Cherenkov exists in all media except vacuum. It is when electrons travel faster than speed of light in the media (in this case air). In vacuum they are limited

Cherenkov exists in all media except vacuum. It is when electrons travel faster than speed of light in the media (in this case air). In vacuum they are limited by the speed of light. For example: https://www.mpi-hd.mpg.de/hfm/CosmicRay/ChLight/Cherenkov.html#:~:text=Charged%20particles%20moving%20through%20the,the%20direction%20of%20the%20particle https://www.mpi-hd.mpg.de/hfm/CosmicRay/ChLight/Cherenkov.ht.... Also, hello! my name is Ryan & we are likely in the same field.