The effect of the electric conductivity of the Earth's mantle on the Ekman-Hartman hydromagnetic boundary layer and on the magnetic diffusion region

Summary The effect of the electrical conductivity of the Earth's mantle on the non-stationary Ekman-Hartman hydromagnetic boundary layer is investigated under the conditions in the Earth's core. It is shown that under an impulsive change of rotation of the mantle Alfvén waves can only be excited if the Ekman-Hartman hydromagnetic boundary layer is in a non-stationary state, i.e. at a time when its structure is developing. The intensity of the Alfvén waves is very small, because the excitation is more of a mechanic nature than magnetic.     

Models of electric conductivity in the upper mantle

Summary The accessible data of the apparent resistivity _T(T) were collected and supplemented by the spectral analysis of daily means of geomagnetic components observed at world geomagnetic observatories. The data were least-squares fitted; four models of the resistivity in the upper mantle which agree with the experimental data were obtained by solving the inverse problem using Marquardt's gradient-expansion algorithm.     

科学家首次绘制地幔电导率全球三维图像

家用电器警示标签中通常会注明,水是很好的导电材料。现在,科学家发现,地幔局部电导率的增大很可能意味着地表下含水量的增大。研究人员首次绘制了地幔电导率全球三维图像,他们的研究结果发表在近期的Nature杂志上。     

Axially asymmetric velocities in the boundary layer of the nearly symmetric hydromagnetic dynamo

Abstract

An attempt has been made to include the axially asymmetric velocities into the calculation of Braginsky's Z-model of the nearly symmetric hydromagnetic dynamo. In this axisymmetric non-linear model dominated by Lorentz and Coriolis forces and maintained by a specified convection, the α-effect is prescribed. An example is shown of the axially asymmetric Archimedean buoyancy, which can imply an arbitrary alpha effect in the model with viscous core-mantle coupling. The formalisms of Tough and Roberts (1968) is also discussed and a modified α-effect in the Z-model is suggested.     

Chemical structure of the Earth's mantle defined by fast diffusion elements like helium

A theoretical model and numerical simulation(i.e.,Zhu et al.2019)demonstrates that the Earth’s lower mantle was intrinsically stratified for helium(He)by thermal diffusion owing to adiabatic temperature gradient in the Earth.This finding helps to explain the two longstanding problems:the helium concentration paradox and the heathelium paradox.     

Effect of composition,structure, and spin state on the thermal conductivity of the Earth's lower mantle

The change in electronic structure of iron at high pressures to spin-paired states in ferropericlase, silicate perovskite, and post-perovskite may have a profound influence on the thermal conductivity of the lower mantle. Here, we present optical absorption data for lower mantle minerals to assess the effect of composition (including iron oxidation state), structure, and iron spin state on radiative heat transfer. We confirm that the presence of ferric iron in ferropericlase strongly affects the optical properties, while the effect of the spin-pairing transition may be more secondary. We also show that post-perovskite exhibits larger optical absorption in the near infrared and visible spectral ranges than perovskite which may have a profound effect on the dynamics the lowermost mantle. We present preliminary results from measurements of the phonon thermal conductivity of perovskite at 125 GPa using a pulsed laser heating technique. The available data suggest a larger value than what previously estimated, although the uncertainty is large.     

Hall effect on magnetic reconnection at the Earth's magnetopause

We analyze in situ observations of magnetic reconnection at the Earth magnetopause to estimate the importance of the Hall current during the merging of interplanetary and magnetospheric magnetic field lines. The reconnection process is studied through numerical simulations, integrating the Hall MHD equations in 2.5 dimensions. A large influence of the Hall effect is found, which can be measured by a significant increase of the reconnection rate.     

The cooling models of Earth's early mantle

The thermal state of the early Earth’s interior and its way of cooling are crucial for its subsequent evolution. Earth is initially hot as it acquired enormous heat in response to violent processes during its formation, e.g., the Moon-forming giant impact, the segregation and formation of its metallic core, the tidal interaction with the early Moon, and the decay of radioactive elements, etc. In the meantime, the cooling mechanisms of early Earth’s mantle remain elusive despite their importance,...     

南极上地幔结构异常对长周期自由振荡的影响

2012年发生的苏门答腊大地震激起了强烈的环形地球自由振荡,本文利用南极26个地震台站记录的此次大地震激发的₀T₁₀简正模,分析南极上地幔结构异常对长周期自由振荡的影响,我们将震后20 h的自由振荡观测数据与利用PREM地球模型模拟的结果进行比较,结果表明：在震后较短时间内区域性（区域的范围远远小于简正模的波长）上地幔结构异常能显著影响长周期自由振荡质点的偏振,引起简正模强烈的偏振异常.以往研究主要关注地球自转和全球范围的上地幔结构异常对长周期自由振荡简正模的影响,局部范围的上地幔结构异常对长周期自由振荡的影响并未被重视.对南极地区₀T₁₀垂向偏振异常的进一步分析表明：南极大陆上地幔存在方位各向异性,上地幔各向异性主要分布在横贯南极山脉下方,深度范围约为70~660 km.利用震后较短时间内的长周期地球自由振荡观测资料可以对局部区域上地幔的各向异性及其深度范围提供约束,可作为剪切波分裂和面波层析成像技术的补充.     

Influence of the crystal field effect on chemical transport in Earth's mantle: Cr3+ and Ga3+ diffusion in periclase

Experiments were performed to determine concentration-dependent diffusion coefficients of Cr³⁺ and Ga³⁺ in periclase at temperatures of 1563–2273 K. Diffusion profiles measured in the quenched samples are consistent with a theoretical model in which the mobile species is a bound M³⁺-vacancy pair, and each profile was fitted to determine the binding energy and diffusion coefficient of the pair. Trivalent chromium-vacancy pairs diffuse more slowly than Ga³⁺-vacancy pairs, and with higher migration energy, 237 kJ/mol vs. 190 kJ/mol. Cation vacancies also bind less tightly to Cr³⁺ than to Ga³⁺, with average binding free energies of ?22 and ?83 kJ/mol, respectively. At all concentrations and temperatures, Cr³⁺ diffuses much more slowly than Ga³⁺, by up to two orders of magnitude. The differences between Cr³⁺ and Ga³⁺ cannot be explained by differences in ionic radius or dipole polarizability, but are consistent with the influence of the crystal field on the partially occupied 3d orbitals of Cr³⁺. The crystal field splitting stabilizes Cr³⁺ on the octahedral cation site, increasing the energy required for Cr³⁺ to exchange positions with an adjacent vacancy. It also makes Cr³⁺-vacancy pairs less favorable, with the presence of a nearest-neighbor vacancy disrupting the symmetry of the octahedral site, thus diminishing the crystal field stabilization. Trends in the diffusion of first-row divalent transition metals in periclase can also be explained by the crystal field effect. High-spin to low-spin transitions in Fe²⁺, Co²⁺ or Mn²⁺ would significantly enhance their crystal field stabilization in periclase, and if such spin transitions occur in the deep mantle, they would be expected to slow the diffusivity of these ions significantly, perhaps by several orders of magnitude.     

The effect of the irregularities of the shape of the Earth's core-mantle boundary on the observed acceleration of gravity

Summary The disturbing gravitational effect of the irregularities of the shape of the coremantle boundary are studied at a point on the Earth's surface. The effect is computed for models of local and zonal distribution of the disturbing masses under the assumption that the total mass of the Earth remains constant. Formulae giving the disturbing gravitational effect as a function of the distribution of the disturbing masses and of the position of the point of observation on the Earth's surface are derived for the individual models. Numerical values of the disturbances have been calculate in all cases.     

Stability of the boundary layer between the lithosphere and convecting mantle and the steady-state lithospheric geotherm

In the steady state, the convective boundary layer (CBL) (the transition from the lithosphere to the convecting mantle, the lithosphere-asthenosphere boundary) is on the verge of stability. This determines its depth, thickness, and the steady-state temperature distribution in the lithosphere. Had the mantle been homogeneous, the base of the lithosphere at the current potential temperature would lie globally at the same depth H _rh of 50 to 70 km. Actually, the regime of interaction of the mantle convection with the lithosphere is determined by the relationship between this depth and the thickness H _depl of the chemical boundary layer including the crust and the layer of the depleted rock. If the thickness of the chemical boundary layer is small H _depl < H _rh, as it is the case in the present-day oceanic mantle, the suboceanic regime is established with the mantle convection that does not reach the base of the chemical boundary layer. In this case, the top of CBL is located at depth H _rh, while the oceanic heat flow and the depth of the seafloor only depend on the potential temperature T _p and, within the areas where the crust is older than 60 to 70 Ma, are the same everywhere far from the disturbed territories (the hot points and the subduction zones). The absence of noticeable distinctions between the heat flow in the different oceanic basins suggests a global constancy of the potential temperature. If H _depl > H _rh, the subcontinental regime of the interaction of the mantle convection with the lithosphere is established. In this case, the CBL is immediately adjacent to the depleted lithosphere, its top is located at depth H _depl, and the surface heat flow q(T _p, H _depl) not only depends on the potential temperature T _p but also on the the thickness of the depleted lithosphere H _depl; it decreases with increasing H _depl and, therefore, with the age of the lithosphere. Given the potential temperature, the dependence q(T _p, H _depl) agrees well with the envelope of the results of kimberlite xenolith thermobarometry presented in the diagram of the deepest xenolith depth as a function of the heat flow. It is likely that in the lowest part of the continental lithosphere there is a zone of horizontal shear deformation, from where kimberlites entrain the strongly deformed and, at the same time, the deepest xenoliths. Besides, the azimuthal anisotropy of seismic velocities can be associated with this zone. The change in its direction with depth can be observed as the Lehmann discontinuity.     

On Poisson's ratio and composition of the Earth's lower mantle

Poisson's ratio of the lower mantle, calculated from recently published values of seismic wave velocities and extrapolated to atmospheric pressure and room temperature is found to be in the range 0.23 ⩽ ν ⩽ 0.25. These values are compared with the values of Poisson's ratio calculated for binary mixtures of MgSiO₃ perovskite and magnesiowüstite with various iron contents. Current values of the experimental error on measured elastic moduli give little hope to be able to discriminate between pyrolite and chondritic lower mantles: both are acceptable if the shear modulus of perovskite is in the upper range of Liebermann et al. estimates. A similar calculation using the seismic parameter φ confirms the results obtained by considering Poisson's ratio and further constrains the value of the shear modulus of perovskite to lie between 1600 and 1700 kilobars for current mantle models to remain plausible. Chemical stratification of the mantle is, therefore, possible but not required by seismological data.     

On the diffusion of the perturbing toroidal magnetic field from the core to the mantle

Zusammenfassung Es werden die Wurzeln einer charakteristischen Gleichung für die Randbedingung der Diffusion des toroidalen magnetischen Störungsfeldes berechnet, das in dünner Schicht unterhalb der Grenze Kern — Mantel, als eine Folge von Störung der Winkelgeschwindigkeit dieser Schicht in Anwesenheit des magnetischen Dipolfeldes induziert wird. Die Wurzeln werden für verschiedene Werte des Verhältnisses der Leitfähigkeit des Erdmantels und Erdkerns ausgerechnet. Es wird gezeigt, dass das aus dem Milieu mit der elektrischen Leitfähigkeit in das Milieu mit elektrischer Leitfähigkeit _m diffundierende toroidale magnetische Störungsfeld vom Werte _m/<10^–2 an von dem Verhältnis elektrischer Leitfähigkeiten beider Milieus unabhänging bleibt.     

The effect of an interface boundary layer on the resonance properties of a buried structure

This paper presents a model for the analysis of the diffraction of plane waves at a cavity in an infinite homogeneous linear elastic medium supported by a segmented lining. An elastic boundary layer is introduced between the cavity lining and the infinite medium. The boundary layer is simulated by ‘elastic boundary conditions’ in which the stress is proportional to the relative displacement of the lining and of the surrounding medium boundary. A closed‐form analytical solution of the problem was obtained using the Fourier–Bessel series, the convergence of which was proven. It was shown that the number of series terms required to obtain a desired level of accuracy can be determined in advance. Using amplitude–frequency response analysis it was shown that the boundary layer produces additional ‘pseudo‐resonance’ frequencies that depend on the layer properties. These frequencies are almost identical to the eigenvalues obtained from the simple analysis of a segmented elastically supported lining. Copyright © 2003 John Wiley & Sons, Ltd.     

The effect of turbulent flow structures on saltation sand transport in the atmospheric boundary layer

The effect of turbulent flow structures on saltation sand transport was studied during two convective storms in Niger, West Africa. Continuous, synchronous measurements of saltation fluxes and turbulent velocity fluctuations were made with a sampling frequency of 1 Hz. The shear stress production was determined from the vertical and streamwise velocity fluctuations. The greatest stress-bearing events were classified as turbulent structures, with sweep, ejection, inward interaction, and outward interaction described according to the quadrant technique. The classified turbulent structures accounted for 63·5 per cent of the average shear stress during the first storm, and 56·0 per cent during the second storm. The percentage of active time was only 20·6 per cent and 15·8 per cent, respectively. High saltation fluxes were associated with sweeps and outward interactions. These two structures contribute positively (sweeps) and negatively (outward interactions) to the shear stress, but have in common that the streamwise velocity component is higher than average. Therefore, the horizontal drag force seems primarily responsible for saltation sand transport, and not the shear stress. This was also reflected by the low correlation coefficients (r) between shear stress and saltation flux (0·12 and 0·14, respectively), while the correlation coefficients between the streamwise velocity component and saltation flux were much higher (0·65 and 0·57, respectively). © 1998 John Wiley & Sons, Ltd.     

地幔热导率的选取对动力学数值模拟的影响——以岩石圈张裂过程为例

目前存在有多种地幔热导率模型,不同模型在数值和随温压变化的特征上有明显的差异.为探究不同热导率模型对动力学数值模拟结果的影响,本文对不同模型下的岩石圈张裂过程进行模拟研究,探讨地幔热导率对岩石圈热传输、变形和熔融过程的影响及其作用机理.结果显示,不同热导率模型下,岩石圈的变形和熔融特征表现出明显差异.高热导率模型下,岩石圈破裂较晚,形成陆缘较为宽阔,地壳熔融强烈而地幔熔融较弱;低热导率模型下,岩石圈破裂较早,形成陆缘较为狭窄,地幔熔融强烈而地壳熔融较弱.这种差异源于不同地幔热导率下岩石圈和地幔热状态的变化及相应力学性质的改变.高热导率下,热传导的增温效应显著,岩石圈呈现较热的状态,其强度整体较低,壳幔耦合减弱;而低热导率下,热对流的增温效应显著,岩石圈呈较冷的状态,其强度整体较高,壳幔耦合增强.基于模拟结果,本文认为地幔热导率的选取对动力学模拟的结果有着较为显著的影响,相对于随温压的变化,热导率数值的差异对动力学数值模拟的结果影响更大,尤其是对于地幔熔融过程的影响.