Magnetic diffusivity

The magnetic diffusivity is a parameter in plasma physics which appears in the magnetic Reynolds number. It has SI units of m²/s and is defined as:
while in Gaussian units it can be defined as
In the above, formula_3 is the permeability of free space, formula_4 is the speed of light, and formula_5 is the electrical conductivity of the material in question. In case of a plasma, this is the conductivity due to Coulomb or neutral collisions: formula_6, where