Editing Magnetohydrodynamics (section)

=== Applicability of ideal MHD to plasmas ===

Ideal MHD is only strictly applicable when:

# The plasma is strongly collisional, so that the time scale of collisions is shorter than the other characteristic times in the system, and the particle distributions are therefore close to [[Maxwell–Boltzmann distribution|Maxwellian]].
# The resistivity due to these collisions is small. In particular, the typical magnetic diffusion times over any scale length present in the system must be longer than any time scale of interest.
# Interest in length scales much longer than the ion [[Plasma parameters#Lengths|skin depth]] and [[Larmor radius]] perpendicular to the field, long enough along the field to ignore [[Landau damping]], and time scales much longer than the ion gyration time (system is smooth and slowly evolving).