Atmospheric acoustics

Andi Petculescu

Recent missions such as Cassini-Huygens, Mars Phoenix, and Venus Express have shown the growing interest in detecting and analyzing signatures of wave motion in planetary atmospheres. At UL Lafayette, we are combining data analysis, theoretical and computational modeling to predict the characteristics of acoustic and gravity wave propagation in terrestrial atmospheres. A comparative analysis of the atmospheric acoustics of Earth, Mars, Venus, and Titan can be found here. These studies are important in planetary science because they guide instrumentation development and data analysis. We have recently completed a study of the generation and propagation of thunder on Titan. The results, published here, show the optimum frequency bands in which future detectors should look. Limiting the working bandwidth helps save onboard power and optimize the measurement process.

Current projects:

• Infrasound absorption and dispersion in Earth's lower thermosphere (between 80 and 160 km). The premise of this project, whose preliminary results were published here, is that at thermospheric altitudes the molecular mean-free-path is large enough to require non-continuum fluid mechanics. Therefore, in our pilot study, we have developed a framework based on the Boltzmann transport equations whereby themal conduction and viscous stress are coupled. Furthermore, the lower thermosphere corresponds to the D and E ionospheric layers, for which reason, in the current project stage we include magnetohydrodynamics to treat the mixture of neutral and charged particles.
• Acoustic dispersion and absorption profiles in Venus' lower and middle atmospheres (0 to 100 km). Venus Express has shown evidence of distinct cold and warm layers in Venus' atmosphere, and has also probed the planet's polar vortex in detail. The measurements show a dynamic and yet mysterious environment. Acoustic measurements will help to better constrain the Venusian atmospheric dynamics, fluxes, as well as cloud composition. For instance, fast and robust active acoustic sensors based on molecular acoustics could help identify the mean molecular weight and the geometry of an unknown UV-absorbing molecule at around 50 km. A subject of current interest is the possibility of using long-lived aerial platforms (e.g. free-drifting balloons) to detect infrasound signals generated by venusquakes or volcanoes. The instrumented balloons will likely be deployed at an altitude of 50-60 km where they will be advected by the strong zonal winds. In order to constrain the expected strength of infrasonic arrivals in Venus's upper troposphere, we are developing a theoretical--computational model to predict the acoustic attenuation coefficient and speed of sound as a function of height throough the main cloud layer of Venus. The initial results are available here.