Zebra case from 21 June 2011. (A) Zebra-stripe frequencies as in Fig. 9C, but with the δf variation caused by the magnetosonic wave (amplitude δn/n = δB/B = 0.0015, wavelength of 1370 km, and the velocity 214 km s⁻¹). (B) δf. (C) Map of Δf_n + 1 − Δf_n, where Δf_n = f_n + 1 − f_n, and f_n is the frequency of the nth stripe numbered from the lowest frequency in the spectrum (part A). Asterisks mean maxima of these differences. Connecting dashed lines show the frequency drift.