Longitudinal (left) and transverse (middle) velocities multiplied by and , respectively, in a region around the null point (a part of the simulation domain). The dashed closed curve is the contour where β = 1 and solid lines are magnetic field lines. Right: fluctuations of the total gas pressure, that is, the sum of the equilibrium and perturbed gas pressure, in the same region around the null point. Units are [dyn/cm²]. Time increases from bottom to top. For a better visualization, a movie of the propagation is available online.