Atmospheric tides significantly influence the dynamics of Mars' upper atmosphere. The impact of tides on the mean state of the present-day Martian atmosphere is especially large at high altitudes.
Certain tides can propagate away from the region of generation in the lower atmosphere and reach the upper atmosphere, where they can achieve significant amplitudes. Such vertically propagating tides constitute one of the primary mechanisms by which energy and momentum are transferred between atmospheric layers. Much of the initial evidence of tides reaching the upper atmosphere came from the Mars Global Surveyor mission (MGS). The MGS aerobraking densities revealed large-amplitude large-scale wavenumber-2 signature attributed to a class of tides known as nonmigrating tides. Recent observations from the Mars Atmosphere and Volatile Evolution mission (MAVEN) suggest that tides producing wavenumber-2 and wavenumber-3 structures are strongest in the upper atmosphere in a fixed local time reference frame. However, the energy carried by these tides and the region of deposition has not been well characterized. Moreover, it has been challenging to obtain a global understanding of the behavior of tides due to observations being limited in altitude combined with sparse geographical coverage.
Over the recent years, multiple missions have been active simultaneously, presenting an excellent opportunity to understand the nature and behavior of vertically propagating tides from an observational lens. This dissertation aims to infer the vertical propagation characteristics of tides by combining the relative strengths of in situ and remotely sensed data from multiple instruments on different spacecrafts over a broad range of altitudes. Estimates of tidal amplitudes for five cases around the equator are presented. Hemispherical differences in the dominant wavenumber are reported in the middle atmosphere. It is seen that the wavenumber structure in the upper atmosphere reflects that seen in the lower atmosphere about half the time. Of note is that most of the energy carried by the wave is dissipated by ~90 km. This analysis is also extended to high latitudes, where wave signatures are identified in the upper atmosphere using solar occultation observations for the first time. The eastward propagating non-migrating tides are shown to dominate the tidal spectrum. A key finding is that the relative importance of the tides with different periods is more significant at high latitudes, leading to a change in the observed wavenumber structure with local time. Comparison to physics-based models reveal that the model performs generally better at low latitudes than high latitudes. / Doctor of Philosophy / Various waves exist in nature, some visible like ocean tides, others unseen like sound waves, but their effects are undoubtedly perceptible. Often when we think of waves, we envision those that ripple across the ocean, but the atmosphere also hosts a multitude of waves, driving a large part of our weather systems. If you consider the atmosphere a fluid, it carries waves of different sizes.
One such category of waves, on a scale comparable to the planet's size, is called atmospheric tides.
These atmospheric tides are classified into 'migrating tides' and 'non-migrating tides'. 'Nonmigrating tides' are generated near the surface. Some of these tides can propagate upward, reaching what is referred to as the 'upper atmosphere'. As they ascend, these tides grow in size, similar to how an ocean wave lifts a boat higher as the wave itself grows larger. The tides that reach the upper atmosphere can cause considerable displacement of atoms and molecules. These tides are particularly large on Mars, presenting a challenge for spacecraft that rely on precise knowledge of the total amount of molecules in the upper atmosphere for slowing down the spacecraft.
This study aims to understand the nature of these tides as they propagate into the upper atmosphere and how they evolve as they pass through different regions of the Martian atmosphere. To do this, combining observations from multiple spacecraft is necessary, as a single spacecraft's observations are insufficient for probing these tides. One notable finding is that the tides lose most of their energy by the time they reach an altitude of 90 km, but they are still noticeable in the upper atmosphere. Previous work has relied on 'snapshots' in time to identify the strongest wave. This approach may work well near the equator, but this study reveals that closer to the poles, the strongest wave can change due to the presence of tides with different periods (24 hr, 12 hr, and so on).
| Identifer | oai:union.ndltd.org:VTETD/oai:vtechworks.lib.vt.edu:10919/116293 |
| Date | 18 September 2023 |
| Creators | Kumar, Aishwarya |
| Contributors | Aerospace and Ocean Engineering, England, Scott Leslie, Earle, Gregory D., Bailey, Scott M., Ross, Shane D., Thurairajah, Brentha |
| Publisher | Virginia Tech |
| Source Sets | Virginia Tech Theses and Dissertation |
| Language | English |
| Detected Language | English |
| Type | Dissertation |
| Format | ETD, application/pdf |
| Rights | In Copyright, http://rightsstatements.org/vocab/InC/1.0/ |
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