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Direct measurement of the acoustic wave field in the stratosphere

Low frequency acoustic waves (infrasound) are generated by a variety of natural and anthropogenic phenomena. Although infrasound propagates throughout the atmosphere, the vast majority of acoustic studies utilize sensors on or near the Earth's surface. This paper describes results from two infrasound arrays launched into the stratosphere, one in August 2014 and the other in September 2015. The observations presented here are the first stratospheric infrasound measurements reported in scientific literature in 50 years. Acoustic signals recorded on the balloon borne sensors were different than those recorded by nearby infrasound stations on the ground. The 0.2 Hz ocean microbarom was detected in the stratosphere, but was not observed on nearby ground stations. A series of narrow band signals were also observed in the stratosphere, some of which varied in frequency over tens of minutes. The source of these signals is unclear. Wind noise decreased with altitude during the ascent, becoming negligible above 20 km. It was absent when the balloon was neutrally buoyant, although it was pervasive on ground stations operating in the same region during the day. Spectral characteristics of stratospheric infrasound were similar between the two flights and also resembled the last experiment in the early 1960s, but spatiotemporal variations in signal strength and frequency were also observed. Future efforts should focus on characterizing infrasound sensor operation in extreme environments and increasing spatial and temporal frequency of acoustic measurements in the free atmosphere. Results from this study have implications for long range detection of events such as nuclear blasts, the quantification of acoustic energy that heats the upper atmosphere, and calibration of a proposed mission to place airborne acoustic sensors on Venus.