Energy barriers in three-dimensional micromagnetic models and the physics of thermoviscous magnetization

This article has been accepted for publication in Geophysical Journal International © 2018. Published by Oxford University Press on behalf of the Royal Astronomical Society. All rights reserved. Source at https://doi.org/10.1093/gji/ggy285.A first-principle micromagnetic and statistical calculation of viscous remanent magnetization (VRM) in an ensemble of cubic magnetite pseudo-single domain (PSD) particles is presented. The theoretical methods developed apply to all magnetic particle sizes, from single domain to multidomain. The numerical implementation is based on a fast relaxation algorithm for finding optimal transition paths between micromagnetic local energy minima. The algorithm combines a nudged-elastic-band technique with action minimization. Initial paths are obtained by repetitive minimizations of modified energy functions. For a cubic PSD particle, 60 different local energy minima are identified, and all optimal energy barriers between them are numerically calculated for the case of zero external field. These results are used to estimate the energy barriers in weak external fields. Based on these, time-dependent transition matrices are constructed, which fully describe the continuous homogeneous Markov processes of VRM acquisition and decay. By spherical averaging, the acquisition of remanent magnetization in an isotropic PSD ensemble is calculated from laboratory to geological timescales. The modelled particle ensemble shows a physically meaningful overshooting of magnetization during VRM acquisition. The results also clarify why VRM acquisition in PSD particles can occur much faster than VRM decay, and therefore explain occurrence of extremely stable VRM as found in some palaeomagnetic studies

Applicability of the Peclet number approach to blow-off and flashback limits of common steelworks process gases

The ever-increasing importance of energy efficiency has given rise to numerous areas of concern for operators and developers of combustion plants; as the need to utilise fuel gases of increasingly poor quality and variability is essential for sustainability, while emission standards continuing to become more stringent. Swirl combustors are ubiquitous in industry owing to their great stability range which occurs due to the formation of a CRZ, which through the recycling of heat and active chemical species to the root of the flame enhances stability over a wide range of operating conditions. Alternative fuels containing hydrogen offer the possibility of reduced greenhouse gas emissions; however flashback is of special concern with hydrogen enriched fuels, owing to the very high flame speed of hydrogen. Many by-products of process and waste industries can include a high proportion of hydrogen, for example Coke Oven Gas. Alternatively, many by-product process gases can contain a high proportion of non-combustible species such as nitrogen and carbon dioxide which can substantially reduce their flame speed and as a consequence increase the possibility of the flame extinguishing through blow-off. This paper examines the blow-off and flashback potential of common steelworks process gases (including one which contains hydrogen) in a compact, premixed swirl burner in swirl number regimes representative of those found in practical systems. Methane is used as a base fuel for comparison. All results are obtained at atmospheric pressure without air preheat. The Peclet number modelling approach incorporating a flame quenching parameter was applied to the results obtained for each of the fuel gases. Using this model, the quench factor value was seen to be dependent on burner configuration as well as fuel composition. It was found that the stable burner operating conditions significantly change from fuel to fuel; with the operating points at which flashback occurs with Coke Oven Gas producing blow-off with weaker process gases such as Blast Furnace Gas and Basic Oxygen Steelmaking gas

MANGANESE MINERALISATIONS AT THE BASE OF MIOCENE SEDIMENTS IN NORTHERN SARDINIA (ITALY)

During the eastward drift of the Palaeozoic-Mesozoic block formed by Sardinia and Corsica in the Oligocene-Miocene, calc-alkaline volcanism developed mostly in the western part of the island. Most Tertiary metallogenic phoenomena are related to hydrothermal activity associated with this volcanism. Following volcanic and related hydrothermal activity, sediments were deposited during the Oligocene-Miocene as a consequence of a marine transgression. The basal part of this series is clastic and includes elements derived from erosion of unaltered volcanics as well as hydrothermally altered rocks and hydrothermal vein quartz. Inside the Tertiary volcanics manganese ore-minerals occur as nodules, veinlets, and stockworks and mainly include Mn and Fe oxides; quartz in different forms is the most common gangue mineral. The mineralisations at the contact between volcanics and Miocene sediments are the most homogeneous, the ore-minerals occur in the cement, but also as fairly continuous thin beds, nodules and veinlets containing pyrolusite, frequent ramsdellite, less frequent manganite, psilomelane, cryptomelane-manjiroite, rare ranciéite, and todorokite. The nature of the ore-bearing beds indicate a near-shore clastic environment along the ancient coastal lines of the Miocene sea. Genetic considerations point to a supergenic transport and redeposition after erosion of primary dispersion and residual concentrations of Mn in the volcanics

Reshaping the African Internet: From scattered islands to a connected continent

There is an increasing awareness amongst developing regions on the importance of localizing Internet traffic in the quest for fast, affordable, and available Internet access. In this paper, we focus on Africa, where 37 IXPs are currently interconnecting local ISPs, but mostly at the country level. An option to enrich connectivity on the continent and incentivize content providers to establish presence in the region is to interconnect ISPs present at isolated IXPs by creating a distributed IXP layout spanning the continent. The goal of this paper is to investigate whether such IXP interconnection would be possible, and if successful, to estimate the best-case benefits that could be realized in terms of traffic localization and performance. Our hope is that quantitatively demonstrating the benefits will provide incentives for ISPs to intensify their peering relationships in the region. However, it is challenging to estimate this best-case scenario, due to numerous economic, political, and geographical factors influencing the region. Towards this end, we begin with a thorough analysis of the environment in Africa. We then investigate a naive approach to IXP interconnection, which shows that a theoretically optimal solution would be infeasible in practice due to the prevailing socio-economic conditions in the region. We therefore provide an innovative, realistic four-step interconnection scheme to achieve the distributed IXP layout that considers and parameterizes external socio-economic factors using publicly available datasets. We demonstrate that our constrained solution doubles the percentage of continental intra-African paths, reduces their lengths, and drastically decreases the median of their RTTs as well as RTTs to ASes hosting the top 10 global and top 10 regional Alexa websites. Our approach highlights how, given real-world constraints, a solution requires careful considerations in order to be practically realizable.Rodérick Fanou was partially supported by IMDEA Networks Institute, US NSF grant CNS-1414177, and the project BRADE (P2013/ICE-2958) from the Directorate General of Universities and Research, Board of Education, Madrid Regional Governement. Francisco Valera was partially funded by the European Commission under FP7 project LEONE (FP7-317647). Amogh Dhamdhere was partially funded by US NSF grants CNS-1414177 and CNS-1513847.Publicad

Frequency-dependent mobilization of heterogeneous pools of synaptic vesicles shapes presynaptic plasticity

The segregation of the readily releasable pool of synaptic vesicles (RRP) in sub-pools that are differentially poised for exocytosis shapes short-term plasticity. However, the frequency-dependent mobilization of these sub-pools is poorly understood. Using slice recordings and modeling of synaptic activity at cerebellar granule cell to Purkinje cell synapses of mice, we describe two sub-pools in the RRP that can be differentially recruited upon ultrafast changes in the stimulation frequency. We show that at low frequency stimulations, a first sub-pool is gradually silenced, leading to full blockage of synaptic transmission. Conversely, a second pool of synaptic vesicles that cannot be released by a single stimulus is recruited within milliseconds by high-frequency stimulation and support an ultrafast recovery of neurotransmitter release after low-frequency depression. This frequency-dependent mobilization or silencing of sub-pools in the RRP in terminals of granule cells may play a role in the filtering of sensorimotor information in the cerebellum

Primera valoración genética Blup en el caballo trotador español

Tras la aprobación del Esquema de Selección específico para el Caballo Trotador Español (l de septiembre de 2005), se ha realizado la primera Valoración Genética para carreras de trote con metodología BLUP modelo animal en España, en base a los resullados deportivos obtenidos por los animales de esta raza en 4.180 carreras celebradas durante los años 2003-2005. Trabajando con un total de 42.993 registros de participación pertenecientes a 1.580 animales (832 machos, 593 hembras y 155 castrados), se estimaron los parámetros genéticos y los valores de cría para los tres caracteres seleccionados: ganancias anuales (GA), mejor tiempo anual (MTPA) y porcentaje de puestos clasificado entre el primero y el cuarto (PPC). El modelo genético-estadístico empleado incluye el sexo del animal, el año de nacimiento, el año de carrera y el número de participaciones registradas cada año (covariable de GA) como efectos fijos; y el efecto aditivo directo y el efecto ambiental permanente como factores aleatorios