Magnetic-to-thermal diffusivity ratio ($\zeta =\eta /\kappa$)

The ratio of magnetic diffusivity $\eta$ to thermal diffusivity $\kappa$ controls their relative relaxation times. In nonrotating vertical-field magnetoconvection, oscillatory linear onset in the simple stress-free layer requires $0<\zeta <1$, so thermal perturbations diffuse faster than magnetic perturbations. This is distinct from the magnetic Prandtl number, $\nu /\eta$.