Leveraging infrasound for estimating the characteristics of shockwaves generated by large bolides

Natural and artificial impulsive sources in the atmosphere can generate infrasound, or very low frequency (f<20 Hz) acoustic waves. Infrasound can travel over long distances with minimal attenuation, and for that reason it is used in global monitoring efforts. Unlike other sensing modalities that might have geographic (e.g., inaccessible regions), time-of-day (e.g., optical), or other limitations, infrasound can be utilized continuously (day and night). Volcanoes, lightning, chemical explosions, re-entry vehicles, space debris, and bolides are among the sources producing infrasound. Among these, bolides present a particularly intriguing scientific challenge due to their varying parameters (e.g., velocity, entry angle, and physical properties). Theoretically, bolide infrasound signatures should carry information about the source, potentially also informing about the type (hypersonic or spherical) and altitude of the shock. To fully leverage infrasound in characterization of bolides and sources alike, it is important to have both the detailed event ground truth and accurate atmospheric specifications. However, atmospheric specifications might not always accurately portray the real conditions. For example, it is well-established that the dynamic changes in the atmosphere that occur on temporal scales of minutes to hours might lead to the degradation of information carried by the infrasonic wave from the source. Moreover, unexpected propagation paths might also exist, where signals could be detected despite not being anticipated to reach certain areas. Despite these issues, there are cases where infrasound can provide a more complete picture about possible propagation conditions as well as the origin of the shock (altitude and type). One such example is the 23 July 2008 bolide over Tajikistan. This event was detected at distant infrasound stations, between 1500 and 2100 km from the source. While propagation modeling using realistic atmospheric specifications indicated that the signal would readily arrive at one station, the opposite was true for the other station. The presence of the detectable signal where such is not expected is attributed to the acoustic energy being trapped in a leaky stratospheric AtmoSOFAR duct. This acoustic channel was previously theoretically predicted to exist but only recently validated through high-altitude balloon-borne infrasound experiments. The primary mode of shock production was a spherical blast generated by the main gross fragmentation episode at an altitude of 35 km. The utility of infrasound in characterization of this and similar events will be discussed.

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