A baryon sweep in progress.
A baryon sweep is a procedure used to eliminate baryon particles from a starship. The baryons accumulate as a natural side-effect of warp travel and generally need to be removed every five years to keep harmful radiation from building up.
The sweep can be accomplished at specialized space stations such as the Remmler Array. The sweep is green in color and resembles a forcefield, capable of passing through solid matter. The strength of the sweep can be adjusted and made more effective for a ship which has made heavy use of its warp drive. The sweep is deadly to organic matter, and all lifeforms aboard a ship are evacuated to a safe location before the sweep is initiated. Additionally, the sweep can be damaging to certain areas such as the bridge and computer core, so field diverters can be installed in order to deflect the sweep and protect the sensitive equipment. Once the diverters are installed and the crew evacuated, an auto shutdown sequence is initiated, the ship is powered down, and the sweep begins. The sweep takes several hours to complete and is generally conducted from the rear of the ship to the front.
The USS Enterprise-D docked at the Remmler Array for its first sweep in 2369. It needed a stronger than usual sweep because it had spent more time at warp in five years than most ships did in ten. (TNG: "Starship Mine")