Modeling of nutation-precession: very long baseline interferometry results

Analysis of over 20 years of very long baseline interferometry data (VLBI) yields estimates of the coefficients of the nutation series with standard deviations ranging from 5 microseconds of arc (μas) for the terms with periods <400 days to 38 µas for the longest-period terms. The largest deviations between the VLBI estimates of the amplitudes of terms in the nutation series and the theoretical values from the Mathews-Herring-Buffett (MHB2000) nutation series are 56 ± 38 μas (associated with two of the 18.6 year nutations). The amplitudes of nutational terms with periods <400 days deviate from the MHB2000 nutation series values at the level standard deviation. The estimated correction to the IAU-1976 precession constant is -2.997 ± 0.008 mas yr-1 when the coefficients of the MHB2000 nutation series are held fixed and is consistent with that inferred from the MHB2000 nutation theory. The secular change in the obliquity of the ecliptic is estimated to be -0.252 ± 0.003 mas yr-1. When the coefficients of the largest-amplitude terms in the nutation series are estimated, the precession constant correction and obliquity rate are estimated to be -2.960 ± 0.030 and -0.237 ± 0.012 mas yr-1. Significant variations in the freely excited retrograde free core nutation mode are observed over the 20 years. During this time the amplitude has decreased from -300 ± 50 μas in the mid-1980s to nearly zero by the year 2000. There is evidence that the amplitude of the mode in now increasing again

Effects of anisotropy in geostrophic turbulence

The Boussinesq model of convection in a flat layer with heating from below is considered. We analyze the effects of anisotropy caused by rapid rotation in physical and wave spaces and demonstrate the suppression of energy transfer by rotation. We also examine the structure of the wave triangle in nonlinear interaction. The range of parameters is adapted to the models of convection in the geodynamo

Thermal and electrical conductivity of iron at Earth's core conditions

The Earth acts as a gigantic heat engine driven by decay of radiogenic isotopes and slow cooling, which gives rise to plate tectonics, volcanoes, and mountain building. Another key product is the geomagnetic field, generated in the liquid iron core by a dynamo running on heat released by cooling and freezing to grow the solid inner core, and on chemical convection due to light elements expelled from the liquid on freezing. The power supplied to the geodynamo, measured by the heat-flux across the core-mantle boundary (CMB), places constraints on Earth's evolution. Estimates of CMB heat-flux depend on properties of iron mixtures under the extreme pressure and temperature conditions in the core, most critically on the thermal and electrical conductivities. These quantities remain poorly known because of inherent difficulties in experimentation and theory. Here we use density functional theory to compute these conductivities in liquid iron mixtures at core conditions from first principles- the first directly computed values that do not rely on estimates based on extrapolations. The mixtures of Fe, O, S, and Si are taken from earlier work and fit the seismologically-determined core density and inner-core boundary density jump. We find both conductivities to be 2-3 times higher than estimates in current use. The changes are so large that core thermal histories and power requirements must be reassessed. New estimates of adiabatic heat-flux give 15-16 TW at the CMB, higher than present estimates of CMB heat-flux based on mantle convection; the top of the core must be thermally stratified and any convection in the upper core driven by chemical convection against the adverse thermal buoyancy or lateral variations in CMB heat flow. Power for the geodynamo is greatly restricted and future models of mantle evolution must incorporate a high CMB heat-flux and explain recent formation of the inner core.Comment: 11 pages including supplementary information, two figures. Scheduled to appear in Nature, April 201

Agrárpiaci Jelentések TEJ ÉS TEJTERMÉKEK

Magyarországon a nyerstej országos termelői átlagára 82,84 forint/kg volt 2015 áprilisában, ami 23 százalékos csökkenést jelent az előző év azonos hónapjának átlagárához képest. A zsírtartalom 0,02 százalékpontos, a fehérje-tartalom 0,04 százalékpontos mérséklődése hozzájárult a nyerstej árának 5 százalékos csökkenéséhez áprilisban a márciusihoz képest. A nyerstej felvásárlása 13 százalékkal nőtt ugyanekkor. Az Európai Bizottság adatai szerint az év első két hónapját tekintve az Európai Unió tagországai közül Magyarországon nőtt a legerőteljesebben, 10 száza-lékkal a nyerstej felvásárlása az egy évvel korábbihoz viszonyítva. A nyerstej kiviteli ára 74,92 forint/kg volt áprilisban, egy év alatt 29 százalékkal esett, és 10 százalékkal maradt el a belpiaci ártól. Az AKI PÁIR adatai szerint nyerstej kiszállítása 19 százalékkal nőtt a vizsgált időszakban, ezen belül a termelők és a kereskedők 10 százalékkal kevesebb, míg a feldolgozók 121 százalékkal több nyerstejet expor-táltak. A termelők és a kereskedők nyerstejkivitele 41 százalékkal haladta meg feldolgozókét. Az Európai Unió a tejkvóta megszűntetése után, figyelembe véve a tejtermelés májusi szezonális csúcspontját, jelentős mennyiségű tejterméket exportálhat a világpiacra. Az új-zélandi Westland tejfeldolgozó szerint az európai kínálat bővülésével a következő három hónapban túlkínálatra és az árak jelentős ingadozására lehet számítani. Előzetes adatok szerint a zsírtartalommal korrigált nyerstejfelvásárlás Ausztriában és Lengyelországban egyaránt 5,8 százalékkal, Írországban 4,3 százalékkal, Hollandiában 4,1 százalékkal, Németországban 3,7 százalékkal, Dáni-ában 1,26 százalékkal haladta meg, míg Csehországban 1,16 százalékkal, Franciaországban 4,3 százalékkal maradt el a 2014/2015. tejkvótaévben (április-március) a rendelkezésre álló tejkvótától. Olaszországban a kvótaév első 11 hónapjában a túllépés 2,69 százalékos volt

Multidisciplinary Constraints on the Thermal-Chemical Boundary Between Earth's Core and Mantle

Abstract: Heat flux from the core to the mantle provides driving energy for mantle convection thus powering plate tectonics, and contributes a significant fraction of the geothermal heat budget. Indirect estimates of core‐mantle boundary heat flow are typically based on petrological evidence of mantle temperature, interpretations of temperatures indicated by seismic travel times, experimental measurements of mineral melting points, physical mantle convection models, or physical core convection models. However, previous estimates have not consistently integrated these lines of evidence. In this work, an interdisciplinary analysis is applied to co‐constrain core‐mantle boundary heat flow and test the thermal boundary layer (TBL) theory. The concurrence of TBL models, energy balance to support geomagnetism, seismology, and review of petrologic evidence for historic mantle temperatures supports QCMB ∼15 TW, with all except geomagnetism supporting as high as ∼20 TW. These values provide a tighter constraint on core heat flux relative to previous work. Our work describes the seismic properties consistent with a TBL, and supports a long‐lived basal mantle molten layer through much of Earth's history

Back-arc strain in subduction zones: Statistical observations versus numerical modeling

International audience1] Recent statistical analysis by Lallemand et al. (2008) of subduction zone parameters revealed that the back-arc deformation mode depends on the combination between the subducting (nu(sub)) and upper (nu(up)) plate velocities. No significant strain is recorded in the arc area if plate kinematics verifies nu(up) = 0.5 vsub - 2.3 (cm/a) in the HS3 reference frame. Arc spreading ( shortening) occurs if nu(up) is greater ( lower) than the preceding relationship. We test this statistical law with numerical models of subduction, by applying constant plate velocities far away from the subduction zone. The subducting lithosphere is free to deform at all depths. We quantify the force applied on the two converging plates to sustain constant surface velocities. The simulated rheology combined viscous (non-Newtonian) and brittle behaviors, and depends on water content. The influence of subduction rate vs is first studied for a fixed upper plate. After 950 km of convergence ( steady state slab pull), the transition from extensional to compressive stresses in the upper plate occurs for vs similar to 1.4 cm/a. The effect of upper plate velocity is then tested at constant subduction rate. Upper plate retreat ( advance) with respect to the trench increases extension ( compression) in the arc lithosphere and increases ( decreases) the subducting plate dip. Our modeling confirms the statistical kinematic relationship between vsub and nu(up) that describes the transition from extensional to compressive stresses in the arc lithosphere, even if the modeled law is shifted toward higher rates of upper plate retreat, using our set of physical parameters ( e. g., 100 km thick subducting oceanic plate) and short- term simulations. Our results make valid the choice of the HS3 reference frame for assessing plate velocity influence on arc tectonic regime. The subduction model suggests that friction along the interplate contact and the mantle Stokes reaction could be the two main forces competing against slab pull for upper mantle subductions. Besides, our simulations show that the arc deformation mode is strongly time dependent

Inertia-less convectively-driven dynamo models in the limit of low Rossby number and large Prandtl number

Compositional convection is thought to be an important energy source for magnetic field generation within planetary interiors. The Prandtl number, Pr , characterizing compositional convection is significantly larger than unity, suggesting that the inertial force may not be important on the small scales of convection as long as the buoyancy force is not too strong. We develop asymptotic dynamo models for the case of small Rossby number and large Prandtl number in which inertia is absent on the convective scale. The relevant diffusivity parameter for this limit is the compositional Roberts number, q=D/η, which is the ratio of compositional and magnetic diffusivities. Dynamo models are developed for both order one q and the more geophysically relevant low q limit. For both cases the ratio of magnetic to kinetic energy densities, M , is asymptotically large and reflects the fact that Alfvén waves have been filtered from the dynamics. Along with previous investigations of asymptotic dynamo models for Pr=O(1), our results show that the ratio M is not a useful indicator of dominant force balances in the momentum equation since many different asymptotic limits of M can be obtained without changing the leading order geostrophic balance. Furthermore, the present models show that inertia is not a requirement for driving low q, large-scale dynamos