On the Structure and Dynamics of the Martian Middle Atmosphere.

The martian middle atmosphere (∼50-130 km altitude) is notoriously understudied, yet important for several reasons. It passes upward propagating planetary waves, gravity waves, and thermal tides from the lower atmosphere to the upper atmosphere. It also communicates the seasonal and dust-heating driven expansion and contraction of the lower atmosphere to the upper atmosphere. The middle atmosphere itself can be affected by and affect these dynamical processes. Characterization of this coupling region is therefore necessary if we are to fully understand the lower and upper atmospheres. It is also important for ensuring the safety of spacecraft that pass through this region. In part one of this thesis we characterize the nightside density and thermal structure in the 80-130 km altitude region as observed by stellar occultations. The nightside mesopause ranges from warm and high (115 K at 118 km) over the tropics during southern summer to cool and low (105 K at 103 km) over middle-southern winter latitudes. Using a general circulation model we find that the temperature and density of the 80-130 km region is significantly affected by even short-term increases in loweratmospheric dust loading. In part two, we quantitatively characterize dynamical polar warming (PW) over three martian years. Unexpectedly we find that during a typical dust year the magnitude of PW is larger during the Ls = 180◦ equinox than during northern winter, and it is comparable during southern winter and northern winter. Moreover, during equinoxes, the magnitude of PW is larger in the southern hemisphere than in the northern hemisphere. Results from this work provide quantitative constraints for model calculations of middle-atmosphere temperatures and winds. We close with part three, in which we use a general circulation model to investigate the role that the forcings generated by dust heating and by gravity wave momentum deposition play in producing the observed PW trends from part two. We show that while the effects of dust heating upon the mean meridional circulation contribute significantly to PW, they are insufficient for producing its precise magnitude, vertical distribution, and seasonality. Gravity wave momentum deposition shows promise for improving modeled PW.PhDAtmospheric and Space ScienceUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/93828/1/tmcdunn_1.pd