Cherenkov Radiation is radiation emitted when a charged particle, e.g., electron, passes through a Dielectric Medium at a speed faster than the speed of light in that medium. The Electromagnetic (EM) disturbance at the point where the particle travels moves faster than light and causes radiation in an expanding cone shape (Cherenkov Cone), with an emphasis on shorter Wavelengths. It is the cause of the blue glow in the water cooling a nuclear reactor.
Spaceborne Gamma Rays can trigger showers of particles (such as electron-positron pairs) traveling at high speeds in the Atmosphere, generating Cherenkov Radiation that provides a mechanism for Earth-bound Gamma Ray detectors.
Cherenkov Telescope Array (CTA)
Very-High-Energy Gamma Rays (VHEGR)