# 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:[1]

${\displaystyle \eta ={\frac {1}{\mu _{0}\sigma _{0}}}}$,

where ${\displaystyle \mu _{0}}$ is the permeability of free space and ${\displaystyle \sigma _{0}}$ is the electrical conductivity of the material in question. In case of a plasma, this is the conductivity due to Coulomb or neutral collisions: ${\displaystyle \sigma _{0}={\frac {n_{e}e^{2}}{m_{e}\nu _{c}}}}$, where

• ${\displaystyle n_{e}}$ is the electron density.
• ${\displaystyle e}$ is the electron charge.
• ${\displaystyle m_{e}}$ is the electron mass.
• ${\displaystyle \nu _{c}}$ is the collision frequency.