Bores fall into the category of "gravity waves"—so called because gravity acts as the restoring force essential to wave motion. Analogy: boats in water. When a boat goes tearing across a lake, water in front of the boat is pushed upward. Gravity pulls the water back down again and this sets up a wave. In this case, instead of water, rarefied air is the medium through which the wave propagates. The sudden boundary in the airglow layer is probably akin to a hydraulic jump. But what created the disturbance in the first place? (What is the 'boat'?) No one knows.
"There may be more updates in the coming days as scientists from NASA and the Chinese Academy of Science check data from satellites to learn more about this event," says Jeff Dai, who has been helping Xiao Shuai process and communicate his extraordinary images.