567 research outputs found
Cosmological Magnetic Fields from Primordial Helicity
Primordial magnetic fields may account for all or part of the fields observed
in galaxies. We consider the evolution of the magnetic fields created by
pseudoscalar effects in the early universe. Such processes can create
force-free fields of maximal helicity; we show that for such a field magnetic
energy inverse cascades to larger scales than it would have solely by flux
freezing and cosmic expansion. For fields generated at the electroweak phase
transition, we find that the predicted wavelength today can in principle be as
large as 10 kpc, and the field strength can be as large as 10^{-10} G.Comment: 13 page
Groundwater–rock interactions in crystalline rocks: evidence from SIMS oxygen isotope data
The diffusive exchange of dissolved material between fluid flowing in a fracture and the enclosing wallrocks (rock matrix diffusion) has been proposed as a mechanism by which radionuclides derived from a radioactive waste repository may be removed from groundwater and incorporated into the geosphere. To test the effectiveness of diffusive exchange in igneous and metamorphic rocks, we have carried out an investigation of veins formed at low temperatures (<100°C), comparing the oxygen isotopic composition of vein calcite with that of secondary calcite in the wallrocks. Two examples of veins from the Borrowdale Volcanic Group, Cumbria, and one from the Mountsorrel Granodiorite, Leicestershire, UK, have remarkably similar vein calcite compositions, ca. +20‰(SMOW) or greater, substantially heavier than the probable compositions of the host rocks, and these vein calcite compositions are inferred to reflect the infiltrating fluid and the temperature of vein formation. Calcites from the wallrocks are similar to those in veins, with little evidence for exchange with the wallrocks. The results support existing models for this type of vein which suggest low-temperature growth from formation brines originally linked to Permian or Triassic evaporites. The results are consistent with flow through fractures being attenuated through a damage zone adjacent to the fracture and provide no evidence of diffusional exchange with pore waters from wallrocks
Natural analogue evidence for controls on radionuclide uptake by fractured crystalline rock
Fractured Crystalline Rocks (FCR) are being considered in several countries as hosts for radioactive waste repositories. In FCR, radionuclides may be transported relatively rapidly by bulk groundwater flow through open fractures, but much more slowly by diffusion through porewater in the rock matrices. Rock matrix diffusion (RMD) is the diffusion of radionuclides in the aqueous phase, between open fractures and rock matrices. Sorption or co-precipitation on the fracture surfaces and walls of the matrix pores causes further radionuclide retardation. RMD may be important in a repository's safety case and has been investigated by many published short-term (to a few years) laboratory and in-situ experiments. To improve understanding over longer timescales, we investigated evidence for RMD of several natural radioelements, and radioelement analogues, in five exemplar fractured crystalline rock (FCR) samples aged between c. 70 Ma and c. 455 Ma. The sample suite consisted of two samples of Borrowdale Volcanic Group (BVG) meta-tuff from northwest England, a sample of Carnmenellis Granite from southwest England and two samples of Toki Granite from central Japan. Uptake or loss of the studied elements is limited to an altered damage zone in each sample, coupled to mineral alteration processes. These zones are most extensive (a few tens of millimetres) in the Toki Granite samples. We also found unstable primary igneous minerals to persist in the immediate wallrocks of fractures in studied granite samples, suggesting that pores were not permanently water saturated in these samples. Although only a small sample suite was studied, the results show that while RMD may be important in some kinds of FCR, in others it may be negligible. Site-specific information is therefore needed to determine how much reliance can be placed on RMD when developing a safety case
On the variation of the gauge couplings during inflation
It is shown that the evolution of the (Abelian) gauge coupling during an
inflationary phase of de Sitter type drives the growth of the two-point
function of the magnetic inhomogeneities. After examining the constraints on
the variation of the gauge coupling arising in a standard model of inflationary
and post-inflationary evolution, magnetohydrodynamical equations are
generalized to the case of time evolving gauge coupling. It is argued that
large scale magnetic fields can be copiously generated. Other possible
implications of the model are outlined.Comment: 5 pages in RevTex style, one figur
Columbus’ footprint in Hispaniola:A paleoenvironmental record of indigenous and colonial impacts on the landscape of the central Cibao Valley, northern Dominican Republic
The 1100-year sedimentary record of Laguna Biajaca reveals human-driven landscape changes in the central Cibao Valley, Dominican Republic, Hispaniola. This sediment-filled cutoff meander is located in close proximity to pre-Colonial archaeological sites and a Colonial urban hub. It provided a nutrient-rich floodable locus for agricultural activities for indigenous communities and for the first introduction of Old World crops and cattle in the Americas. Integration of paleoecological proxies revealed the formation of a clear-water body surrounded by a palm-rich forested landscape around 1100 cal yr BP. Changes in the drainage system were linked to human-driven deforestation, which also changed the composition of the vegetation and fungal communities around the site between AD 1150 and 1500 (800 and 700 cal yr BP). Pre-Colonial modifications of the landscape were primarily the result of fire-use and small-scale clearings. Crop cultivation developed between AD 1250 and 1450 (700–500 cal yr BP). Within decades after Columbus’ arrival in Hispaniola in AD 1492, the first impacts of European colonization included the abandonment of indigenous sites and the introduction of Old World domesticated animals. During the 15th and 16th centuries the area underwent intensive land-clearing that allowed for larger scale crop cultivation. An increase of aquatic vegetation points to sediment-filling around AD 1700 (250 cal yr BP). At that time, cattle breeding expanded and rapidly provoked eutrophication while, concurrently, monocultures became regionally established. This paper provides a framework of past environmental dynamics and offers an opportunity to place archaeological findings in a context of natural and anthropogenic change
Nucleation of a sodium droplet on C60
We investigate theoretically the progressive coating of C60 by several sodium
atoms. Density functional calculations using a nonlocal functional are
performed for NaC60 and Na2C60 in various configurations. These data are used
to construct an empirical atomistic model in order to treat larger sizes in a
statistical and dynamical context. Fluctuating charges are incorporated to
account for charge transfer between sodium and carbon atoms. By performing
systematic global optimization in the size range 1<=n<=30, we find that Na_nC60
is homogeneously coated at small sizes, and that a growing droplet is formed
above n=>8. The separate effects of single ionization and thermalization are
also considered, as well as the changes due to a strong external electric
field. The present results are discussed in the light of various experimental
data.Comment: 17 pages, 10 figure
Magnetogenesis and the dynamics of internal dimensions
The dynamical evolution of internal space-like dimensions breaks the
invariance of the Maxwell's equations under Weyl rescaling of the (conformally
flat) four-dimensional metric. Depending upon the number and upon the dynamics
of internal dimensions large scale magnetic fields can be created. The
requirements coming from magnetogenesis together with the other cosmological
constraints are examined under the assumption that the internal dimensions
either grow or shrink (in conformal time) prior to a radiation dominated epoch.
If the internal dimensions are growing the magnitude of the generated magnetic
fields can seed the galactic dynamo mechanism.Comment: 27 in RevTex style, four figure
Grain Surface Models and Data for Astrochemistry
AbstractThe cross-disciplinary field of astrochemistry exists to understand the formation, destruction, and survival of molecules in astrophysical environments. Molecules in space are synthesized via a large variety of gas-phase reactions, and reactions on dust-grain surfaces, where the surface acts as a catalyst. A broad consensus has been reached in the astrochemistry community on how to suitably treat gas-phase processes in models, and also on how to present the necessary reaction data in databases; however, no such consensus has yet been reached for grain-surface processes. A team of ∼25 experts covering observational, laboratory and theoretical (astro)chemistry met in summer of 2014 at the Lorentz Center in Leiden with the aim to provide solutions for this problem and to review the current state-of-the-art of grain surface models, both in terms of technical implementation into models as well as the most up-to-date information available from experiments and chemical computations. This review builds on the results of this workshop and gives an outlook for future directions
Large-scale magnetic fields from inflation due to a -even Chern-Simons-like term with Kalb-Ramond and scalar fields
We investigate the generation of large-scale magnetic fields due to the
breaking of the conformal invariance in the electromagnetic field through the
-even dimension-six Chern-Simons-like effective interaction with a fermion
current by taking account of the dynamical Kalb-Ramond and scalar fields in
inflationary cosmology. It is explicitly demonstrated that the magnetic fields
on 1Mpc scale with the field strength of G at the present time
can be induced.Comment: 18 pages, 6 figures, version accepted for publication in Eur. Phys.
J.
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