The AtmoSOFAR Channel: First Direct Observations of an Elevated Acoustic Duct

The Sound Fixing and Ranging (SOFAR) channel in the ocean allows for low frequency sound to travel thousands of kilometers, making it particularly useful for detecting underwater nuclear explosions. Suggestions that an elevated SOFAR-like channel should exist in the stratosphere date back over half a century and imply that sources within this region can be reliably sensed at vast distances. However, this theory has not been supported with evidence of direct observations from sound within this channel. Here we show that an infrasound sensor on a solar hot air balloon recorded the first infrasound detection of a ground truth airborne source while within this acoustic channel, which we refer to as the AtmoSOFAR channel. Our results support the existence of the AtmoSOFAR channel, demonstrate that acoustic signals can be recorded within it, and provide insight into the characteristics of recorded signals. Results also show a lack of detections on ground-based stations, highlighting the advantages of using balloon-borne infrasound sensors to detect impulsive sources at altitude. A layer exists in the ocean that allows sound to travel great distances. Something like this could exist in our atmosphere, too, but until now no one had verified this. Low frequency sound, known as infrasound, travels efficiently through the atmosphere. Specialized instruments, called infrasound sensors, are microphones that can record low frequency sound. We attached an infrasound sensor to a solar hot air balloon to make the first recording of a known object within the theorized atmospheric layer. Our results show that this layer exists, sound can be recorded within it, and analysis can give us more information about what generated that sound. The rocket launch was not well recorded by infrasound sensors on the ground, highlighting the advantages of our method for detecting objects that travel through this region of the atmosphere.

Automated summary

Researchers have recorded the first infrasound detection of a ground truth airborne source within the theorized atmospheric layer called the AtmoSOFAR channel, using an infrasound sensor on a solar hot air balloon. This study confirms the existence of the AtmoSOFAR channel and demonstrates the ability to record acoustic signals within it. The lack of detections on ground-based stations highlights the advantages of using balloon-borne infrasound sensors to detect objects in the atmosphere.