respectively, to form a closed set. The displacement current is neglected in Equation (7.7) on the reasonable assumption that MHD motions are slow compared to the velocity of light. Equation (7.8) guarantees that , provided that this relation is presumed to hold initially. Furthermore, the assumption of quasi-neutrality renders the Poisson-Maxwell equation, , redundant.

Equations (7.2) and (7.7) can be combined to give the MHD equation of motion:

(7.9) |

where

(7.10) |

Suppose that the magnetic field is approximately uniform, and directed along the -axis. In this case, the previous equation of motion reduces to

(7.11) |

where

(7.12) |

It can be seen that the magnetic field increases the plasma pressure, by an amount , in directions perpendicular to the magnetic field, and decreases the plasma pressure, by the same amount, in the parallel direction. Thus, the magnetic field gives rise to a