> ... We can consider two regimes which are categorized by the magnetic Reynolds number (Rm). ... In the simple case with low Rm then the field strongly influences the flow but the flow does not strongly influence the field. The high Rm case is more difficult as the field and flow are strongly coupled and Alfvens frozen flux theorem is applicable (which essentially says that the fluid flow is frozen to the magnetic field and vice versa).

Huh ... I'm curious, is there any intermediate regime, however small or poorly-understood it might be, where there is some substantial back-reaction of the flow onto the field but not enough to freeze the fluid flow to the magnetic field ... or is there essentially a hard phase transition between the two behaviors?

Cheers!