 Thermal-diffusion magnetic-field parameter

With thermal diffusivity $\kappa$ and magnetic diffusivity $\eta$, the standard Chandrasekhar number uses $\nu \eta$ in its denominator. A thermal-diffusion alternative uses $\nu \kappa$ and is $\zeta =\eta /\kappa$ times the standard parameter. In thermal-time magnetoconvection equations, the linear magnetic-force coefficient is $\sigma Q_{\kappa }=\sigma \zeta Q_{\mathrm{Ch}}$. Specifying the convention prevents a missing diffusivity factor in a dispersion relation.