stationary planetary wav es are responsible for some aspects of the temperature and geopotential fie lds, particularly at pressures exceeding 100-200 Pa. On the basis of strong similarities between a GCM simulation and the observations, we conclude th at 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 g eopotential field provides evidence for the presence of the resonantly enha nced, diurnal, wave-1 Kelvin mode.