Abstract
Magnetic fields may introduce structure (inhomogeneity) into an otherwise uniform medium and thus change the nature of wave propagation in that medium. As an example of such structuring, wave propagation in an isolated magnetic slab is considered. It is supposed that disturbances outside the slab are laterally non-propagating. The effect of gravity is ignored.
The field can support the propagation of both body and surface waves. The existence and nature of these waves depends upon the relative magnitudes of the sound speed c o and Alfvén speed ς A inside the slab, and the sound speed c e in the field-free environment.
In general terms the slow mode can always propagate, and does so both as a surface wave and as a body wave. On the other hand, the fast mode may propagate only in slabs that are not hotter than their surroundings (c e ≥ c 0), and then it is a body wave or a surface wave accordingly as c e is greater than or less than ς A. For example, if c e > c o > ς A then a fast body wave propagates with phase-speed between c e and c o, a slow body wave between ς A and c T = c o ς A/(c o 2 + ς A 2)1/2, and a slow surface wave with phase-speed below c T . There are no modes between c o and ς A. As a second illustration, if ς A > c e > c o, then in addition to the slow body and slow surface waves, as before, there is a fast surface wave with phase-speed between c o and c e. There is no fast body wave.
The special case of a slender field is also investigated and it is shown how the slender flux tube approximation relates to the more general results described above. In particular, the tube wave with phase-speed c T studied by Defouw (1976) and Roberts and Webb (1978) is shown to be a slow surface wave (sausage mode). Finally, we discuss briefly the generation of resonant modes in a slender slab.
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Roberts, B. Wave propagation in a magnetically structured atmosphere. Sol Phys 69, 39–56 (1981). https://doi.org/10.1007/BF00151254
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DOI: https://doi.org/10.1007/BF00151254