Čerenkov radiation
From Halopedia, the Halo wiki
Cherenkov radiation is a type of radiation emitted by plutonium, such as in HORNET mines, when transitioning from Slipspace to realspace. It is extremely powerful, reaching across light-minutes. This is what gives Slipspace entry and exit the unique blue coloration, and usually alerts in-system enemy ships to their arrival. Covenant ships apparently also give off Cherenkov radiation, though whether it is emitted by plutonium or some other material on their ships is unknown.
It also seems that either a material or the fusion core in the MJOLNIR armor produces Cherenkov radiation, with steps made with the Recon Armor to reduce the radiation emmited.
