In plasma physics, the Lundquist number (denoted by ) is the dimensionless ratio of an Alfvén wave crossing timescale to a resistive diffusion timescale. In SI units, it is given by
- ,
where is a typical length scale, is the permeability of free space, is the Alfvén speed for a plasma with magnetic field, density, ion mass and is the plasma resistivity. High Lundquist numbers indicate highly conducting plasmas, while low Lundquist numbers indicate more resistive plasmas. Laboratory plasma experiments typically have Lundquist numbers between, while in astrophysical situations the Lundquist number can be greater than . Considerations of Lundquist number are especially important in magnetic reconnection.
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