A sudden change and recovery in the magnetic environment around a repeating fast radio burst

Fast radio bursts (FrBs) are millisecond-duration radio transients from extragalactic sources. Some repeating FrBs exhibit variations in their Faraday rotation measure (rM) due to changes in their magneto-ionic environment. We report magneto-ionic variations of FrB 20220529, a repeating FrB from a disk galaxy at redshift 0.18. For the first 17 months of observations, the rM had a median of 17 radians per square meter (rad m⁻²) and a scatter of 101 rad m⁻². In December 2023, the rM jumped to 1977 ± 84 rad m⁻², then gradually returned to typical values within 2 weeks. This sudden rM variation indicates that a dense magnetized clump of plasma passed across the line of sight. We discuss potential explanations, including a coronal mass ejection from a companion star, high plasma turbulence, or binary orbital motion.