Ponderomotive Forces, Stokes Drift, and Momentum in Acoustic and Electromagnetic Waves

Abstract: We reveal universal connections between three important phenomena in classical wave physics: (i) the ponderomotive force acting on the medium particles in an oscillatory wavefield, (ii) the Stokes drift of free medium particles in a wave field, and (iii) the canonical wave momentum in a medium. We analyse these phenomena for (a) longitudinal sound waves in a gas or fluid and (b) transverse electromagnetic waves interacting with electrons in a plasma or metal. In both cases, assuming quasi-monochromatic yet arbitrarily inhomogeneous wavefields, the connections between the ponderomotive force, Stokes drift velocity, kinetic energy, and canonical wave momentum carried by the medium particles are given by the same simple expressions. This sheds light on the nature of these phenomena, their fine interplay, and can be useful for applications to dynamical transport phenomena in various types of waves.