Change in signal strength between the start and the end of the transit as a function of the observed deviation from the expected systemic velocity. The error bars denote 1σ uncertainties calculated via Gaussian error propagation. The grey vertical region indicates range (−24.452 ± 0.012km s⁻¹). We note that Sr and Sr⁺ are not included in this figure due to large uncertainties. We also note that Fe⁺ is an outlier (see Fig. 9). The hotter atmosphere rotating into view (see Fig. 1) causes most signal strengths to increase over the course of transit. However, further ionisation (Ca⁺, Ni, Ba⁺, and H) or dissociation (TiO) on the dayside can also cause some signals to decrease over time.