MHD Wave (Strong magnetic field)
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.