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[1]. It is extremely long-ranged, reaching across light-minutes.
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[2].
