Is there any chance something is deflecting the rotating particles onto the axis? Similar to the particle jets emitted at the poles of black holes and other large objects - I don't think that process is fully understood either.
Unless I misunderstand your meaning, I guess in a certain sense you could frame it in that way, where you have a rotational drift which provides energy to an axially-propagating wave. I'm not well-versed in black hole plasma physics. It is worth noting that anomalous cross-field transport is a problem that exists in lots of plasmas, including fusion plasmas like tokamaks and probably also in astrophysical plasmas.