PSE (Photon Sensitive Effect) is a mechanic that only appears in battle sectors. It is represented by a meter that appears on the minimap which shows the current level of the PSE. This will increase in level by defeating enemies in the battle sector and successful trials, but will also decrease at a slow yet steady rate over time.
As the level goes up, the appearance rate of rare enemies will increase, and when it reaches the maximum, a PSE Burst will occur. PSE levels are affected by weather conditions, such as thunderstorms. During thunderstorms, the PSE levels in all applicable PSE Burst regions will always be at a minimum of 4 levels. This effect will not subside after completing a PSE Climax until the thunderstorm is over.
During the PSE Burst, the PSE meter will slowly drain, representing the amount of time the PSE Burst is active for. Many enemies will appear at a special rainbow "E" marker during the PSE Burst. Once the PSE meter empties, a PSE Burst Climax will occur, and a boss with the Silver or Golden variant will appear.