The induction equation, one of the magnetohydrodynamic equations, is a partial differential equation that relates the magnetic field and velocity of an electrically conductive fluid such as a plasma. It can be derived from Maxwell's equations and Ohm's law, and plays a major role in plasma physics and astrophysics, especially in dynamo theory.
Maxwell's equations describing the Faraday's and Ampere's laws read
where the displacement current has been neglected as it usually has small effects in astrophysical applications as well as in most of laboratory plasmas. Here, , and are, respectively, electric and magnetic fields, and is the electric current. The electric field can be related to the current density using the Ohm's law, where is the velocity field, and is the electric conductivity of the fluid. Combining these three equations, eliminating and , yields the induction equation for an electrically resistive fluid:
Here, is the magnetic diffusivity (in the literature, the electrical resistivity, defined as , is often identified with the magnetic diffusivity).
If the fluid moves with a typical speed and a typical length scale , then
The ratio of these quantities, which is a dimensionless parameter, is called the magnetic Reynolds number:
Perfectly conducting limit
For a fluid with infinite electric conductivity, , the first term in the induction equation vanishes. This is equivalent to a very large magnetic Reynolds number. For example, it can be of order in a typical star. In this case, the fluid can be called a perfect or ideal fluid. So, the induction equation for an ideal conductive fluid such as most astrophysical plasmas is
For very small magnetic Reynolds numbers, the diffusive term overcomes the convective term. For example, in an electrically resistive fluid with large values of , the magnetic field is diffused away very fast, and the Alfvén's Theorem cannot be applied. This means magnetic energy is dissipated to heat and other types of energy. The induction equation then reads
It is common to define a dissipation time scale which is the time scale for the dissipation of magnetic energy over a length scale .