Infrasound in the stratosphere captured with balloon-borne sensors

Infrasound sensors launched into the stratosphere on high altitude balloons experience zero wind noise, travel across regions inaccessible to ground stations, and access a unique geoacoustic wave field that has not been examined in half a century. Recent flights in the American Southwest have detected one to several events per hour, a capability that may be due to the lack of wind noise and the balloons' presence in a stratospheric wave guide. The provenance of these events is currently uncertain, but a known explosive source was detected at greater ranges compared to nearby ground stations. A 14 day circumnavigation of Antarctica revealed that high altitude sensors have lower noise floors than International Monitoring System (IMS) infrasound stations. The ocean microbarom was continuously recorded during this flight. The sensors flew directly over spatially extended microbarom source regions circulating along the Antarctic Circumpolar Current; these signals are also detected on IMS stations. Other signals that resemble far-field explosions were detected in the stratosphere, as well as many whose sources are unknown. We look forward to the development of a new branch of geoacoustics focusing on signals recorded in the free atmosphere at extreme range, above the open ocean, and over phenomena such as severe storms that preclude ground based observation.