Three branches of dynamo action

Dormy, Emmanuel; Oruba, Ludivine; Petitdemange, Ludovic

doi:10.1088/1873-7005/aa769c

Cited by 42 publications

(27 citation statements)

References 43 publications

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“…For a series of dynamos, Dormy et al (2018) calculated the vorticity length scale 2 ω = u 2 / ω 2 (introduced in Oruba & Dormy 2014), where the angle brackets denote time and volume averages. For the helical columnar convection exhibited in these simulations, Figure 20.…”

Section: Discussionmentioning

confidence: 99%

“…For the helical columnar convection exhibited in these simulations, Figure 20. The vorticity length scale ω plotted against n for a dataset from Dormy et al (2018). The shaded region is bounded by the lines n = 4 ω and n = 6 ω , and the dashed black line is n = 5 ω .…”

Section: Discussionmentioning

confidence: 99%

“…The length scale at which inertial effects are in contention with the effects of global rotation shifts into the energy-containing scales. This causes a loss of columnar structures and flow helicity, which results in the dipole collapse (Christensen & Aubert 2006;Dormy et al 2018). This paper addresses the question: what mechanism causes the transition from columnar to three-dimensional flow?…”

Section: An Observed Dipole-multipole Transition In Numerical Dynamosmentioning

confidence: 99%

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A physical conjecture for the dipolar–multipolar dynamo transition

McDermott

Davidson

2019

J. Fluid Mech.

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In numerical simulations of planetary dynamos there is an abrupt transition in the dynamics of both the velocity and magnetic fields at a 'local' Rossby number of 0.1. For smaller Rossby numbers there are helical columnar structures aligned with the rotation axis, which efficiently maintain a dipolar field. However, when the thermal forcing is increased, these columns break down and the field becomes multi-polar. Similarly, in rotating turbulence experiments and simulations there is a sharp transition at a Rossby number of ∼ 0.4. Again, helical axial columnar structures are found for lower Rossby numbers, and there is strong evidence that these columns are created by inertial waves, at least on short timescales. We perform direct numerical simulations of the flow induced by a layer of buoyant anomalies subject to strong rotation, inspired by the equatorially biased heat flux in convective planetary dynamos. We assess the role of inertial waves in generating columnar structures. At high rotation rates (or weak forcing) we find columnar flow structures that segregate helicity either side of the buoyant layer, whose axial length scale increases linearly, as predicted by the theory of low-frequency inertial waves. As the rotation rate is weakened and the magnitude of the buoyant perturbations is increased, we identify a portion of the flow which is more strongly three-dimensional. We show that the flow in this region is turbulent, and has a Rossby number above a critical value Ro crit ∼ 0.4, consistent with previous findings in rotating turbulence. We suggest that the discrepancy between the transition value found here (and in rotating turbulence experiments), and that seen in the numerical dynamos (Ro crit ∼ 0.1), is a result of a different choice of the length scale used to define the local Ro. We show that when a proxy for the flow length scale perpendicular to the rotation axis is used in this definition, the numerical dynamo transition lies at Ro crit ∼ 0.5. Based on this we hypothesise that inertial waves, continually launched by buoyant anomalies, sustain the columnar structures in dynamo simulations, and that the transition documented in these simulations is due to the inability of inertial waves to propagate for Ro > Ro crit .

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Section: Discussionmentioning

confidence: 99%

Section: Discussionmentioning

confidence: 99%

Section: An Observed Dipole-multipole Transition In Numerical Dynamosmentioning

confidence: 99%

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A physical conjecture for the dipolar–multipolar dynamo transition

McDermott

Davidson

2019

J. Fluid Mech.

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“…Since increasing Pm not only increases Rm but also decreases E λ , figure 5 suggests that this offers an alternative (and certainly numerically cheaper) path to more Earth-like simulations. Dormy (2016) and Dormy et al (2018) report that a third dynamo branch can indeed be found when increasing the magnetic Prandtl number at low Rayleigh numbers. This branch is characterised by a particularly strong dipole-dominated magnetic field.…”

Section: Towards Earth-like Solutionsmentioning

confidence: 94%

“…One reason is that ⊥ spans little more than one order of magnitude in typical current dynamo simulations. Another reason is that the results can depend on the specific method used for estimating ⊥ (Dormy et al 2018). When using, for example, equation 36, the additional small scales that are excited at smaller Ekman numbers could already decrease the ⊥ estimate, even when the dominant flow scale remains unchanged.…”

Section: The Mac Balancementioning

confidence: 99%

Advances in geodynamo modelling

Wicht

Sanchez

2019

Geophysical & Astrophysical Fluid Dynamics

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This paper reviews the remarkable developments in numerical geodynamo simulations over the last few years. Simulations with Ekman numbers as low as E = 10 −8 are now within reach and more and more details of the observed field are recovered by computer models. However, some newer experimental and ab initio results suggest a rather large thermal conductivity for the liquid iron alloy in Earth's core. More heat would then simply be conducted down the core adiabat and would not be available for driving the dynamo process. The current status of this topic is reported and alternative driving scenarios are discussed. The paper then addresses the question whether dynamo simulations obey the magnetostrophic force balance that characterises the geodynamo and proceeds with discussing related problems like scaling laws and torsional oscillations. Finally, recent developments in geomagnetic data assimilation are reviewed, where geomagnetic data and dynamo simulations are coupled to form a tool for interpreting observations and predicting the future evolution of Earth's magnetic field.

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Magnetic effects on fields morphologies and reversals in geodynamo simulations

Petitdemange

Galtier

2020

Physics of the Earth and Planetary Interiors

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The dynamo effect is the most popular candidate to explain the non-primordial magnetic fields of astrophysical objects. Although many systematic studies of parameters have already been made to determine the different dynamical regimes explored by direct numerical geodynamo simulations, it is only recently that the regime corresponding to the outer core of the Earth characterized by a balance of forces between the Coriolis and Lorentz forces is accessible numerically. In most previous studies, the Lorentz force played a relatively minor role. For example, they have shown that a purely hydrodynamic parameter (the local Rossby number Ro) determines the stability domain of dynamos dominated by the axial dipole (dipolar dynamos). In this study, we show that this result cannot hold when the Lorentz force becomes dominant. We model turbulent geodynamo simulations with a strong Lorentz force by varying the important parameters over several orders of magnitude. This method enables us to question previous results and to argue on the applications of numerical dynamos in order to better understand the geodynamo problem. Strong dipolar fields considerably affect the kinetic energy distribution of convective motions which enables the maintenance of this field configuration. The relative importance of each force depends on the spatial length scale, whereas Ro is a global output parameter which ignores the spatial dependency. We show that inertia does not induce a dipole collapse as long as the Lorentz and the Coriolis forces remain dominant at large length scales.

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Three branches of dynamo action

Cited by 42 publications

References 43 publications

A physical conjecture for the dipolar–multipolar dynamo transition

A physical conjecture for the dipolar–multipolar dynamo transition

Advances in geodynamo modelling

Magnetic effects on fields morphologies and reversals in geodynamo simulations

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