Multimodal rotating magnetoconvection in liquid metals

Early experiments of rotating magnetoconvection in a liquid mercury layer and linear stability analysis suggested that at a critical field strength, the horizontal length scale of the convection cells is significantly enlarged (Nakagawa, Proc. Roy. Soc. A, 1959; Chandrasekhar, 1961). We have numerically reproduced those experiments as close as possible, using a cylinder filled with a fluid with Prandtl number Pr = 0.025, a rotation rate corresponding to an Ekman number of Ek = 1.2 × 10^-4, and a wide diameter-to-height aspect ratio of Γ = 8. We varied both the magnetic field strength and the thermal forcing in the Chandrasekhar and Rayleigh number ranges 9.5 × 10¹ ≤ Ch ≤ 5.5 × 10⁴ and 10⁵ ≤ Ra ≤ 10⁷, respectively. Employing polar spectral analysis, we find that the pronounced multimodality of liquid metal flows leads to a rich variety of flow regimes with strongly varying distinct length scales. However, contrary to the early laboratory experiments, none of these regimes is characterised by a single large-scale mode. Moreover, we detect a strong boundary zonal flow which does not originate from a nonlinear wallmode.