Laboratory Measurement of Non-Rotating Magnetoconvection in Liquid Gallium: Wall-mode Onset and Supercritical Precessional Mode

Turbulent flows in the Earth's molten outer core, driven by convection, generate a planetary-scale, nearly axial, dipole-dominated magnetic field. The behaviors of strongly turbulent convection in the presence of strong Lorentz forces are mostly unknown. Thus, we present results of laboratory experiments on non-rotating Rayleigh-B\'enard convection of liquid gallium in the presence of a vertical magnetic field. Our heat transfer survey in a diameter-to-height aspect ratio $\Gamma=1$ tank, with $10^6 < Ra<10^8$ and $0< Ch< 3\times 10^5$, shows that the convection onsets well below the predicted $Ra$ for an infinite fluid layer. Magnetoconvection in our finite cylindrical tanks likely onsets via stationary wall-attached modes (Houchens et al., 2002; Busse, 2008). In a $\Gamma=2$ tank and with $Ra\approx 2\times10^6$, we vary the applied magnetic field corresponding to interaction parameter numbers $N$ from $0$ to $10$. Our thermal measurements show the existence of a novel precessional mode at $N\approx 0.5$ with electrically conducting boundaries (Cu), but not with electrically insulated boundaries (Teflon coated Al). This finding suggests the possibility of slowly traveling magneto-precessional modes attached to the electrically conductive boundaries of Earth's outer core.