Rotation and dispersion measure evolution of repeating fast radio bursts propagating through a magnetar flare ejecta

Rotation measure (RM) and dispersion measure (DM) are characteristic properties of fast radio bursts (FRBs) that contain important information of their source environment. The time evolution of RM and DM is more inclined to be ascribed to local plasma in the host galaxy rather than the intergalactic medium or free electrons in the Milky Way. Recently a sudden drastic RM change was reported for an active repeating FRB 20220529, implying that some kind of mass ejection happened near the source. In this work I suggest that magnetar flare ejecta could play this role and give rise to the significant RM change. I introduce a toy structured ejecta model and calculate the contribution to RM and DM by a typical flare event. I find that this model could reproduce the RM behaviour of FRB 20220529 well under reasonable parameters, and similar sudden change is expected as long as this source maintains its activity.