Aeronomy of Mars: An observational study of the dynamic martian upper atmosphere

Abstract

The Martian upper atmosphere is highly dynamic over both short and long temporal and spatial scales. Our understanding of this region primarily stems from a wealth of data gathered both in-situ and remotely from spacecraft orbiting the Red Planet. In 2014 NASA's Mars Atmosphere and Volatile Evolution (MAVEN) spacecraft began orbiting Mars with the primary objective to probe and characterise the upper atmosphere using composition data. Data from the Neutral Gas and Ion Mass Spectrometer (NGIMS) on board MAVEN has been utilised throughout this thesis. Daily, monthly and seasonal density and temperature variations have been explored. An apparent day-night asymmetry is observed in temperature with the dayside typically 50-100 K warmer due to solar-EUV heating. Seasonal analysis has found densities to be significantly enhanced around perihelion compared to other times throughout a Martian year. Perturbations in density and temperature profiles have been interpreted as vertically propagating gravity waves. Diurnal and seasonal variations of gravity wave characteristics have been examined with enhanced activity on the nightside. Fully understanding these features is required if the complexities of the upper atmosphere are to be fully grasped. The Martian science community was fortunate to have a plethora of spacecraft at Mars during the June 2018 global dust storm. This thesis has examined the response of the upper atmosphere to such an important and rare event. A notable expansion of the atmosphere is observed whereby the upper atmosphere is raised by several kilometres, due to heating in the lower atmosphere. It is hoped that results can inform efforts made to model and predict the effects of a dust storm. For the first time, gravity waves at Mars during a global dust storm have been studied. Atmospheric perturbations are found to be significantly enhanced during the dust storm event. During 2017/2018, ESA's Trace Gas Orbiter (TGO) undertook its aerobraking phase to circularise itself into its science orbit. During this period, density data were able to be retrieved in the lower thermosphere from accelerometer measurements. The retrieval process was not part of this thesis; however, these data are used for the first time. Standalone analyses are performed with these data but are also combined with results from MAVEN owing to the two spacecraft sampling similar regions concurrently. This overlapping period is exploited, and densities are hydrostatically connected to understand the entire thermosphere structure. By combining wave data, it has been inferred that shorter wavelengths are saturated within the thermosphere, whereas larger wavelengths continue to grow with height.