The transition to Earth-like torsional oscillations in magnetoconvection simulations

Teed, Robert; Jones, C. A.; Tobias, Steven M.

doi:10.1016/j.epsl.2015.02.045

Cited by 62 publications

(45 citation statements)

References 30 publications

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“…cylinder away from the boundary (region named tc). This hints to the Lorentz force close to the inner-core as the main source for torsional waves, which is in line with recent findings (Teed et al, 2015).…”

Section: Torsional Alfvén Wavessupporting

confidence: 91%

Turbulent geodynamo simulations: a leap towards Earth’s core

Schaeffer¹,

Jault²,

Nataf³

et al. 2017

Geophysical Journal International

203

287

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We present an attempt to reach realistic turbulent regime in direct numerical simulations of the geodynamo. We rely on a sequence of three convection-driven simulations in a rapidly rotating spherical shell. The most extreme case reaches towards the Earth's core regime by lowering viscosity (magnetic Prandtl number P m = 0.1) while maintaining vigorous convection (magnetic Reynolds number Rm > 500) and rapid rotation (Ekman number E = 10 −7 ), at the limit of what is feasible on today's supercomputers. A detailed and comprehensive analysis highlights several key features matching geomagnetic observations or dynamo theory predictions -all present together in the same simulation -but it also unveils interesting insights relevant for Earth's core dynamics.In this strong-field, dipole-dominated dynamo simulation, the magnetic energy is one order of magnitude larger than the kinetic energy. The spatial distribution of magnetic intensity is highly heterogeneous, and a stark dynamical contrast exists between the interior and the exterior of the tangent cylinder (the cylinder parallel to the axis of rotation that circumscribes the inner core).In the interior, the magnetic field is strongest, and is associated with a vigorous twisted polar vortex, whose dynamics may occasionally lead to the formation of a reverse polar flux patch at the surface of the shell. Furthermore, the strong magnetic field also allows accumulation of light material within the tangent cylinder, leading to stable stratification there. Torsional Alfvén waves are frequently triggered in the vicinity of the tangent cylinder and propagate towards the equator.Outside the tangent cylinder, the magnetic field inhibits the growth of zonal winds and the kinetic energy is mostly non-zonal. Spatio-temporal analysis indicates that the low-frequency, non-zonal flow is quite geostrophic (columnar) and predominantly large-scale: an m=1 eddy spontaneously emerges in our most extreme simulations, without any heterogeneous boundary forcing.Our spatio-temporal analysis further reveals that (i) the low-frequency, largescale flow is governed by a balance between Coriolis and buoyancy forces -magnetic field and flow tend to align, minimizing the Lorentz force; (ii) the high-frequency flow obeys a balance between magnetic and Coriolis forces; (iii) the convective plumes mostly live at an intermediate scale, whose dynamics is driven by a 3-term 1 arXiv:1701.01299v3 [physics.geo-ph] 15 Jun 2017 MAC balance -involving Coriolis, Lorentz and buoyancy forces. However, smallscale ( E 1/3 ) quasi-geostrophic convection is still observed in the regions of low magnetic intensity.

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Section: Torsional Alfvén Wavessupporting

confidence: 91%

Turbulent geodynamo simulations: a leap towards Earth’s core

Schaeffer¹,

Jault²,

Nataf³

et al. 2017

Geophysical Journal International

203

287

View full text Add to dashboard Cite

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“…Interestingly, the acceleration of the jet that we find from 2004 onwards is coincident with an abrupt alteration in the rotation direction of the inner core from eastwards to westwards [29] that has been inferred at about the same time. Lastly, mounting evidence [30,31] suggests that torsional waves may be launched from the tangent cylinder, perhaps by the dynamics associated with the cylindrical jet [16]. An accelerating phase of the jet is consistent with independent studies [32] showing an increase in torsional-wave magnitudes over the last decade.…”

Section: Implications For Core Dynamicssupporting

confidence: 78%

An accelerating high-latitude jet in Earth’s core

2016

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Observations of the change in Earth's magnetic field, the secular variation, provide information on the motion of liquid metal within the core that is responsible for its generation. The very latest high-resolution observations from ESA's Swarm satellite mission show intense field change at high-latitude localised in a distinctive circular daisy-chain configuration centred on the north geographic pole. Here we explain this feature with a localised, non-axisymmetric, westwards jet of 420 km width on the tangent cylinder, the cylinder of fluid within the core that is aligned with the rotation axis and tangent to the solid inner core.We find that the jet has increased in magnitude by a factor of three over the period 2000-2016 to about 40 km/yr, and is now much stronger than typical large-scale flows inferred for the core. The current accelerating phase may be a part of a longer term fluctuation of the jet causing both eastwards and westwards movement of magnetic features over historical

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“…Among the rapid dynamics present in Earth's core (Gillet et al, 2010) and also observed in advanced numerical geodynamo simulations, axisymmetric torsional waves have been repeatedly exhibited before (Wicht & Christensen, 2010;Teed et al, 2014Teed et al, , 2015Schaeffer et al, 2017) with various simulation set-ups and are now well understood. The non-axisymmetric, quasi-geostrophic Alfvén waves highlighted in this study are perhaps more surprising, because the classical theoretical views operate a strong dichotomy between axisymmetric and non-axisymmetric waves (see e.g.…”

Section: Dynamic Variability Versus Kinematic Invariance Along the Pathmentioning

confidence: 89%

Geomagnetic acceleration and rapid hydromagnetic wave dynamics in advanced numerical simulations of the geodynamo

Aubert

2018

Geophysical Journal International

107

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Geomagnetic secular acceleration, the second temporal derivative of Earth's magnetic field, is a unique window on the dynamics taking place in Earth's core. In this study, the behaviours of the secular acceleration and underlying core dynamics are examined in new numerical simulations of the geodynamo that are dynamically closer to Earth's core conditions than earlier models. These new models reside on a theoretical path in parameter space connecting the region where most classical models are found to the natural conditions. The typical time scale for geomagnetic acceleration is found to be invariant along this path, at a value close to 10 years that matches Earth's core estimates. Despite this invariance, the spatio-temporal properties of secular acceleration show significant variability along the path, with an asymptotic regime of rapid rotation reached after 30% of this path (corresponding to a model Ekman number E = 3 10 −7 ). In this regime, the energy of secular acceleration is entirely found at periods longer than that of planetary rotation, and the underlying flow acceleration patterns acquire a two-dimensional columnar structure representative of the rapid rotation limit. The spatial pattern of the secular acceleration at the coremantle boundary shows significant localisation of energy within an equatorial belt. Rapid hydromagnetic wave dynamics is absent at the start of the path because of insufficient time scale separation with convective processes, weak forcing and excessive damping but can be clearly exhibited in the asymptotic regime. This study reports on ubiquitous axisymmetric geostrophic torsional waves of weak amplitude relatively to convective transport, and also stronger, laterally limited, quasi-geostrophic Alfvén waves propagating in the cylindrical radial direction from the tip of convective plumes towards the core-mantle boundary. In a system similar to Earth's core where the typical Alfvén velocity is significantly larger than the typical convective velocity, quasi-geostrophic Alfvén waves are shown to be an important carrier of flow acceleration to the core surface that links with the generation of strong, short-lived and intermittent equatorial pulses in the secular acceleration energy. The secular acceleration time scale is shown to be insensitive to magnetic signatures from torsional waves because of their weak amplitude, and from quasi-geostrophic Alfvén waves because of their intermittent character, and is therefore only indicative of convective transport phenomena that remain invariant along the parameter space path.

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The transition to Earth-like torsional oscillations in magnetoconvection simulations

Cited by 62 publications

References 30 publications

Turbulent geodynamo simulations: a leap towards Earth’s core

Turbulent geodynamo simulations: a leap towards Earth’s core

An accelerating high-latitude jet in Earth’s core

Geomagnetic acceleration and rapid hydromagnetic wave dynamics in advanced numerical simulations of the geodynamo

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