An undular bore is an atmospheric phenomenon characterized by waves traveling in a fixed pattern behind a wave- front. Usually, this phenomenon occurs in the troposphere -- the lowest part of the atmosphere extending up 6 miles.
In 1993, Michael Taylor of Utah State University used an airglow imager to snap images of an undular bore at the US Air Force Maui Optical Station in Haleakala, Hawaii. After further inspection of the images several years later, researchers at the US Air Force Research Laboratory at Hanscom Air Force Base, Mass., have presented a strong case that these bores occurred about 50 miles above Earth's surface and far above the troposphere.