Magnetohydrodynamic (MHD) waves
spaceweb@oulu.fi - last update: 13 June 1996, 0740 UT (RR)
Magnetohydrodynamic (MHD) plasma theory deals
with a compressible, conducting fluid immersed in a magnetic field. The
wave
modes derived using this theory are called magnetohydrodynamic or MHD waves.
In the general case, with the wave propagation in an arbitrary direction with
respect to external magnetic field B, three MHD wave modes
are found:
- Pure (or oblique) Alfven wave
- Slow MHD wave
- Fast MHD wave
(There will be a figure here showing how the different wave modes are related.)
These relate with the three different oblique
shock types in ideal MHD.
All these waves have constant phase velocities for all frequencies, and hence
there is no dispersion. At the limits when the angle a between the wave
propagation vector k and magnetic field B is either 0 (180) or
90 degrees, the very same wave modes are called as:
- Sound wave
- longitudinal wave
- propagates parellel to B
- phase velocity is the adiabatic sound velocity
- no associated E, j, or B
- found also in a compressible, nonconducting fluid
- Alfven wave
- transverse wave
- propagates also parellel to B
- phase velocity is the Alfven velocity
- associated B found
- known also as the shear Alfven wave or the slow Alfven wave, and sometimes
as the torsional Alfven wave
- Magnetosonic wave
- longitudinal wave
- propagates perpendicular to B
- associated B and E found
- known also as the compressional Alfven wave or the fast Alfven wave, and
sometimes as the magnetoacoustic wave
When the fluid is not perfectly conducting, but has a finite conductivity, or if
viscous effects are present, the MHD oscillations will be damped.
See also: