|There is more information available on this subject at Čerenkov Radiation on the English Wikipedia.|
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 Čherenkov, the 1958 Nobel Prize winner who was the first to characterize it rigorously.
During transitions from Slipspace to normal space, or vice versa, the glow of Cherenkov radiation emitted by a ship's fissile materials, such as plutonium, can give its presence away to enemy forces. 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.
Cherenkov radiation only occurs in a medium such as air or water because the speed of light in the medium is slowed. Contrary to popular belief, this phenomena could occur in space as mentioned in the book Halo: Ghosts of Onyx, due to there being an extremely low density of particles, along with the container of the fissile material itself.