Further information

The wave is launched below the equipartition level by the piston as a predominantly acoustic wave. Upon reaching the equipartition level, where the Alfven speed reaches the sound speed, $c_\mathrm{s}=c_\mathrm{A}$, the acoustic wave is partially transmitted, and changes from the fast to the slow branch of the magneto-acoustic waves. This slow mode is then guided along the magnetic field lines into the higher layers of the atmosphere . The part of the wave energy that is not transmitted, is converted into the fast magnetic mode. This fast mode emerges from the equipartition level and is then refracted due to the high gradient of the Alfven speed.

Further reading

Dissertation: Numerical experiments on the propagation of waves in a convectively unstable magneto-atmosphere: A new interpretation of the measured reduction of wave travel-times in magnetic regions of the Sun. (Figure 4.5)