Electric field and plasma flow: What drives what?

The MHD approximation connects the plasma bulk how velocity and the electri c field, but it does not say whether one of them can be considered as causi ng or producing the other, and if so, which one. This question is often vie wed as one having no unambiguous answer and possibly no physical meaning. H owever, a definite answer can be obtained by solving the basic equations wi th appropriate initial values, with the result that, for the commonly consi dered case where the Alfven speed is small compared to the speed of light, (1) a given plasma bulk flow produces an electric field, (2) a given electr ic field does not produce a plasma bulk flow. The general result can also b e derived as a simple consequence of conserving the total (plasma plus elec tromagnetic field) linear momentum.