Cherenkov radiation

Cherenkov radiation is electromagnetic radiation (EMR) emitted when a charged particle, e.g., electron, passes through a dielectric medium at a speed faster than the speed of light within that medium. The electromagnetic disturbance at the point where the particle travels moves faster than light and causes EMR in an expanding cone shape (Cherenkov cone), with an emphasis on shorter wavelengths. A well-known example is the blue glow in pictures of cooling water pools within nuclear-reactor facilities.

Spaceborne gamma rays can trigger showers of particles (such as electron-positron pairs) traveling at high speeds in the atmosphere, in turn, generating Cherenkov radiation in the air that can be detected by Earth-bound detectors, enabling them to indirectly detect the gamma rays (Cherenkov detectors).