Acoustic-Gravity Waves from Impulsive Sources in the Atmosphere

Abstract

A numerical model for acoustic-gravity waves (AGWs) has been developed incorporating wave periods from ∼3 h to 0.10 s that are typically produced during meteor-fireball (or bolide) entry into the atmosphere. With relatively minor extensions, this model works equally well for rockets and missiles and other impulsive, elevated atmospheric sources. This chapter represents an extension of work originally presented at the Meteoroids 2007 Conference in Barcelona, Spain, and that originally began long ago. The equations being solved (currently written explicitly in Cartesian coordinates) were developed self consistently to model conditions applicable to the smallest bolides detectable at ground level at close ranges for any source height as well as the largest, most deeply penetrating bolides, at ranges up to ∼one earth radius from the source. Source modeling expressed in terms of the size of the line source blast wave relaxation radius is allowed to vary reliably from ∼1 m to as large as ∼36 km. In this work, an entry modeling capability has also been used that links satellite and/or ground-based near-optical pass-band photometry of the bolide in the near-field to either linear or quasilinear far-field locations. The geometrical acoustics, wave normal limit as well as the full wave theory developed previously for kt class and larger nuclear explosions in steady state, range-independent, perfectly stratified model atmospheres with horizontal mean winds have been used. Geometrical theory alternatives approaches such as Gaussian beam tracing theory are also included as needed to accurately model the signals from the smallest bolides at the closest ranges. Atmospheric waves produced during bolide entry (assuming a Dirac Delta function or a Heaviside step function type source) include Lamb waves, weak shock waves at relatively close range, infrasonic acoustical waves, and ducted acoustical signals propagating in the stratospheric or thermospheric sound channels. The ducted waves were modeled initially using the predicted near-field, line source, wave amplitude time series, allowing for weakly nonlinear propagation effects, with AGW dispersion as a function of range, etc. For the ducted waves made use of exact, ideal waveguide mode theory solutions. These later solutions predict the ducted modes that are allowed for a specified atmospheric sound and horizontal wind speed profile. In this chapter, predictions for the small blast wave, close range limit as well for a much larger blast wave at much longer range are both illustrated. The latter limit is indicative of the relatively low rate of detections made by the International Monitoring System (IMS) global infrasound network, whereas the former limit is indicative of the numerous AGWs detected by the Southern Ontario Meteor Network (SOMN) operated by the Physics and Astronomy Department at the University of Western Ontario.