Oscillation-centre fluid theory of the ponderomotive force

A new fluid-theory derivation of the time-dependent ponderomotive force is presented in the oscillation-centre representation. It is shown that the velocity renormalization adopted is equivalent to the oscillation-centre representation and is the velocity required so that the mean density obeys the continuity equation exactly. It is also shown that previous fluid theory results may be recovered by adopting a different velocity renormalization and by redefining the ponderomotive force. (author)