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. We show that the velocity renormalization adopted in this paper is equivalent to the oscillation-centre representation and is the velocity required so that the mean density obeys the continuity equation exactly. We also show that previous fluid theory results may be recovered by adopting a different velocity renormalization and by redefining the ponderomotive force.