Planetary magnetic fields: Observations and models

Schubert, G.; Soderlund, K. M.

doi:10.1016/j.pepi.2011.05.013

Cited by 119 publications

(110 citation statements)

References 210 publications

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“…Uranus and Neptune are each estimated to have E = 10 −16 (53), and therefore Rf t = 10 32 . The Ice Giants, therefore, are close to the transition between turbulent states, with Rf = OðRf t Þ.…”

Section: Methodsmentioning

confidence: 99%

“…Thus, despite the fact that the flux-Rayleigh numbers for the dynamo regions of the Ice Giants are smaller than those of the Gas Giants, the influence of Pr on the nature of the regime transition indicates that convection in Ice Giants may be weakly affected by rotation. Dynamo action within the Ice Giants produces off-axis, multipolar magnetic fields that differ dramatically from the axial dipoles of the Gas Giants (53). Because little is known of the interiors of these planets, the cause(s) of this division in field morphology is not well understood, and several hypotheses have been put forth, which usually rely on a geometric restriction of the Ice Giant dynamo region (16,54).…”

Section: Methodsmentioning

confidence: 99%

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Turbulent convection in liquid metal with and without rotation

King

Aurnou

2013

Proc. Natl. Acad. Sci. U.S.A.

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The magnetic fields of Earth and other planets are generated by turbulent, rotating convection in liquid metal. Liquid metals are peculiar in that they diffuse heat more readily than momentum, quantified by their small Prandtl numbers, Pr = 1. Most analog models of planetary dynamos, however, use moderate Pr fluids, and the systematic influence of reducing Pr is not well understood. We perform rotating Rayleigh-Bénard convection experiments in the liquid metal gallium (Pr = 0:025) over a range of nondimensional buoyancy forcing (Ra) and rotation periods (E). Our primary diagnostic is the efficiency of convective heat transfer (Nu). In general, we find that the convective behavior of liquid metal differs substantially from that of moderate Pr fluids, such as water. In particular, a transition between rotationally constrained and weakly rotating turbulent states is identified, and this transition differs substantially from that observed in moderate Pr fluids. This difference, we hypothesize, may explain the different classes of magnetic fields observed on the Gas and Ice Giant planets, whose dynamo regions consist of Pr < 1 and Pr > 1 fluids, respectively.T he interiors of Earth and other terrestrial bodies, as well as the Gas Giant planets, contain vast oceans of flowing metals. Mixed by turbulent convection as these planets cool, the flowing conductors produce electric currents that maintain planetary magnetic fields. These bodies also rotate, and it is expected that the resulting Coriolis forces strongly influence convective dynamo processes. Beyond linear theory (1), however, not much is known about the dynamics of rotating convection in liquid metal that underlie planetary magnetic field generation.Theory and experiments often focus on a simplified analog of geophysical and astrophysical systems, Rayleigh-Bénard convection (RBC). The RBC system consists of a fluid layer contained between flat, horizontal plates separated by distance h, the bottom of which is warmer than the top by ΔT, and with downward pointing gravity, g. Near the bottom boundary, the fluid warms and expands by a volumetric factor α (per degree kelvin) and similarly cools and contracts near the top boundary, such that the fluid is unstably stratified.Rotating convection is explored by spinning the RBC system about a vertical axis at an angular rate Ω, which is adopted as the reference frame for the model. The fluid dynamics of the system are characterized by three dimensionless parameters. The Prandtl number, Pr ≡ ν=κ, defines the diffusive transport properties of the fluid, where ν and κ are its viscous and thermal diffusivities. The Rayleigh number, Ra ≡ αgΔTh 3 =ðνκÞ, prescribes the magnitude of the buoyancy force. Finally, the rotation period is specified by the Ekman number, E ≡ ν=ð2Ωh 2 Þ. The primary diagnostic used in many convection studies, including this one, is the Nusselt number, which characterizes the efficiency of heat transport by convection as Nu ≡ qh=ðkΔTÞ, where q is total heat flux and k is the fluid's thermal conductivi...

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Section: Methodsmentioning

confidence: 99%

Section: Methodsmentioning

confidence: 99%

Turbulent convection in liquid metal with and without rotation

King

Aurnou

2013

Proc. Natl. Acad. Sci. U.S.A.

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“…In many geophysical settings, the Ekman number is extremely small, implying that rotational effects massively overwhelm global-scale viscous forces. For instance, the Ekman number is estimated to be of order 10 −15 in Earth's core (Schubert & Soderlund 2011). At such low values, the system-scale flows are expected to be essentially unaffected by fluid viscosity (e.g.…”

Section: Rotating Convectionmentioning

confidence: 99%

“…Typical estimates of the core Rayleigh number range between 10 20 Ra 10 30 (e.g. Gubbins 2001;Kono & Roberts 2002;Aurnou et al 2003;Schubert & Soderlund 2011;Roberts & King 2013). However, some estimates are as low as 10 15 (Roberts & Aurnou 2012) while others are as high as 10 38 (Gubbins 2001).…”

Section: E X T R a P O L At I O N T O P L A N E Ta Ry C O R E S E T Tmentioning

confidence: 99%

Laboratory-numerical models of rapidly rotating convection in planetary cores

Cheng

Stellmach

Ribeiro

et al. 2015

Geophysical Journal International

116

203

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“…6 The fluid layer response to the librational forcing through viscous, 7-9 topographic, 10,11 and electromagnetic coupling [12][13][14] is important for understanding the thermal, magnetic, and orbital evolution of the body. Importantly, while it is often assumed that thermo-compositional convection drives the fluid motions responsible for dynamo generation, [15][16][17] recent studies [18][19][20][21] have characterized how mechanical forcing can also drive dynamos by injecting a portion of the vast quantity of rotational energy from primary-satellite orbital systems into driving fluid motions. a) Electronic mail: agrannan@ucla.edu…”

Section: Introductionmentioning

confidence: 99%

Experimental study of global-scale turbulence in a librating ellipsoid

et al. 2014

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We present laboratory experimental results demonstrating that librational forcing of an ellipsoidal container of water can produce intense motions through the mechanism of a libration driven elliptical instability (LDEI). These libration studies are conducted using an ellipsoidal acrylic container filled with water. A particle image velocimetry method is used to measure the 2D velocity field in the equatorial plane over hundreds libration cycles for a fixed Ekman number, E = 2 × 10 −5 . In doing so, we recover the libration induced base flow and a time averaged zonal flow. Further, we show that LDEI in non-axisymmetric container geometries is capable of driving both intermittent and saturated turbulent motions in the bulk fluid. Additionally, we measure the growth rate and amplitude of the LDEI induced excited flow in a fully ellipsoidal container at more extreme parameters than previously studied [Noir

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Planetary magnetic fields: Observations and models

Cited by 119 publications

References 210 publications

Turbulent convection in liquid metal with and without rotation

Turbulent convection in liquid metal with and without rotation

Laboratory-numerical models of rapidly rotating convection in planetary cores

Experimental study of global-scale turbulence in a librating ellipsoid

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