Light curves of the blue (left panel) and red (right panel) wings of the Lyman-α emission of K2-18 during the transit of planet b. Time = 0 corresponds to the transit center according to the ephemeris of Benneke et al. (2019). The vertical red lines represent the ingress and egress of K2-18 b. Visit A was affected by strong geocoronal contamination, and therefore the measured stellar fluxes are likely inaccurate.