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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]

$$\eta = \frac{1}{\mu_0 \sigma_0},$$

while in Gaussian units it can be defined as

$${\displaystyle \eta ={\frac {c^{2}}{4\pi \sigma _{0}}}}.$$

In the above, $$\mu _{0}$$ is the permeability of free space, c is the speed of light, and $$\sigma _{0}$$ is the electrical conductivity of the material in question. In case of a plasma, this is the conductivity due to Coulomb or neutral collisions: $$\sigma_0=\frac{n_ee^2}{m_e\nu_c}$$ , where

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

Electrical resistivity and conductivity

References

W. Baumjohann and R. A. Treumann, Basic Space Plasma Physics, Imperial College Press, 1997.

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