On cataloguing the possible waveforms for impulse propagation along ground to ground thermospheric paths

Abstract

Infrasound propagation in the thermosphere is characterized by very low mean density, a very steep mean temperature gradient, and tidal winds. The steep temperature gradient means that there is always a thermospheric duct and this always provides ground to ground propagation paths that pass through the thermosphere. These paths are modulated by the atmospheric tides, while the very low density in the thermosphere causes severe attenuation and non-linear distortion. For an impulsive signal initially in the form of an N-like wave, such as the signal produced by a large explosion, non-linear propagation effects cause the positive pressure phases to propagate more rapidly than the negative pressure phases, in turn causing the duration of the impulse to increase. This effect can be quite significant in the thermosphere and continues until a caustic is encountered transforming the N-wave into a U-wave. The severe attenuation causes the high frequency components of the signal to attenuate resulting in a long period and fairly clean waveform that propagates back to the ground. The combination of steep temperature gradient and atmospheric tides produce a canonical thermospheric propagation environment with a diurnal cycle. In this presentation, it is shown that non-linear ray theory (weak shock theory) can be used to catalogue the waveform shapes of ground returns from explosive signals traveling along thermospheric paths. Qualitative comparisons to observed thermospheric returns are made.