6,178 research outputs found
Dynamics of methane ebullition from a peat monolith revealed from a dynamic flux chamber system
Methane (CH4) ebullition in northern peatlands is poorly quantified in part due to its high spatiotemporal variability. In this study, a dynamic flux chamber (DFC) system was used to continuously measure CH4 fluxes from a monolith of near‐surface Sphagnum peat at the laboratory scale to understand the complex behavior of CH4 ebullition. Coincident transmission ground penetrating radar measurements of gas content were also acquired at three depths within the monolith. A graphical method was developed to separate diffusion, steady ebullition, and episodic ebullition fluxes from the total CH4 flux recorded and to identify the timing and CH4 content of individual ebullition events. The results show that the application of the DFC had minimal disturbance on air‐peat CH4 exchange and estimated ebullition fluxes were not sensitive to the uncertainties associated with the graphical model. Steady and episodic ebullition fluxes were estimated to be averagely 36 ± 24% and 38 ± 24% of the total fluxes over the study period, respectively. The coupling between episodic CH4 ebullition and gas content within the three layers supports the existence of a threshold gas content regulating CH4 ebullition. However, the threshold at which active ebullition commenced varied between peat layers with a larger threshold (0.14 m3 m−3) observed in the deeper layers, suggesting that the peat physical structure controls gas bubble dynamics in peat. Temperature variation (23°C to 27°C) was likely only responsible for small episodic ebullition events from the upper peat layer, while large ebullition events from the deeper layers were most likely triggered by drops in atmospheric pressure
Magnetoresistance, Micromagnetism and Domain Wall Effects in Epitaxial Fe and Co Structures with Stripe Domains
We review our recent magnetotransport and micromagnetic studies of
lithographically defined epitaxial thin film structures of bcc Fe and hcp Co
with stripe domains. Micromagnetic structure and resistivity anisotropy are
shown to be the predominant sources of low field magnetoresistance (MR) in
these microstructures, with domain wall (DW) effects smaller but observable
(DW-MR ). In Fe, at low temperature, in a regime in which fields
have a significant effect on electron trajectories, a novel negative DW
contribution to the resistivity is observed. In hcp Co microstructures,
temperature dependent transport measurements for current perpendicular and
parallel to walls show that any additional resistivity due to DW scattering is
very small.Comment: 7 pages, 8 figures, to appear in Journal of Applied Physics 199
Smooth and Non-Smooth Dependence of Lyapunov Vectors upon the Angle Variable on a Torus in the Context of Torus-Doubling Transitions in the Quasiperiodically Forced Henon Map
A transition from a smooth torus to a chaotic attractor in quasiperiodically
forced dissipative systems may occur after a finite number of torus-doubling
bifurcations. In this paper we investigate the underlying bifurcational
mechanism which seems to be responsible for the termination of the
torus-doubling cascades on the routes to chaos in invertible maps under
external quasiperiodic forcing. We consider the structure of a vicinity of a
smooth attracting invariant curve (torus) in the quasiperiodically forced Henon
map and characterize it in terms of Lyapunov vectors, which determine
directions of contraction for an element of phase space in a vicinity of the
torus. When the dependence of the Lyapunov vectors upon the angle variable on
the torus is smooth, regular torus-doubling bifurcation takes place. On the
other hand, the onset of non-smooth dependence leads to a new phenomenon
terminating the torus-doubling bifurcation line in the parameter space with the
torus transforming directly into a strange nonchaotic attractor. We argue that
the new phenomenon plays a key role in mechanisms of transition to chaos in
quasiperiodically forced invertible dynamical systems.Comment: 24 pages, 9 figure
Chemical Enrichment RGS cluster sample (CHEERS): Constraints on turbulence
Feedback from AGN, galactic mergers, and sloshing are thought to give rise to
turbulence, which may prevent cooling in clusters. We aim to measure the
turbulence in clusters of galaxies and compare the measurements to some of
their structural and evolutionary properties. It is possible to measure the
turbulence of the hot gas in clusters by estimating the velocity widths of
their X-ray emission lines. The RGS Spectrometers aboard XMM-Newton are
currently the only instruments provided with sufficient effective area and
spectral resolution in this energy domain. We benefited from excellent 1.6Ms
new data provided by the CHEERS project. The new observations improve the
quality of the archival data and allow us to place constraints for some
clusters, which were not accessible in previous work. One-half of the sample
shows upper limits on turbulence less than 500km/s. For several sources, our
data are consistent with relatively strong turbulence with upper limits on the
velocity widths that are larger than 1000km/s. The NGC507 group of galaxies
shows transonic velocities, which are most likely associated with the merging
phenomena and bulk motions occurring in this object. Where both low- and
high-ionization emission lines have good enough statistics, we find larger
upper limits for the hot gas, which is partly due to the different spatial
extents of the hot and cool gas phases. Our upper limits are larger than the
Mach numbers required to balance cooling, suggesting that dissipation of
turbulence may prevent cooling, although other heating processes could be
dominant. The systematics associated with the spatial profile of the source
continuum make this technique very challenging, though still powerful, for
current instruments. The ASTRO-H and Athena missions will revolutionize the
velocity estimates and discriminate between different spatial regions and
temperature phases.Comment: 16 pages, 18 figures, 3 tables, accepted for publications in
Astronomy and Astrophysic
Genomic and biologic comparisons of cyprinid herpesvirus 3 strains
Cyprinid herpesvirus 3 (CyHV-3) is the archetypal fish alloherpesvirus and the etiologic agent of a lethal disease in common and koi carp. To date, the genome sequences of only four CyHV-3 isolates have been published, but no comparisons of the biologic properties of these strains have been reported. We have sequenced the genomes of a further seven strains from various geographical sources, and have compared their growth in vitro and virulence in vivo. The major findings were: (i) the existence of the two genetic lineages previously described as European and Asian was confirmed, but inconsistencies between the geographic origin and genotype of some strains were revealed; (ii) potential inter-lineage recombination was detected in one strain, which also suggested the existence of a third, as yet unidentified lineage; (iii) analysis of genetic disruptions led to the identification of non-essential genes and their potential role in virulence; (iv) comparison of the in vitro and in vivo properties of strains belonging to the two lineages revealed that inter-lineage polymorphisms do not contribute to the differences in viral fitness observed; and (v) a negative correlation was observed among strains between viral growth in vitro and virulence in vivo. This study illustrates the importance of coupling genomic and biologic comparisons of viral strains in order to enhance understanding of viral evolution and pathogenesis
Constraints on the perturbed mutual motion in Didymos due to impact-induced deformation of its primary after the DART impact
Binary near-Earth asteroid (65803) Didymos is the target of the proposed NASA
Double Asteroid Redirection Test (DART), part of the Asteroid Impact &
Deflection Assessment (AIDA) mission concept. In this mission, the DART
spacecraft is planned to impact the secondary body of Didymos, perturbing
mutual dynamics of the system. The primary body is currently rotating at a spin
period close to the spin barrier of asteroids, and materials ejected from the
secondary due to the DART impact are likely to reach the primary. These
conditions may cause the primary to reshape, due to landslides, or internal
deformation, changing the permanent gravity field. Here, we propose that if
shape deformation of the primary occurs, the mutual orbit of the system would
be perturbed due to a change in the gravity field. We use a numerical
simulation technique based on the full two-body problem to investigate the
shape effect on the mutual dynamics in Didymos after the DART impact. The
results show that under constant volume, shape deformation induces strong
perturbation in the mutual motion. We find that the deformation process always
causes the orbital period of the system to become shorter. If surface layers
with a thickness greater than ~0.4 m on the poles of the primary move down to
the equatorial region due to the DART impact, a change in the orbital period of
the system and in the spin period of the primary will be detected by
ground-based measurement.Comment: 8 pages, 7 figures, 2 tables, accepted for publication in MNRA
A low-temperature phase of bis(tetrabutylammonium) octa-l3-chloridohexachlorido- octahedro-hexatungstate
The article discusses the low-temperature phase of bis(tetrabutylammonium) octa-µ3-chlorido-hexachlorido-octahedro-hexa-tungstate, which undergoes a reversible phase transition at 268 K. The unit cells of the room- and low-temperature polymorphs of this compound are found to be closely related. The hydrocarbon chain of one of the tetrabutylammonium cations is found to be disordered at both 150 and 200 K
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