Zonal shear and super-rotation in a magnetized spherical Couette flow experiment

We present measurements performed in a spherical shell filled with liquid sodium, where a 74 mm-radius inner sphere is rotated while a 210 mm-radius outer sphere is at rest. The inner sphere holds a dipolar magnetic field and acts as a magnetic propeller when rotated. In this experimental set-up called DTS, direct measurements of the velocity are performed by ultrasonic Doppler velocimetry. Differences in electric potential and the induced magnetic field are also measured to characterize the magnetohydrodynamic flow. Rotation frequencies of the inner sphere are varied between -30 Hz and +30 Hz, the magnetic Reynolds number based on measured sodium velocities and on the shell radius reaching to about 33. We have investigated the mean axisymmetric part of the flow, which consists of differential rotation. Strong super-rotation of the fluid with respect to the rotating inner sphere is directly measured. It is found that the organization of the mean flow does not change much throughout the entire range of parameters covered by our experiment. The direct measurements of zonal velocity give a nice illustration of Ferraro's law of isorotation in the vicinity of the inner sphere where magnetic forces dominate inertial ones. The transition from a Ferraro regime in the interior to a geostrophic regime, where inertial forces predominate, in the outer regions has been well documented. It takes place where the local Elsasser number is about 1. A quantitative agreement with non-linear numerical simulations is obtained when keeping the same Elsasser number. The experiments also reveal a region that violates Ferraro's law just above the inner sphere.Comment: Phys Rev E, in pres

Firms' Main Market, Human Capital and Wages

Recent international trade literature emphasizes two features in characterizing the current patterns of trade: efficiency heterogeneity at the firm level and quality differentiation. This paper explores human capital and wage differences across firms in that context. We build a partial equilibrium model predicting that firms selling in more-remote markets employ higher human capital and pay higher wages to employees within each education group. The channel linking these variables is firms’ endogenous choice of quality. Predictions are tested using Spanish employer-employee matched data that classify firms according to four main destination markets: local, national, European Union, and rest of the World. Employees’ average education is increasing in the remoteness of firm’s main output market. Market–destination wage premia are large, increasing in the remoteness of the market, and increasing in individual education. These results suggest that increasing globalization may play a significant role in raising wage inequality within and across education groups

On the influence of a translating inner core in models of outer core convection

It has recently been proposed that the hemispheric seismic structure of the inner core can be explained by a self-sustained rigid-body translation of the inner core material, resulting in melting of the solid at the leading face and a compensating crystallisation at the trailing face. This process induces a hemispherical variation in the release of light elements and latent heat at the inner-core boundary, the two main sources of thermochemical buoyancy thought to drive convection in the outer core. However, the effect of a translating inner core on outer core convection is presently unknown. In this paper we model convection in the outer core with a nonmagnetic Boussinesq fluid in a rotating spherical shell driven by purely thermal buoyancy, incorporating the effect of a translating inner core by a time-independent spherical harmonic degree and order 1 (View the MathML sourceY11) pattern of heat-flux imposed at the inner boundary. The analysis considers Rayleigh numbers up to 10 times the critical value for onset of nonmagnetic convection, a parameter regime where the effects of the inhomogeneous boundary condition are expected to be most pronounced, and focuses on varying q∗q∗, the amplitude of the imposed boundary anomalies. The presence of inner boundary anomalies significantly affects the behaviour of the model system. Increasing q∗q∗ leads to flow patterns dominated by azimuthal jets that span large regions of the shell where radial motion is significantly inhibited. Vigorous convection becomes increasingly confined to isolated regions as q∗q∗ increases; these regions do not drift and always occur in the hemisphere subjected to a higher than average boundary heat-flux. Effects of the inner boundary anomalies are visible at the outer boundary in all inhomogeneous models considered. At low q∗q∗ the expression of inner boundary effects at the core surface is a difference in the flow speed between the two hemispheres. As q∗q∗ increases the spiralling azimuthal jets driven from the inner boundary are clearly visible at the outer boundary. Finally, our results suggest that, when the system is heated from below, a View the MathML sourceY11 heat-flux pattern imposed on the inner boundary has a greater overall influence on the spatio-temporal behaviour of the flow than the same pattern imposed at the outer boundary

Cooling history of Earth's core with high thermal conductivity

Thermal evolution models of Earth's core constrain the power available to the geodynamo process that generates the geomagnetic field, the evolution of the solid inner core and the thermal history of the overlying mantle. Recent upward revision of the thermal conductivity of liquid iron mixtures by a factor of 2-3 has drastically reduced the estimated power available to generate the present-day geomagnetic field. Moreover, this high conductivity increases the amount of heat that is conducted out of the core down the adiabatic gradient, bringing it into line with the highest estimates of present-day core-mantle boundary heat flow. These issues raise problems with the standard scenario of core cooling in which the core has remained completely well-mixed and relatively cool for the past 3.5. Ga. This paper presents cooling histories for Earth's core spanning the last 3.5. Ga to constrain the thermodynamic conditions corresponding to marginal dynamo evolution, i.e. where the ohmic dissipation remains just positive over time. The radial variation of core properties is represented by polynomials, which gives good agreement with radial profiles derived from seismological and mineralogical data and allows the governing energy and entropy equations to be solved analytically. Time-dependent evolution of liquid and solid light element concentrations, the melting curve, and gravitational energy are calculated for an Fe-O-S-Si model of core chemistry. A suite of cooling histories are presented by varying the inner core boundary density jump, thermal conductivity and amount of radiogenic heat production in the core. All models where the core remains superadiabatic predict an inner core age of ≲600Myr, about two times younger than estimates based on old (lower) thermal conductivity estimates, and core temperatures that exceed present estimates of the lower mantle solidus prior to the last 0.5-1.5. Ga. Allowing the top of the core to become strongly subadiabatic in recent times pushes the onset of inner core nucleation back to ~1.5Gyr, but the ancient core temperature still implies a partially molten mantle prior to ~2Ga. Based on these results, the scenario of a long-lived basal magma ocean and subadiabatic present-day core seems hard to avoid

Age and skill bias of trade liberalisation? : heterogeneous employment effects of EU Eastern Enlargement

This study analyses the 2004 Eastern Enlargement to the European Union to obtain evidence on the employment effects of an increase in trade liberalisation. The Enlargement is thought to generate a trade-induced demand shock with no (or only limited) supply effects. Besides the variation over time induced by the Enlargement, identification of the effects is based on a Melitz (2003) type productivity term to differentiate firms by the extent of exposure to the demand shock. The idea is that the effects of the demand shock should be driven by differences in firm-level productivity from the period before the new member countries actually entered the EU. German linked employer-employee data allow to observe the relation of initial establishment productivity with employment changes over a long panel from 1995 to 2009. The estimates show that the Enlargement had a negative effect on establishment-level employment growth, which is driven by increased worker separations and increased job destruction. Besides the overall employment effect, the study focuses on effect heterogeneity across age and skill groups of the workforce. These estimates point to a skill bias in the effect of the Enlargement that disadvantages low- and medium-skilled workers in terms of higher worker separation and job destruction. In addition, lowskilled workers suffer fewer accessions by firms, where against medium-skilled workers enjoy increased accessions and creation of new jobs. Besides this indication for a skill bias, there are no clear indications that point to an age bias in the employment effect of the Eastern Enlargement

Solubility of Rock in Steam Atmospheres of Planets

Extensive experimental studies show that all major rock-forming elements (e.g., Si, Mg, Fe, Ca, Al, Na, K) dissolve in steam to a greater or lesser extent. We use these results to compute chemical equilibrium abundances of rocky-element-bearing gases in steam atmospheres equilibrated with silicate magma oceans. Rocky elements partition into steam atmospheres as volatile hydroxide gases (e.g., Si(OH)4, Mg(OH)2, Fe(OH)2, Ni(OH)2, Al(OH)3, Ca(OH)2, NaOH, KOH) and via reaction with HF and HCl as volatile halide gases (e.g., NaCl, KCl, CaFOH, CaClOH, FAl(OH)2) in much larger amounts than expected from their vapor pressures over volatile-free solid or molten rock at high temperatures expected for steam atmospheres on the early Earth and hot rocky exoplanets. We quantitatively compute the extent of fractional vaporization by defining gas/magma distribution coefficients and show that Earth's subsolar Si/Mg ratio may be due to loss of a primordial steam atmosphere. We conclude that hot rocky exoplanets that are undergoing or have undergone escape of steam-bearing atmospheres may experience fractional vaporization and loss of Si, Mg, Fe, Ni, Al, Ca, Na, and K. This loss can modify their bulk composition, density, heat balance, and interior structure