Lundquist number
In plasma physics, the Lundquist number (denoted by S {\displaystyle S} ) is a dimensionless ratio which compares the timescale of an Alfvén wave crossing to the timescale of resistive diffusion. It is a special case of the magnetic Reynolds number when the Alfvén velocity is the typical velocity scale of the system, and is given by S = L v A η , {\displaystyle S={\frac {Lv_{A}}{\eta }},} where L {\displaystyle L} is the typical length scale of the system, η {\displaystyle \eta } is the magnetic diffusivity and v A {\displaystyle v_{A}} is the Alfvén velocity of the plasma.