Date of Award

12-2016

Document Type

Thesis - Open Access

Degree Name

Master of Science in Engineering Physics

Department

Physical Sciences

Committee Chair

Dr. Jonathan B. Snively

First Committee Member

Dr. Michael P. Hickey

Second Committee Member

Dr. Jeremy A. Riousset

Third Committee Member

Dr. Christopher J. Heale

Abstract

Acoustic waves in the infrasonic frequency range, that is below 10 Hertz, have been observed to propagate to high altitudes in Earth's atmosphere. These waves have many sources, both natural and artificial, such as seismic events, convective storm systems, and nuclear explosions. Here, we seek to better understand the characteristics of atmospheric infrasound- below 0.1 Hz in particular- so as to improve the ability to detect their presence and sources. It is well-known that ambient attributes of an atmosphere, such as temperature, density, and composition, directly affect the propagation and growth of waves, and therefore it is likely that these dynamic phenomena are present (and may be detected) on other terrestrial planets with similar atmospheric structures.

Using a one-dimensional, nonlinear, compressible atmospheric acoustics model, this thesis seeks to investigate the propagation and dissipation of atmospheric acoustic waves in different terrestrial planetary atmospheres. The model, which includes gravity, molecular viscosity, and thermal conduction, has been developed using numerical solutions in Fortran, and is validated for the atmospheric conditions of Earth, Mars, and Venus. Empirical profiles for these planets are provided by the NASA Global Reference Atmospheric Model (GRAM) packages developed by Marshall Spaceflight Center. The terrestrial planets selected for investigation in this thesis exhibit similar atmospheric structures with very different temperatures, pressures, and compositions, which makes them ideal for a comparative study.

The model is used to determine the maximum achieved wave amplitude and propagation time to several altitudes of note as they vary with atmospheric conditions and wave parameters; sensitivity to these parameters on the three planets under investigation are determined. Furthermore, by establishing these sensitivities we may identify conditions that are favorable for detection of infrasound in the upper atmospheres of Earth, Mars, and Venus.

By performing large model run sweeps of parameters such as latitude and longitude, time of day, and solar activity, we have drawn correlations between the atmospheric profile of each planet and the maximum achieved amplitude of propagating infrasound. The variations of temperature and gas composition due to ambient conditions directly affect damping of waves by viscosity and thermal conduction, and thus affect the growth of infrasonic wave packets. Venusian waves were found to be the most sensitive to ambient conditions, while waves on Earth were found to be the lease sensitive. Results indicate that upward-propagating atmospheric acoustic waves are readily detectible from the middle and upper atmospheres of Earth and Venus, however those on Mars may only be detectible if they have energetic sources.

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