Dr. Elizabeth A. Silber

DOCTORAL RESEARCH

My PhD dissertation project encompassed research in several distinct, yet interconnected topics pertaining to meteor infrasound. I studied infrasound generated by both large bolides and small (centimetre-sized) meteoroids. The former are detectable on a global scale, while the latter are detectable on a much smaller scale (several hunderd kilometers). Another significant aspect of my project was to in detail examine the weak shock model, which was originally developed by ReVelle (1974) to predict meteor infrasound signal amplitude and period at the receiver. Due to the lack of well constrained and statistically meaningful meteor events, the weak shock model has not been validated against observations until now. A more detailed description of my doctoral work and major conclusions are given in my thesis abstract.

PhD Thesis Abstract

During their passage through the atmosphere meteoroids produce a hypersonic shock which may be recorded at the ground in the form of infrasound. The first objective of this project was to use global infrasound measurements to estimate the influx of large (meter/decameter) objects to Earth and investigate which parameters of their ablation and disruption can be determined using infrasound records. A second objective was to evaluate and extend existing cylindrical line source blast theory for meteoroids by combining new observations with earlier analytical models, and validate these against centimetre-sized optical meteor observations.

The annual terrestrial influx of large meteoroids (kinetic energies above a threshold E) was found to be N=4.5E^(–0.6) where E is expressed in kilotons of TNT equivalent. This indicates that estimates of the influx derived from telescopic surveys of small asteroids near Earth are too low. Infrasound records from an event over Indonesia in 2009 were used to develop a technique to estimate the altitude of meteoroid terminal bursts and their energies. The burst altitude in this case was determined to be near 20 kilometers and the energy between 8 – 67 kilotons of TNT equivalent.

Using a network of optical cameras and an Infrasound Array in southern Ontario, Canada, 71 centimetre-sized meteoroids were optically detected and associated with infrasonic signals recorded at the ground. The shock source height and its uncertainty along the meteor trail from raytracing was determined including wind effects due to gravity waves perturbations, which were found to be significant for such short range (<300 km) infrasound propagation. Approximately 75% of signals were attributed to cylindrical line source geometry, while ray deviation angles greater than 117° were associated with spherical shocks. The ReVelle (1974) meteor infrasound model was found to be accurate when using infrasound period measurements, but systematically under-predicted blast radii when amplitude is used. The latter can be better modelled assuming the wave distortion distance is <6%, as opposed to the 10% adopted by ReVelle. Infrasonic masses found from ReVelle’s theory deviate from photometric estimates largely due to meteoroid fragmentation.

My dissertation can be found here.

Q & A

Q: How do meteoroids generate infrasound?

A: Meteoroids can generate shock waves in the form of a cylindrical line source or a spherical shock (point source).

Q: What is infrasound?

A: Infrasound is low frequency sound extending below the human hearing range of 20 Hz and down to the natural oscillation of the atmosphere.

Downloads for PhD Thesis

Other Research

Over the years, I have been involved in various infrasound studies (bolide events, the Windsor Hum, missile infrasound). I am also interested in other sources of infrasound (e.g. lightning, rockets, ocean waves).