Fluid Motion of the Outer Core in Response to a Temperature Heterogeneity at the Core-Mantle Boundary and Its Dynamo Action

Yoshida, Shigeo; Hamano, Yozo

doi:10.5636/jgg.45.1497

Cited by 8 publications

(13 citation statements)

References 16 publications

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“…In general such a heat flux anomaly introduces a global azimuthal and latitudinal temperature gradient, which tends to be equilibrated by a dominantly geostrophic mean two-cell circulation given by a broad cyclonic structure in the eastern hemisphere and an anticyclone in the west, which converge into a radial inward flow. As it was previously suggested and in agreement with the linear theory, see Section 3.1 in this paper (or Zhang and Gubbins, 1992;Yoshida and Hamano, 1993) the radial flows are not located where the outer boundary heat is maximal, but where the azimuthal temperature gradient is maximised. For the linear model this phase shift is 90 eastward or a quarter of the azimuthal wavelength of the anomaly.…”

Section: Discussionsupporting

confidence: 89%

“…4 shows the time-averaged flow and temperature distribution found in the equatorial plane for several Ra h ranging from Ra h ¼ 10 to 3:2 Á 10 8 . As extensively discussed above, for the smallest Ra h (top row) the flow structure is in agreement with numerical and analytical results (Zhang and Gubbins, 1992;Yoshida and Hamano, 1993). Note, a few plots from Fig.…”

Section: Nonlinear Regimes: Advective and Convectivesupporting

confidence: 85%

“…The modelling strategy of this paper attempts to cover all possible linear and nonlinear flow states. We start off with deriving the analytical description like Yoshida and Hamano (1993), but for spherical shells rather than a cylindrical annulus, adding the ageostrophic part of the flow and without magnetic fields. These results are verified by numerical simulations set in the almost linear (conductive) regime (Section 3.1).…”

Section: à2xmentioning

confidence: 99%

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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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Section: Discussionsupporting

confidence: 89%

Section: Nonlinear Regimes: Advective and Convectivesupporting

confidence: 85%

Section: à2xmentioning

confidence: 99%

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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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“…The pattern was predicted by theoretical consideration [Busse, 1970] and was later confirmed by numerical and laboratory experiments [e.g., Zhang, 1991;Cardin and Olson, 1994]. Moreover, we can interpret the pattern of the observed geomagnetic field in terms of the columnar convection [Gubbins and Bloxham, 1987;Yoshida and Hamano, 1993]. As a result, we expect that the inner core growth rate will be different between the polar regions and equatorial belt due to a difference in the heat flux extracted from the inner core surface.…”

Section: Coupled Inner Core and Outer Core Dynamics Anisotropic Innersupporting

confidence: 75%

Growth model of the inner core coupled with the outer core dynamics and the resulting elastic anisotropy

Yoshida¹,

Sumita²,

Kumazawa³

1996

J. Geophys. Res.

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206

212

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We present a growth tectonic model of Earth's inner core and the resulting model of the seismic anisotropy. The inner core grows anisotropically if the convection in the outer core is of Taylor column type. The anisotropic growth produces a flow field of the poloidal zonal order 2 type as a result of the isostatic adjustment of the viscous inner core. Crystals in the inner core align themselves under the stress field produced by the flow. We model the anisotropic structure of the inner core, using the theory of Kamb [1959] and elastic constants of Stixrude and Cohen [1995b]. We consider models for both hcp iron and fcc iron, which are the probable crystal structures for the inner core iron according to Stixrude and Cohen [1995a]. We have found that the c axis for hcp iron and [111] direction for fcc iron align in the polar direction. The alignment is consistent with seismic observations, which have revealed that the P wave velocity is faster in the polar direction. Our model predicts that the degree of the alignment decreases near the inner core boundary in accord with recent body wave observations. The radial dependence of the alignment would result from the following three effects: (1) crystals near the surface have not undergone stressed state long enough to acquire anisotropy after precipitation, (2) stress near the surface is different from that in the interior of the inner core due to shear stress free boundary condition, and (3) partially molten structure results in transversely isotropic stress condition near the inner core surface due to compaction. Thus the application of Kamb's theory successfully explains the seismic anisotropy in the inner core provided that the crystals have been subjected under the same stress condition for the timescale of the order of 109 years.

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“…The past decade has seen the emergence of a perspective of the lowermost mantle as a region of complex structures ranging in size from 10 to 1000 km in horizontal extent within the bottom several hundred kilometers of the mantle [Lay, 1989]. The difference in mantle and core timescales is important, as mantle temperature variations impose a long-term, spatially variable heat flux boundary condition on the core flow regime [Jones, 1977;Bloxham andGubbins, 1985, 1987;Gubbins, 1988;King and Hager, 1989; Bloxham and Jackson, 1990; Zhang and Gubbins, 1992; Honkura eta/., 1993; Yoshida and Hamano, 1993].…”

mentioning

confidence: 99%

The core‐mantle boundary region

Loper¹,

Lay²

1995

J. Geophys. Res.

160

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The 75 year history of the American Geophysical Union has accompanied great advances in our understanding of the physics and chemistry of the transition zone between the Earth's core and mantle. The core-mantle boundary (CMB) is the most significant internal boundary within our planet, buried at remote depths and probably forever hidden from direct observation; yet this region is very important to our understanding of the dynamic Earth system. The thermal and chemical processes operating near the CMB have intimate relationships to fundamental events in Earth history, such as core formation, and continue to play a major role in the planet' s evolution, influencing the magnetic field behavior, chemical cycling in the mantle, irregularities in the rotation and gravitation of the planet, and the mode of thermal convection of the Earth. The D" region, comprising the lowermost 300 km of the mantle, is known to be highly heterogeneous in material properties on large and small scales, presumably due to thermal and chemical variations, while the outermost core is much more uniform. The substantial knowledge that we now have about this remote region is testimony to the remarkable progress made in geophysical remote sensing, prompted by prodigious increases in data, computational power, and experimental methodologies used to investigate deep earth structure and processes. In the past decade in particular there have been unprecedented multidisciplinary advances in understanding the CMB region, and there are excellent prospects for developing a comprehensive understanding of this region in the next few decades.Fundamental issues yet to be resolved include the causes of layering extensively observed in D' ', whether or not downwelling slabs accumulate at the base of the mantle, whether plumes arise from the CMB region to feed hotspots at Earth' s surface, the extent of core-mantle chemical reactions, the relative importance of topographic versus electromagnetic coupling across the CMB, and the degree to which mantle structure influences the geomagnetic field and its reversals. This overview highlights the progress and future directions in geophysical investigations of the CMB region. 1919-1994 IntroductionOur Earth is a dynamic body in continual evolution. An important event in this evolution was separation of the ironalloy core and silicate-oxide mantle early in the history of i_•111.11, [.JIKJOO. UIJ ULIIIlI• I. ll•, lllO. lll stage ,,, o.,• ,, ......... process [Anderson and Hanks, 1972' Ruff and Anderson, 1980; Stevenson, 1981' Ringwood,' 1984; Jones and Drake, 1986; Ida etak, 1987; Stevenson, 1990; Honda et al., 1993a]. This chemical separation profoundly impacted the subsequent mechanical, thermal, and chemical evolution of Earth, as the large density contrast between the core and mantle prohibits convective motions across the core-mantle boundary (CMB).Consequently, boundary layers have formed in the CMB region, and these control the transfer of heat, angular momentum, and material between the core and mantle. The goal ...

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Fluid Motion of the Outer Core in Response to a Temperature Heterogeneity at the Core-Mantle Boundary and Its Dynamo Action

Cited by 8 publications

References 16 publications

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

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

Growth model of the inner core coupled with the outer core dynamics and the resulting elastic anisotropy

The core‐mantle boundary region

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