MHD Wave (Strong magnetic field)

Wave propagation

Time variation of gas pressure. Strong magnetic field transports wave energy preferentially along the magnetic field.

In this case wave propagation does not occur isotropically. In the direction perpendicular to magnetic field, there is a fast mode wave which propagates at the fastest speed (v_f = sqrt(cs^2 + va^2)). On the other hand, there are two kinds of waves propagating along the magnetic field, one of which is a slow mode wave (sound wave) and the other is a tube mode wave. The propagation speeds of these waves are smaller than the fast mode wave although most of the initially input energy is transported along the magnetic field.

The next viewgraph shows the characteristics of fast and slow mode waves. The gas pressure and magnetic pressure are in phase for the fast mode wave, however they are out of phase for the slow mode wave.