Time sequences of the propagation of a wave launched at position (ii) of Fig. 1a. Time increases from top to bottom. Left column: perturbation of the absolute velocity, δ|v|. Right column: perturbation of the absolute magnetic field, δ|B|. The green contours (thin white in the print edition) display the equipartition level, c_s = c_A. The white curves indicate the position of a single wave front of the fast magnetic mode and its evolution in time. In the last row, the fast magnetic mode (in the region where c_A/c_s > 1) has already been refracted and wave fronts of constant phase cannot be clearly identified. δ|v|(t) is available as an mpeg animation in the online edition of the article.