Polar Region Variability in the Lower Thermosphere of Mars From Odyssey and Reconnaissance Orbiter Aerobraking Measurements

Accelerometer measurements of lower-thermosphere densities during the aerobraking phases of Mars Odyssey (MO) and Mars Reconnaissance Orbiter (MRO) are analyzed to reveal new perspectives on the longitudinal, latitudinal, local time, and temporal variability of Mars atmosphere. The MO and MRO accelerometers sample the atmosphere similarly, making for interesting comparisons and insights. Periapsis measurements began near 70 degrees N (MO) and 70 degrees S (MRO) latitude, respectively, near winter solstices; slowly approached the poles at near-constant 1730-2000 hr local times (LT); underwent nearly 8-hr shifts in LT near the poles; and then descended in latitude to about 20 degrees N and 20 degrees S near 0330 hr LT. In contrast with most prior studies of Mars thermosphere, emphasis here is placed on the space-based k(s) = 1 (wave-1) and k(s) = 2 (wave-2) components of the longitudinal density structures in the polar regions (>= 70 degrees). Expressed in terms of percent with respect to the zonal mean, k(s) = 1 amplitudes are dominant with mission mean amplitudes of 22%(18.5%) for MO(MRO), and likely arise from the zonally symmetric diurnal tide (D0) and/or a stationary planetary wave with zonal wave number s = 1 (SPW1). Mission-mean k(s) = 2 amplitudes are 17.6%(13.6%) for MO(MRO) and are interpreted as arising from the zonally symmetric semidiurnal tide (S0) and the eastward propagating diurnal tide with s = -1 (DE1). Phases of k(s) = 1 and k(s) = 2 are characterized by zonal migrations with periods between 15 and 45 sols; these features are interpreted in terms of modulations of D0, S0, and DE1 by slow waves in the lower atmosphere that have been reported in the literature.