Action principle derivation of one-fluid models from two-fluid actions

Ideal MHD possesses a well-known action principle formulation when the theory is expressed in terms of Lagrangian (or material) variables [1]. Here we start with a general electromagnetic two-fluid action functional in Lagrangian variables and derive action principles for one-fluid models, including ideal MHD. [1] W.A. Newcomb, Nuclear Fusion: 1962 Suppl. Part 2, p. 451