Field line resonances
For planets with a good conductor boundary near the surface (e.g. the ionosphere), the shear mode reflects back along the ambient field, to the other hemisphere, reflects back into space, and so on. For the right frequencies, this forms field line resonances. These are often likened to resonances on strings (piano, violin, guitar). The resonant frequency (and harmonics) depend on the distance between the north and south ionospheres along the ambient field, the strength of the field and the plasma mass density along the wave path.