Finite-Difference Time-Domain Modeling of Infrasound Propagation in a Realistic Atmosphere

Open Access
Author:
De Larquier, Sebastien
Graduate Program:
Electrical Engineering
Degree:
Master of Science
Document Type:
Master Thesis
Date of Defense:
April 02, 2010
Committee Members:
  • Victor P Pasko, Thesis Advisor
Keywords:
  • auroras
  • infrasound
  • FDTD
  • sprites
Abstract:
Atmospheric infrasonic waves are acoustic waves with frequencies ranging from 0.02 to 10 Hz, slightly higher than the acoustic cut-off frequency (~0.032 Hz), but lower than the audible frequencies (typically 20 Hz-15 kHz). A number of natural events have been identified as generating atmospheric infrasound, such as volcanoes, tornadoes, avalanches, earthquakes, ocean surfaces, lightning, auroral activity, and more recently Transient Luminous Events in the middle atmosphere termed sprites. The importance of infrasound studies has also been emphasized in the past ten years from the Comprehensive Nuclear-Test-Ban Treaty verification perspective. A proper understanding of infrasound propagation in the atmosphere is required for identification and classification of different infrasonic waves and their sources. In this thesis, one-dimensional (1-D) and two-dimensional (2-D) finite-difference time-domain (FDTD) models of infrasound propagation in a realistic atmosphere have been developed. The models are based on linearized equations of acoustics employing the realistic atmospheric structure and infrasound absorption algorithms advanced by <i>Sutherland and Bass</i> [2004]. The absorption is implemented using a recent decomposition technique introduced by <i>de Groot-Hedlin</i> [2008]. The FDTD model is used to provide a quantitative interpretation of the recently reported infrasound signatures from pulsating aurora. The pressure perturbations observed on the ground are analyzed as a function of energy flux of precipitating auroral electrons and geometry and altitude localization of the source. The results indicate that fluxes on the order of 50 erg/cm2/s are needed to explain pressure wave magnitudes of 0.05 Pa observed on the ground. This energy is unlikely to be provided exclusively by precipitating electrons, and Joule heating associated with the electrojet modulated by the pulsating aurora may be responsible for part of the deposited energy. Following recent experimental results on infrasound from Transient Luminous Events (TLEs) in the upper atmosphere termed sprited, the FDTD model is used to provide qualitative explanations of close range ground observations: results suggest that the vertical extent of the sprites combined with the altitude dependency of the transverse extent of filamentary structures in sprites are responsible for the inverted chirp signal observed on the ground. Using HARPA ray-tracing simulations, a mechanism explaining long range observations is proposed based on initial suggestions by <i>Farges et al.</i> [2005]. Efforts to provide FDTD results on long range observations are in progress.