Panels a–d: Running difference images of the streamer wave event at 03:00:00, 04:00:00, 05:00:00, and 06:00:00 (where 00:00:00 corresponds to the introduction of the perturbation). The green line in panel a shows how we performed the wavelength measurements (λ = 7.5 R_⊙). Panel e: Time–distance map along the slit, shown in panel b in Fig. 9. The green line in panel e indicates the half-period measurement (P/2 = 156 min). Panel f: Height–time plot, where the dots correspond to the streamer wave extrema at four times, indicated in panels a-d. The linear fit indicates the measured phase speed (v_ph = 491 km/s).