Čerenkov radiation
From Halopedia, the Halo wiki
Čerenkov radiation (also spelled Cerenkov or Cherenkov) is electromagnetic radiation emitted when a charged particle (such as a proton) passes through an insulator at a speed greater than the speed of light in that medium. The characteristic "blue glow" of nuclear reactors is due to Čerenkov radiation. It is named after Russian scientist Pavel Alekseyevich Čerenkov, the 1958 Nobel Prize winner who was the first to characterise it rigorously.[1]
During transitions from Slipspace to normal space, or vice versa, the glow of Cherenkov radiation emitted by a ships fissile materials, such as plutonium, can give its presence away to enemy forces.[2] It also seems that components of the MJOLNIR armor produce Cherenkov radiation, as the Mark VI MJOLNIR Powered Assault Armor/R variant's design specifically reduced the amount of radiation emitted for stealth purposes[3].
