Slow magnetic Rossby waves in the Earth's core

Hori, K.; Jones, C. A.; Teed, Robert

doi:10.1002/2015gl064733

Cited by 47 publications

(50 citation statements)

References 27 publications

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“…11a) could suggest the presence of Rossby, or magneto-Rossby waves (e.g. Hori et al, 2015Hori et al, , 2017, but this cannot be the case because we observe propagation in the cylindrical radial direction while Rossby waves propagate in the azimuthal direction.…”

Section: Contributions To the Secular Accelerationmentioning

confidence: 79%

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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Section: Contributions To the Secular Accelerationmentioning

confidence: 79%

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

Aubert

2018

Geophysical Journal International

107

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“…MHD Rossby waves in strong magnetic fields in the Earth's outer core have been studied theoretically for at least the 50 years. They may explain the origin of the westward drift of the geomagnetic field (Hori et al, ). Dynamo models of the core also exhibit Rossby wave patterns (Hori et al, ).…”

Section: Concept Of Rossby Wavesmentioning

confidence: 99%

Space Weather Challenge and Forecasting Implications of Rossby Waves

Dikpati

McIntosh

2020

Space Weather

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Rossby waves arise in thin layers within fluid regions of stars and planets. These global wave‐like patterns occur due to the variation in Coriolis forces with latitude. In the past several years observational evidence has indicated that there are also Rossby waves in the Sun. Although Rossby waves have been detected in the Sun's photosphere and corona, they most likely originate in the solar tachocline, the sharp shear layer at the base of the solar convection zone, where the differential rotation driven by convection transitions to the solidly rotating radiative interior. These waves differ from their Earth's counterparts by being strongly modified by toroidal magnetic fields in the solar tachocline. Recent simulations of magnetohydrodynamics of tachocline Rossby waves and magnetic fields are demonstrated to produce strong “tachocline nonlinear oscillations,” which have periods similar to those observed in the solar atmosphere—enhanced periods of solar activity, or “seasons”—occurring at intervals between six months and two years. These seasonal/subseasonal bursts produce the strongest eruptive space weather events. Thus, a key to forecasting the timing, amplitude, and location of future activity bursts, and hence space weather events, could lie in our ability to forecast the phase and amplitude of Rossby waves and associated tachocline nonlinear oscillations. Accurately forecasting the properties of solar Rossby waves and their impact on space weather will require linking surface activity observations to the magnetohydrodynamics of tachocline Rossby waves, using modern data assimilation techniques. Both short‐term (hours to days) and long‐term (decadal to millennial) forecasts of space weather and climate are now being made. We highlight in this article the potential of solar Rossby waves for forecasting space weather on intermediate time scales, of several weeks to months up to a few years ahead.

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“…Incorporating the sloping boundaries conditions (e.g. Busse, 1970;Gillet and Jones, 2006;Hori et al, 2015) gives…”

Section: Linear Conductive Regimementioning

confidence: 99%

Core flows and heat transfer induced by inhomogeneous cooling with sub- and supercritical convection

Dietrich

Hori

2016

Physics of the Earth and Planetary Interiors

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a b s t r a c tThe amount and spatial pattern of heat extracted from cores of terrestrial planets is ultimately controlled by the thermal structure of the lower rocky mantle. Using the most common model to tackle this problem, a rapidly rotating and differentially cooled spherical shell containing an incompressible and viscous liquid is numerically investigated. To gain the physical basics, we consider a simple, equatorial symmetric perturbation of the CMB heat flux shaped as a spherical harmonic Y 11 . The thermodynamic properties of the induced flows mainly depend on the degree of nonlinearity parametrised by a horizontal Rayleigh number Ra h ¼ q Ã Ra, where q Ã is the relative CMB heat flux anomaly amplitude and Ra is the Rayleigh number which controls radial buoyancy-driven convection. Depending on Ra h we identify and characterise three distinctive flow regimes through their spatial patterns, heat transport and flow speed scalings: in the linear conductive regime the radial inward flow is found to be phase shifted 90°eastwards from the maximal heat flux as predicted by a linear quasi-geostrophic model for rapidly rotating spherical systems. The advective regime is characterised by an increased Ra h where nonlinearities become significant, but is still subcritical to radial convection. There the upwelling is dispersed and the downwelling is compressed by the thermal advection into a spiralling jet-like structure. As Ra h becomes large enough for the radial convection to set in, the jet remains identifiable on time-average and significantly alters the global heat budget in the convective regime. Our results suggest, that the boundary forcing not only introduces a net horizontal heat transport but also suppresses the convection locally to such an extent, that the net Nusselt number is reduced by up to 50%, even though the mean CMB heat flux is conserved. This also implies that a planetary core will remain hotter under a non-homogeneous CMB heat flux and is less well mixed. A broad numerical parameter investigation regarding Rayleigh number and the relative heat flux anomaly further fosters these results. Crown

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Slow magnetic Rossby waves in the Earth's core

Cited by 47 publications

References 27 publications

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

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

Space Weather Challenge and Forecasting Implications of Rossby Waves

Core flows and heat transfer induced by inhomogeneous cooling with sub- and supercritical convection

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