Optical Observations of Meteors Generating Infrasound - II: Weak Shock Theory and Validation

Abstract

We have recorded a dataset of 24 centimeter-sized meteoroids detected simultaneously by video and infrasound to critically examine the ReVelle [1974] weak shock meteor infrasound model. We find that the effect of gravity wave perturbations to the wind field and updated absorption coefficients in the linear regime on the initial value of the blast radius (R0), which is the strongly non-linear zone of shock propagation near the body and corresponds to energy deposition per path length, is relatively small. Using optical photometry for ground-truth for energy deposition, we find that the ReVelle model accurately predicts blast radii from infrasound periods (τ), but systematically under-predicts R0 using pressure amplitude. If the weak shock to linear propagation distortion distance is adjusted as part of the modelling process we are able to self-consistently fit a single blast radius value for amplitude and period. In this case, the distortion distance is always much less (usually just a few percent) than the value of 10 percent assumed in the ReVelle model. Our study shows that fragmentation is an important process even for centimeter sized meteoroids, implying that R0, while a good measure of energy deposition by the meteoroid, is not a reliable means of obtaining the meteoroid mass. We derived an empirical period-blast radius relation appropriate to cm sized meteoroids. Our observations suggest that meteors having blast radii as small as 1m are detectable infrasonically at the ground, an order of magnitude smaller than previously considered.