Note 1 to entry: The direction of motion of particles can be either parallel (longitudinal waves) or orthogonal (transverse waves) to the direction of wave propagation given by the wave vector k.

The direction of propagation can be either parallel or orthogonal to the external magnetic flux density B.

Note 2 to entry: There are three types of magnetohydrodynamic wave:

1. sound waves, which are longitudinal, and for which k is parallel to B;
2. magnetosonic waves, which are longitudinal, and for which k is parallel to B;
3. Alfvén waves, which are transverse, and for which k is parallel to B.