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The mechanism responsible for the damping of the large-scale, azimuthally directed jets observed at Jupiter's surface is not well known, but electromagnetic forces are suspected to play a role as the planet's electrical conductivity increases radially with depth. To isolate the jet damping process, we carry out a suite of direct numerical simulations of quasi-two-dimensional, horizontally periodic Rayleigh-Bénard convection with stress-free boundary conditions in the presence of an external, vertical magnetic field. Jets, punctuated by intermittent convective bursts, develop at Rayleigh numbers (Ra, ratio of buoyancy to diffusion) beyond 10^{5} when the magnetic field is relatively weak. Five primary flow regimes are found by varying 10^{3}≤Ra≤10^{10} and the Chandrasekhar number (Ch, ratio of Lorentz to viscosity) 0≤Ch≤10^{6}: (i) steady convection rolls, (ii) steady magneto-columns, (iii) unsteady to turbulent magneto-plumes, (iv) horizontally drifting magneto-plumes, and (v) jets with intermittent turbulent convective bursts. We parse the parameter space using transitions derived from the interaction parameter (N, ratio of Lorentz to inertia). The transition to the regime dominated by jets has the most immediate applications to the magnetic damping of Jovian jet flows, where the separation between jets and a magnetically constrained system occurs at a jet-based interaction parameter value of N_{J}≈1. We approximate the value of the Jovian interaction parameter as a function of depth, and find that the jets may brake at ≈6000 km below the surface, which is deeper than recent estimates from NASA's Juno mission. This suggests that mechanisms in addition to electromagnetic forces are likely required to fully truncate the jets.

In magnetostrophic rotating magnetoconvection, a fluid layer heated from below and cooled from above is equidominantly influenced by the Lorentz and the Coriolis forces. Strong rotation and magnetism each act separately to suppress thermal convective instability. However, when they act in concert and are near in strength, convective onset occurs at less extreme Rayleigh numbers ( Ra , thermal forcing) in the form of a stationary, large-scale, inertia-less, inviscid magnetostrophic mode. Estimates suggest that planetary interiors are in magnetostrophic balance, fostering the idea that magnetostrophic flow optimizes dynamo generation. However, it is unclear if such a mono-modal theory is realistic in turbulent geophysical settings. Donna Elbert first discovered that there is a range of Ekman ( Ek , rotation) and Chandrasekhar ( Ch , magnetism) numbers, in which stationary large-scale magnetostrophic and small-scale geostrophic modes coexist. We extend her work by differentiating five regimes of linear stationary rotating magnetoconvection and by deriving asymptotic solutions for the critical wavenumbers and Rayleigh numbers. Coexistence is permitted if Ek

The interplay between convective, rotational and magnetic forces defines the dynamics within the electrically conducting regions of planets and stars. Yet their triadic effects are separated from one another in most studies, arguably due to the richness of each subset. In a single laboratory experiment, we apply a fixed heat flux, two different magnetic field strengths and one rotation rate, allowing us to chart a continuous path through Rayleigh-Bénard convection (RBC), two regimes of magnetoconvection, rotating convection and two regimes of rotating magnetoconvection, before finishing back at RBC. Dynamically rapid transitions are determined to exist between jump rope vortex states, thermoelectrically driven magnetoprecessional modes, mixed wall- and oscillatory-mode rotating convection and a novel magnetostrophic wall mode. Thus, our laboratory 'pub crawl' provides a coherent intercomparison of the broadly varying responses arising as a function of the magnetorotational forces imposed on a liquid-metal convection system.

We present laboratory measurements of the interaction between thermoelectric currents and turbulent magnetoconvection. In a cylindrical volume of liquid gallium heated from below and cooled from above and subject to a vertical magnetic field, it is found that the large-scale circulation (LSC) can undergo a slow axial precession. Our experiments demonstrate that this LSC precession occurs only when electrically conducting boundary conditions are employed, and that the precession direction reverses when the axial magnetic field direction is flipped. A thermoelectric magnetoconvection (TEMC) model is developed that successfully predicts the zeroth-order magnetoprecession dynamics. Our TEMC magnetoprecession model hinges on thermoelectric current loops at the top and bottom boundaries, which create Lorentz forces that generate horizontal torques on the overturning large-scale circulatory flow. The thermoelectric torques in our model act to drive a precessional motion of the LSC. This model yields precession frequency predictions that are in good agreement with the experimental observations. We postulate that thermoelectric effects in convective flows, long argued to be relevant in liquid metal heat transfer and mixing processes, may also have applications in planetary interior magnetohydrodynamics.

Abstract:

Earth’s magnetic field is generated by convective motions in its liquid metal core. In this fluid, the heat diffuses significantly more than momentum, and thus the Prandtl number $Pr$ is well below unity. The thermally driven convective flow dynamics of liquid metals are very different from moderate-$Pr$ fluids, such as water and those used in current dynamo simulations. In order to characterise rapidly rotating thermal convection in low-$Pr$ number fluids, we have performed laboratory experiments in a cylinder of aspect ratio $\unicode[STIX]{x1D6E4}=1.94$ using liquid gallium ($Pr\simeq 0.025$) as the working fluid. The Ekman number varies from $E\simeq 5\times 10^{-6}$ to $5\times 10^{-5}$ and the Rayleigh number varies from $Ra\simeq 2\times 10^{5}$ to $1.5\times 10^{7}$. Using spectral analysis stemming from point-wise temperature measurements within the fluid and measurements of the Nusselt number $Nu$, we characterise the different styles of low-$Pr$ rotating convective flow. The convection threshold is first overcome in the form of container-scale inertial oscillatory modes. At stronger forcing, sidewall-attached modes are identified for the first time in liquid metal laboratory experiments. These wall modes coexist with the bulk oscillatory modes. At $Ra$ well below the values where steady rotating columnar convection occurs, the bulk flow becomes turbulent. Our results imply that rotating convective flows in liquid metals do not develop in the form of quasisteady columns, as in moderate-$Pr$ fluids, but in the form of oscillatory convective motions. Thus, thermally driven flows in low-$Pr$ geophysical and astrophysical fluids can differ substantively from those occurring in $Pr\simeq 1$ models. Furthermore, our experimental results show that relatively low-frequency wall modes are an essential dynamical component of rapidly rotating convection in liquid metals.

Centrifugal buoyancy affects all rotating turbulent convection phenomena, but is conventionally ignored in rotating convection studies. Here, we include centrifugal buoyancy to investigate what we call Coriolis-centrifugal convection (C^{3}), characterizing two so far unexplored regimes, one where the flow is in quasicyclostrophic balance (QC regime) and another where the flow is in a triple balance between pressure gradient, Coriolis and centrifugal buoyancy forces (CC regime). The transition to centrifugally dominated dynamics occurs when the Froude number Fr equals the radius-to-height aspect ratio γ. Hence, turbulent convection experiments with small γ may encounter centrifugal effects at lower Fr than traditionally expected. Further, we show analytically that the direct effect of centrifugal buoyancy yields a reduction of the Nusselt number Nu. However, indirectly, it can cause a simultaneous increase of the viscous dissipation and thereby Nu through a change of the flow morphology. These direct and indirect effects yield a net Nu suppression in the CC regime and a net Nu enhancement in the QC regime. In addition, we demonstrate that C^{3} may provide a simplified, yet self-consistent, model system for tornadoes, hurricanes, and typhoons.