The magnetic diffusivity has SI units of m²/s and is defined as: η η = 1 μ μ 0 σ σ 0 , while in Gaussian units it can be defined as η η = c 2 4 π π σ σ 0 . In the above, μ μ 0 is the permeability of free space, c is the speed of light, and σ σ 0 is the electrical conductivity of the material in question. In case of a plasma, this is the conductivity due to Coulomb or neutral collisions: σ σ 0 = n e e 2 m e ν ν c , where