2,083 research outputs found
Iron Snow in the Martian Core?
The decline of Mars' global magnetic field some 3.8â4.1 billion years ago is thought to reflect the demise of the dynamo that operated in its liquid core. The dynamo was probably powered by planetary cooling and so its termination is intimately tied to the thermochemical evolution and present-day physical state of the Martian core. Bottom-up growth of a solid inner core, the crystallization regime for Earth's core, has been found to produce a long-lived dynamo leading to the suggestion that the Martian core remains entirely liquid to this day. Motivated by the experimentally-determined increase in the FeâS liquidus temperature with decreasing pressure at Martian core conditions, we investigate whether Mars' core could crystallize from the top down. We focus on the âiron snowâ regime, where newly-formed solid consists of pure Fe and is therefore heavier than the liquid. We derive global energy and entropy equations that describe the long-timescale thermal and magnetic history of the core from a general theory for two-phase, two-component liquid mixtures, assuming that the snow zone is in phase equilibrium and that all solid falls out of the layer and remelts at each timestep. Formation of snow zones occurs for a wide range of interior and thermal properties and depends critically on the initial sulfur concentration, Ο0. Release of gravitational energy and latent heat during growth of the snow zone do not generate sufficient entropy to restart the dynamo unless the snow zone occupies at least 400 km of the core. Snow zones can be 1.5â2 Gyrs old, though thermal stratification of the uppermost core, not included in our model, likely delays onset. Models that match the available magnetic and geodetic constraints have Ο0â10% and snow zones that occupy approximately the top 100 km of the present-day Martian core
A multiple replica approach to simulate reactive trajectories
A method to generate reactive trajectories, namely equilibrium trajectories
leaving a metastable state and ending in another one is proposed. The algorithm
is based on simulating in parallel many copies of the system, and selecting the
replicas which have reached the highest values along a chosen one-dimensional
reaction coordinate. This reaction coordinate does not need to precisely
describe all the metastabilities of the system for the method to give reliable
results. An extension of the algorithm to compute transition times from one
metastable state to another one is also presented. We demonstrate the interest
of the method on two simple cases: a one-dimensional two-well potential and a
two-dimensional potential exhibiting two channels to pass from one metastable
state to another one
The connection between radio and high energy emission in black hole powered systems in the SKA era
Strong evidence exists for a highly significant correlation between the radio
flux density and gamma-ray energy flux in blazars revealed by Fermi. However,
there are central issues that need to be clarified in this field: what are the
counterparts of the about 30% of gamma-ray sources that are as yet
unidentified? Are they just blazars in disguise or they are something more
exotic, possibly associated with dark matter? How would they fit in the
radio-gamma ray connection studied so far?
With their superb sensitivity, SKA1-MID and SKA1-SUR will help to resolve all
of these questions. Even more, while the radio-MeV/GeV connection has been
firmly established, a radio-VHE connection has been entirely elusive so far.
The advent of CTA in the next few years and the expected CTA-SKA1 synergy will
offer the chance to explore this connection, even more intriguing as it
involves the opposite ends of the electromagnetic spectrum and the acceleration
of particles up to the highest energies.
We are already preparing to address these questions by exploiting data from
the various SKA pathfinders and precursors. We have obtained 18 cm European
VLBI Network observations of E>10 GeV sources, with a detection rate of 83%.
Moreover, we are cross correlating the Fermi catalogs with the MWA
commissioning survey: when faint gamma-ray sources are considered, pure
positional coincidence is not significant enough for selecting counterparts and
we need an additional physical criterion to pinpoint the right object. It can
be radio spectral index, variability, polarization, or compactness, needing
high angular resolution in SKA1-MID; timing studies can also reveal pulsars,
which are often found from dedicated searches of unidentified gamma-ray
sources. SKA will be the ideal instrument for investigating these
characteristics in conjunction with CTA.
(abridged)Comment: 12 pages, to be published in the proceedings of "Advancing
Astrophysics with the Square Kilometre Array", PoS(AASKA14)15
Marked long-term decline in ambient CO mixing ratio in SE England, 1997â2014:Evidence of policy success in improving air quality
Atmospheric CO at Egham in SE England has shown a marked and progressive decline since 1997, following adoption of strict controls on emissions. The Egham site is uniquely positioned to allow both assessment and comparison of âclean Atlantic backgroundâ air and CO-enriched air downwind from the London conurbation. The decline is strongest (approximately 50ppb per year) in the 1997â2003 period but continues post 2003. A âlocal CO incrementâ can be identified as the residual after subtraction of contemporary background Atlantic CO mixing ratios from measured values at Egham. This increment, which is primarily from regional sources (during anticyclonic or northerly winds) or from the European continent (with easterly air mass origins), has significant seasonality, but overall has declined steadily since 1997. On many days of the year CO measured at Egham is now not far above Atlantic background levels measured at Mace Head (Ireland). The results are consistent with MOPITT satellite observations and âbottom-upâ inventory results. Comparison with urban and regional background CO mixing ratios in Hong Kong demonstrates the importance of regional, as opposed to local reduction of CO emission. The Egham record implies that controls on emissions subsequent to legislation have been extremely successful in the UK
Prediction of silicate melt viscosity from electrical conductivity : a model and its geophysical implications
Author Posting. © American Geophysical Union, 2013. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Geochemistry, Geophysics, Geosystems 14 (2013): 1685â1692, doi:10.1002/ggge.20103.Our knowledge of magma dynamics would be improved if geophysical data could be used to infer rheological constraints in melt-bearing zones. Geophysical images of the Earth's interior provide frozen snapshots of a dynamical system. However, knowledge of a rheological parameter such as viscosity would constrain the time-dependent dynamics of melt bearing zones. We propose a model that relates melt viscosity to electrical conductivity for naturally occurring melt compositions (including H2O) and temperature. Based on laboratory measurements of melt conductivity and viscosity, our model provides a rheological dimension to the interpretation of electromagnetic anomalies caused by melt and partially molten rocks (melt fraction ~ >0.7).We acknowledge
partial support under NASA USRA subaward 02153â04,
NSF EAR 0739050, and the ASU School of Earth and Space
Exploration (SESE) Exploration Postdoctoral Fellowship
Program.2013-12-1
`Zwicky's Nonet': a compact merging ensemble of nine galaxies and 4C 35.06, a peculiar radio galaxy with dancing radio jets
We report the results of our radio, optical and infra-red studies of a
peculiar radio source 4C~35.06, an extended radio-loud AGN at the center of
galaxy cluster Abell 407 (). The central region of this cluster hosts
a remarkably tight ensemble of nine galaxies, the spectra of which resemble
those of passive red ellipticals, embedded within a diffuse stellar halo of
1~arcmin size. This system (named the `Zwicky's Nonet') provides unique
and compelling evidence for a multiple-nucleus cD galaxy precursor.
Multifrequency radio observations of 4C~35.06 with the Giant Meterwave Radio
Telescope (GMRT) at 610, 235 and 150 MHz reveal a system of 400~kpc scale
helically twisted and kinked radio jets and outer diffuse lobes. The outer
extremities of jets contain extremely steep spectrum (spectral index -1.7 to
-2.5) relic/fossil radio plasma with a spectral age of a few yr. Such ultra-steep spectrum relic radio lobes without definitive
hot-spots are rare, and they provide an opportunity to understand the
life-cycle of relativistic jets and physics of black hole mergers in dense
environments. We interpret our observations of this radio source in the context
of the growth of its central black hole, triggering of its AGN activity and jet
precession, all possibly caused by galaxy mergers in this dense galactic
system. A slow conical precession of the jet axis due to gravitational
perturbation between interacting black holes is invoked to explain the unusual
jet morphology.Comment: Published in MNRAS | No. of pages 12, 10 figures and 4 tables.
Comments are welcom
Clinical trials in children: Equity, quality and relevance
This thesis investigates the equity, quality and relevance of clinical trials in children to inform better evidence-based child healthcare and outcomes worldwide. A comprehensive review of the literature revealed that despite current initiatives to encourage more trials in children, there is still a paucity of safety and efficacy data of many medicines prescribed in this population. An analysis of trials registered in children showed that disease burden was moderately correlated to trials and this scarcity was particularly prevalent in low-and middle-income countries. We explored the contributory factors to this inequity by conducting a systematic review of stakeholdersâ views of trials in children in low-and middle-income countries. In the study evaluating the completeness of protocols of trials in children submitted to ethics committees, we found that protocols are generally comprehensive, but many key domains in trial design and conduct are not reported. Key-informant trial stakeholders who were interviewed proposed strategies to improve trials such as addressing the unique needs of children, embedding trials as part of routine clinical care and streamlining regulatory approvals. Increasing international collaboration, establishing sustainable centralised trials infrastructure, and aligning research to child health priorities were proposed to encourage more high-quality trials that address global child healthcare needs
A Joint ExperimentalâModeling Investigation of the Effect of Light Elements on Dynamos in Small Planets and Moons
We present a joint experimentalâmodeling investigation of core cooling in small terrestrial bodies. Significant amounts of light elements (S, O, Mg, Si) may compose the metallic cores of terrestrial planets and moons. However, the effect of multiple light elements on transport properties, in particular, electrical resistivity and thermal conductivity, is not well constrained. Electrical experiments were conducted at 10 GPa and up to 1850 K on highâpurity powder mixtures in the FeâSâO(±Mg, ±Si) systems using the multianvil apparatus and the fourâelectrode technique. The sample compositions contained 5 wt.% S, up to 3 wt.% O, up to 2 wt.% Mg, and up to 1 wt.% Si. We observe that above the eutectic temperature, electrical resistivity is significantly sensitive to the nature and amount of light elements. For each composition, thermal conductivityâtemperature equations were estimated using the experimental electrical results and a modified WiedemannâFranz law. These equations were implemented in a thermochemical core cooling model to study the evolution of the dynamo. Modeling results suggest that bulk chemistry significantly affects the entropy available to power dynamo action during core cooling. In the case of Mars, the presence of oxygen would delay the dynamo cessation by up to 1 Gyr compared to an Oâfree, FeâS core. Models with 3 wt% O can be reconciled with the inferred cessation time of the Martian dynamo if the coreâmantle boundary heat flow falls from >2 TW to ~0.1 TW in the first 0.5 Gyr following core formation
Domain Dynamics of Magnetic Films with Perpendicular Anisotropy
We study the magnetic properties of nanoscale magnetic films with large
perpendicular anisotropy comparing polarization microscopy measurements on
Co_28Pt_72 alloy samples based on the magneto-optical Kerr effect with Monte
Carlo simulations of a corresponding micromagnetic model. We focus on the
understanding of the dynamics especially the temperature and field dependence
of the magnetisation reversal process. The experimental and simulational
results for hysteresis, the reversal mechanism, domain configurations during
the reversal, and the time dependence of the magnetisation are in very good
qualitative agreement. The results for the field and temperature dependence of
the domain wall velocity suggest that for thin films the hysteresis can be
described as a depinning transition of the domain walls rounded by thermal
activation for finite temperatures.Comment: 7 pages Latex, Postscript figures included, accepted for publication
in Phys.Rev.B, also availible at:
http://www.thp.Uni-Duisburg.DE/Publikationen/Publist_Us_R.htm
- âŠ