Low Frequency Plasma Lagrangian

A formulattion of the plasma Lagrangian for low frequency electromagnetic perturbations is discussed, The analysis is based on a small Larmor radius expansion of the Vlasov equation in which the perpendicular magnetohydrodynamic (MHD) response is separated from the intrinsic parallel particle response. The Euler-Lagrange equations reproduce the linearized MHD and drift-kinetic equations. Hybrid ``fluid-kinetic'' equations are readily derived, with inclusion of kinetic and finite Larmor radius effects. Gauge invariance is preserved, and any representation of the perturbed fields can be accommodated.