Seeing a jet’s shock wave
The image above shows a supersonic jet passing in front of the sun, with the jet’s shockwave visible. A team of NASA researchers generated the images and other like it (here and here), using a modern version of a 150-year-old German photography technique called Schlieren imaging. A NASA webpage explained that, in the images on this page, shock waves appear darker because changes in the air density affect how much light is refracted. NASA wrote:
Shock waves are narrow regions of air where pressure, temperature, and density characteristics are drastically different than surrounding areas. Shock waves occur when objects move faster than the speed of sound, which is 1,236 kilometers (768 miles) per hour.
… Specialized image-processing techniques were required to produce the images. Researchers first collected a series of photographs on a speckled background pattern. They then used computer algorithms and image processing software to deduce the locations of the shock waves based on distortions of the background pattern—an approach called the background-oriented schlieren technique.
While either the land surface or the sun can serve as the background, using a celestial body is simpler and cheaper because the camera can be located on the ground rather than on a second airplane.
Bottom line: Two images of shock waves from supersonic jets, created by NASA researchers using a photography technique called Schlieren imaging.