750 research outputs found
A phase-shift-periodic parallel boundary condition for low-magnetic-shear scenarios
We formulate a generalized periodic boundary condition as a limit of the
standard twist-and-shift parallel boundary condition that is suitable for
simulations of plasmas with low magnetic shear. This is done by applying a
phase shift in the binormal direction when crossing the parallel boundary.
While this phase shift can be set to zero without loss of generality in the
local flux-tube limit when employing the twist-and-shift boundary condition, we
show that this is not the most general case when employing periodic parallel
boundaries, and may not even be the most desirable. A non-zero phase shift can
be used to avoid the convective cells that plague simulations of the
three-dimensional Hasegawa-Wakatani system, and is shown to have measurable
effects in periodic low-magnetic-shear gyrokinetic simulations. We propose a
numerical program where a sampling of periodic simulations at random
pseudo-irrational flux surfaces are used to determine physical observables in a
statistical sense. This approach can serve as an alternative to applying the
twist-and-shift boundary condition to low-magnetic-shear scenarios which, while
more straightforward, can be computationally demanding.Comment: 4 figures, 1 tabl
Glacial geomorphology of the northern Kivalliq region, Nunavut, Canada, with an emphasis on meltwater drainage systems
This paper presents a glacial geomorphological map of glacial lineations, ribbed terrain, moraines, meltwater channels (subglacial and ice-marginal/proglacial), eskers, glaciofluvial deposits, ice-contact outwash fans and deltas and abandoned shorelines on the bed of the former Laurentide Ice Sheet in northern Canada. Mapping was compiled from satellite imagery and digital elevation data and landforms were digitised directly into a Geographical Information System. The map reveals a complex glacial history characterised by multiple ice-flow events, including fast-flowing ice-streams. Moraines record a series of pauses or re-advances during overall SE retreat towards the Keewatin Ice Divide. The distribution of subglacial meltwater landforms indicates that several distinctive scales and modes of drainage system operated beneath the retreating ice sheet. This includes a large (>100 km) integrated network of meltwater channels, eskers, ice-contact outwash fans and deltas and glaciofluvial deposits that originates at the northern edge of Aberdeen Lake. The map comprises zone 66 of the Canadian National Topographic System, which encompasses an area of 160,000 km2. It is presented at a scale of 1:500,000 and is designed to be printed at A0 size
Basement-cover relations and internal structure of the Cape Smith klippe: A 1.9 Ga greenstone belt in northern Quebec, Canada
The Cape Smith Belt is a 380x60 km tectonic klippe composed of greenschistto amphibolite-grade mafic and komatiitic lava flows and fine-grained quartzose sediment, intruded by minor syn- to post-tectonic granitoids. Previously studied transects in areas of relatively high structural level show that the belt is constructed of seven or more north-dipping thrust sheets which verge toward the Superior Province (Archean) foreland in the south and away from an Archean basement massif (Kovik Antiform) external to the Trans-Hudson Orogen (Early Proterozoic) in the north. A field project (mapping and structural-stratigraphic-metamorphic studies) directed by MRS was begun in 1985 aimed at the structurally deeper levels of the belt and underlying basement, which are superby exposed in oblique cross-section (12 km minimum structural relief) at the west-plunging eastern end of the belt. Mapping now complete of the eastern end of the belt confirms that all of the metavolcanic and most of the metasedimentary rocks are allochthonous with respect to the Archean basement, and that the thrusts must have been rooted north of Kovik Antiform. The main findings follow
Electronic band gap reduction and intense luminescence in Co and Mn ion-implanted SiO
Cobalt and manganese ions are implanted into SiO over a wide range of
concentrations. For low concentrations, the Co atoms occupy interstitial
locations, coordinated with oxygen, while metallic Co clusters form at higher
implantation concentrations. For all concentrations studied here, Mn ions
remain in interstitial locations and do not cluster. Using resonant x-ray
emission spectroscopy and Anderson impurity model calculations, we determine
the strength of the covalent interaction between the interstitial ions and the
SiO valence band, finding it comparable to Mn and Co monoxides. Further, we
find an increasing reduction in the SiO electronic band gap for increasing
implantation concentration, due primarily to the introduction of Mn- and
Co-derived conduction band states. We also observe a strong increase in a band
of x-ray stimulated luminescence at 2.75 eV after implantation, attributed to
oxygen deficient centers formed during implantation.Comment: 8 pages, 6 figure
Fluctuation dynamo in a weakly collisional plasma
The turbulent amplification of cosmic magnetic fields depends upon the
material properties of the host plasma. In many hot, dilute astrophysical
systems, such as the intracluster medium (ICM) of galaxy clusters, the rarity
of particle--particle collisions allows departures from local thermodynamic
equilibrium. These departures exert anisotropic viscous stresses on the plasma
motions that inhibit their ability to stretch magnetic-field lines. We present
a numerical study of the fluctuation dynamo in a weakly collisional plasma
using magnetohydrodynamic (MHD) equations endowed with a field-parallel viscous
(Braginskii) stress. When the stress is limited to values consistent with a
pressure anisotropy regulated by firehose and mirror instabilities, the
Braginskii-MHD dynamo largely resembles its MHD counterpart. If instead the
parallel viscous stress is left unabated -- a situation relevant to recent
kinetic simulations of the fluctuation dynamo and to the early stages of the
dynamo in a magnetized ICM -- the dynamo changes its character, amplifying the
magnetic field while exhibiting many characteristics of the saturated state of
the large-Prandtl-number () MHD dynamo. We construct an
analytic model for the Braginskii-MHD dynamo in this regime, which successfully
matches magnetic-energy spectra. A prediction of this model, confirmed by our
simulations, is that a Braginskii-MHD plasma without pressure-anisotropy
limiters will not support a dynamo if the ratio of perpendicular and parallel
viscosities is too small. This ratio reflects the relative allowed rates of
field-line stretching and mixing, the latter of which promotes resistive
dissipation of the magnetic field. In all cases that do exhibit a dynamo, the
generated magnetic field is organized into folds that persist into the
saturated state and bias the chaotic flow to acquire a scale-dependent spectral
anisotropy.Comment: 62 pages, 25 figures, 1 table, accepted for publication to Journal of
Plasma Physic
Contextualising the Permian Sumdo eclogite belt, Lhasa block, Tibet
Abstract HKT-ISTP 2013
A
Seismological structure of the 1.8 Ga Trans-Hudson Orogen of North America
Precambrian tectonic processes are debated: what was the nature and scale of orogenic events on the younger, hotter, and more ductile Earth? Northern Hudson Bay records the Paleoproterozoic collision between the Western Churchill and Superior plates—the ∼1.8 Ga Trans-Hudson Orogeny (THO)—and is an ideal locality to study Precambrian tectonic structure. Integrated field, geochronological, and thermobarometric studies suggest that the THO was comparable to the present-day Himalayan-Karakoram-Tibet Orogen (HKTO). However, detailed understanding of the deep crustal architecture of the THO, and how it compares to that of the evolving HKTO, is lacking. The joint inversion of receiver functions and surface wave data provides new Moho depth estimates and shear velocity models for the crust and uppermost mantle of the THO. Most of the Archean crust is relatively thin (∼39 km) and structurally simple, with a sharp Moho; upper-crustal wave speed variations are attributed to postformation events. However, the Quebec-Baffin segment of the THO has a deeper Moho (∼45 km) and a more complex crustal structure. Observations show some similarity to recent models, computed using the same methods, of the HKTO crust. Based on Moho character, present-day crustal thickness, and metamorphic grade, we support the view that southern Baffin Island experienced thickening during the THO of a similar magnitude and width to present-day Tibet. Fast seismic velocities at >10 km below southern Baffin Island may be the result of partial eclogitization of the lower crust during the THO, as is currently thought to be happening in Tibet
Body composition changes with aging: The cause or the result of alterations in metabolic rate and macronutrient oxidation?
Abstract It has been well documented that as individuals age, body composition changes, even in the absence of changes in body weight. Studies have shown that fat mass increases and muscle mass decreases with age. However, it is unclear why such changes occur. Resting metabolic rate (RMR) and substrate oxidation rates have been examined with aging. It has been proposed that reductions in RMR and fat oxidation may lead to changes in body composition. Alternatively, changes in body composition with aging may lead to reductions in RMR. The purpose of this review is to provide an overview of the literature surrounding the impact of aging on RMR and substrate oxidation. Although long-term longitudinal studies are lacking, most cross-sectional studies or short-term longitudinal studies show a reduction in RMR with aging that cannot be explained by changes in body composition including loss in fat-free mass, where the latter includes atrophy or decreases in the mass of high metabolic rate organs. There is indirect evidence suggesting that the metabolic rate of individual organs is lower in older compared with younger individuals. With aging, we conclude that reductions in the mass of individual organs/tissues and in tissue-specific organ metabolic rate contribute to a reduction in RMR that in turn promotes changes in body composition favoring increased fat mass and reduced fat-free mass
Ptarmigan Fiord basement-cover thrust imbricates, Baffin Island, Nunavut
The rocks at Ptarmigan Fiord on the Hall Peninsula of Baffin Island underwent midcrustal deformation during the formation
of the Paleoproterozoic Trans-Hudson Orogen. The structural style in the region is dominated by imbricate panels of
Archean basement orthogneiss and Paleoproterozoic supracrustal strata, interpreted to have been deformed by thick-
skinned ductile thrusting. Basement rocks comprise amphibolite-facies metatonalite, metagranodiorite, metaquartz-diorite
and metamonzogranite, and cover rocks comprise amphibolite-facies migmatitic pelitic and semipelitic schist, psammitic
schist, amphibolite, calcsilicate and quartzite. The S 1a penetrative foliation is variably present in basement rocks and consis-
tently present in cover rocks, and is defined by alignment of biotite, sillimanite and leucogranite that formed before and dur-
ing the thermal metamorphic peak. The S 1a foliation was deformed by F 1b isoclinal folds with an amplitude of 100 m. These
structures are interpreted as forming during a D 1 east-west crustal shortening event. Basement and cover imbrication oc-
curred after the thermal metamorphic peak and is interpreted as D 2 thick-skinned ductile thrusting. Ductile thrust faults at
the base of seven basement-cover slices are identified on the basis of repetition of units and strain localization, and are inter-
preted as predominantly south-to-southeast verging on the basis of shear-sense indicators. There are two structural panels of
D 2 thrust imbricates, one in the northwestern part of the map area and one in the eastern part of the map area. Map-scale
crosscutting relationships indicate that the northwestern panel overthrusted the eastern panel on a southeasterly T 2c -di-
rected thrust fault, following a F 2b folding event that folded the T 2a basement-cover thrust imbricates in the eastern panel.
The Ptarmigan Fiord area contains a world-class exposure of thick-skinned structures as they are spectacularly delineated
by belts of distinctive grey-weathering Archean basement rocks and brown- to black-weathering Paleoproterozoic supra-
crustal rocks
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