330 research outputs found
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
Intermittent magnetic field excitation by a turbulent flow of liquid sodium
The magnetic field measured in the Madison Dynamo Experiment shows
intermittent periods of growth when an axial magnetic field is applied. The
geometry of the intermittent field is consistent with the fastest growing
magnetic eigenmode predicted by kinematic dynamo theory using a laminar model
of the mean flow. Though the eigenmodes of the mean flow are decaying, it is
postulated that turbulent fluctuations of the velocity field change the flow
geometry such that the eigenmode growth rate is temporarily positive.
Therefore, it is expected that a characteristic of the onset of a turbulent
dynamo is magnetic intermittency.Comment: 5 pages, 7 figure
The Cooperative Participatory Evaluation of Renewable Technologies on Ecosystem Services (CORPORATES)
Publisher PD
Reversals in nature and the nature of reversals
The asymmetric shape of reversals of the Earth's magnetic field indicates a
possible connection with relaxation oscillations as they were early discussed
by van der Pol. A simple mean-field dynamo model with a spherically symmetric
coefficient is analysed with view on this similarity, and a comparison
of the time series and the phase space trajectories with those of paleomagnetic
measurements is carried out. For highly supercritical dynamos a very good
agreement with the data is achieved. Deviations of numerical reversal sequences
from Poisson statistics are analysed and compared with paleomagnetic data. The
role of the inner core is discussed in a spectral theoretical context and
arguments and numerical evidence is compiled that the growth of the inner core
might be important for the long term changes of the reversal rate and the
occurrence of superchrons.Comment: 24 pages, 12 figure
Cattle remain immunocompetent during the acute phase of foot-and-mouth disease virus infection
Infection of cattle with foot-and-mouth disease virus (FMDV) results in the development of long-term protective antibody responses. In contrast, inactivated antigen vaccines fail to induce long-term protective immunity. Differences between susceptible species have also been observed during infection with FMDV, with cattle often developing persistent infections whilst pigs develop more severe symptoms and excrete higher levels of virus. This study examined the early immune response to FMDV in naĂŻve cattle after in-contact challenge. Cattle exposed to FMDV were found to be viraemic and produced neutralising antibody, consistent with previous reports. In contrast to previous studies in pigs these cattle did not develop leucopenia, and the proliferative responses of peripheral blood mononuclear cells to either mitogen or third party antigen were not suppressed. Low levels of type 1 interferon and IL-10 were detected in the circulation. Taken together, these results suggest that there was no generalised immunosuppression during the acute phase of FMDV infection in cattle
Associations between lamb survival and prion protein genotype: analysis of data for ten sheep breeds in Great Britain
BACKGROUND: Selective breeding programmes, based on prion protein (PrP) genotype, have been introduced throughout the European Union to reduce the risk of sheep transmissible spongiform encephalopathies (TSEs). These programmes could have negative consequences on other important traits, such as fitness and production traits, if the PrP gene has pleiotropic effects or is in linkage disequilibrium with genes affecting these traits. This paper presents the results of an investigation into associations between lamb survival and PrP genotype in ten mainstream sheep breeds in Great Britain (GB). In addition, the reasons for lamb deaths were examined in order to identify any associations between these and PrP genotype. RESULTS: Survival times from birth to weaning were analysed for over 38000 lambs (2427 dead and 36096 live lambs) from 128 flocks using Cox proportional hazard models for each breed, including additive animal genetic effects. No significant associations between PrP genotype and lamb survival were identified, except in the Charollais breed for which there was a higher risk of mortality in lambs of the ARR/VRQ genotype compared with those of the ARR/ARR genotype. Significant effects of birth weight, litter size, sex, age of dam and year of birth on survival were also identified. For all breeds the reasons for death changed significantly with age; however, no significant associations between reason for death and PrP genotype were found for any of the breeds. CONCLUSION: This study found no evidence to suggest that a selective breeding programme based on PrP genotype will have a detrimental effect on lamb survival. The only significant effect of PrP genotype identified was likely to be of little consequence because an increased risk of mortality was associated with a genotype that is selected against in current breeding strategies
Variational data assimilation for the initial-value dynamo problem
The secular variation of the geomagnetic field as observed at the Earth's surface results from the complex magnetohydrodynamics taking place in the fluid core of the Earth. One way to analyze this system is to use the data in concert with an underlying dynamical model of the system through the technique of variational data assimilation, in much the same way as is employed in meteorology and oceanography. The aim is to discover an optimal initial condition that leads to a trajectory of the system in agreement with observations. Taking the Earth's core to be an electrically conducting fluid sphere in which convection takes place, we develop the continuous adjoint forms of the magnetohydrodynamic equations that govern the dynamical system together with the corresponding numerical algorithms appropriate for a fully spectral method. These adjoint equations enable a computationally fast iterative improvement of the initial condition that determines the system evolution. The initial condition depends on the three dimensional form of quantities such as the magnetic field in the entire sphere. For the magnetic field, conservation of the divergence-free condition for the adjoint magnetic field requires the introduction of an adjoint pressure term satisfying a zero boundary condition. We thus find that solving the forward and adjoint dynamo system requires different numerical algorithms. In this paper, an efficient algorithm for numerically solving this problem is developed and tested for two illustrative problems in a whole sphere: one is a kinematic problem with prescribed velocity field, and the second is associated with the Hall-effect dynamo, exhibiting considerable nonlinearity. The algorithm exhibits reliable numerical accuracy and stability. Using both the analytical and the numerical techniques of this paper, the adjoint dynamo system can be solved directly with the same order of computational complexity as that required to solve the forward problem. These numerical techniques form a foundation for ultimate application to observations of the geomagnetic field over the time scale of centuries
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