 Field-independent stationary threshold in oblique magnetoconvection

At inclination $\alpha =\arctan \sqrt{2}$ and vertical wavenumber one, the nonmagnetic minimizing wavenumber $k=-1/\sqrt{2}$ also satisfies $\mathbf{c}\cdot \mathbf{k}=0$. The stationary threshold is the sum of $F(k)\ge 27/4$ and a nonnegative magnetic penalty that vanishes there. Therefore $k_c=-1/\sqrt{2}$ and $R_c=27/4$ for every $Q\ge 0$. The selected disturbance is constant along the imposed field, so the linear resistive induction equation produces no magnetic perturbation and there is no field-line tension penalty.