Magnetohydrodynamic equations for systems with large Larmor radius

A set of one-fluid, modified magnetohydrodynamic (MHD) equations is developed that describes magnetized plasmas for which the relevant scale lengths are intermediate between the electron and ion Larmor radii, and the relevant time scales are intermediate between the electron and ion cyclotron frequencies. It is shown that the momentum equation is the same as that of conventional MHD but that the evolution of the magnetic field is strongly affected. The implications of the modified MHD system on the ‘‘frozen-in’’ theorem, magnetosonic waves, Alfvén waves, and the interchange instability are examined.