29,149 research outputs found
Long-term variation in the Sun's activity caused by magnetic Rossby waves in the tachocline
Long-term records of sunspot number and concentrations of cosmogenic
radionuclides (10Be and 14C) on the Earth reveal the variation of the Sun's
magnetic activity over hundreds and thousands of years. We identify several
clear periods in sunspot, 10Be, and 14C data as 1000, 500, 350, 200 and 100
years. We found that the periods of the first five spherical harmonics of the
slow magnetic Rossby mode in the presence of a steady toroidal magnetic field
of 1200-1300 G in the lower tachocline are in perfect agreement with the time
scales of observed variations. The steady toroidal magnetic field can be
generated in the lower tachocline either due to the steady dynamo magnetic
field for low magnetic diffusivity or due to the action of the latitudinal
differential rotation on the weak poloidal primordial magnetic field, which
penetrates from the radiative interior. The slow magnetic Rossby waves lead to
variations of the steady toroidal magnetic field in the lower tachocline, which
modulate the dynamo magnetic field and consequently the solar cycle strength.
This result constitutes a key point for long-term prediction of the cycle
strength. According to our model, the next deep minimum in solar activity is
expected during the first half of this century.Comment: 4 pages, 4 figures, accepted in ApJ
Photophoretic Structuring of Circumstellar Dust Disks
We study dust accumulation by photophoresis in optically thin gas disks.
Using formulae of the photophoretic force that are applicable for the free
molecular regime and for the slip-flow regime, we calculate dust accumulation
distances as a function of the particle size. It is found that photophoresis
pushes particles (smaller than 10 cm) outward. For a Sun-like star, these
particles are transported to 0.1-100 AU, depending on the particle size, and
forms an inner disk. Radiation pressure pushes out small particles (< 1 mm)
further and forms an extended outer disk. Consequently, an inner hole opens
inside ~0.1 AU. The radius of the inner hole is determined by the condition
that the mean free path of the gas molecules equals the maximum size of the
particles that photophoresis effectively works on (100 micron - 10 cm,
depending on the dust property). The dust disk structure formed by
photophoresis can be distinguished from the structure of gas-free dust disk
models, because the particle sizes of the outer disks are larger, and the inner
hole radius depends on the gas density.Comment: 15 pages, 9 figures, Accepted by ApJ; corrected a typo in the author
nam
B-Meson Distribution Amplitudes of Geometric Twist vs. Dynamical Twist
Two- and three-particle distribution amplitudes of heavy pseudoscalar mesons
of well-defined geometric twist are introduced. They are obtained from
appropriately parametrized vacuum-to-meson matrix elements by applying those
twist projectors which determine the enclosed light-cone operators of definite
geometric twist and, in addition, observing the heavy quark constraint.
Comparing these distribution amplitudes with the conventional ones of dynamical
twist we derive relations between them, partially being of Wandzura-Wilczek
type; also sum rules of Burkhardt-Cottingham type are derived.The derivation is
performed for the (double) Mellin moments and then re-summed to the non-local
distribution amplitudes. Furthermore, a parametrization of vacuum-to-meson
matrix elements for non-local operators off the light-cone in terms of
distribution amplitudes accompanying independent kinematical structures is
derived.Comment: 18 pages, Latex 2e, no figure
Discrete Hamiltonian evolution and quantum gravity
We study constrained Hamiltonian systems by utilizing general forms of time
discretization. We show that for explicit discretizations, the requirement of
preserving the canonical Poisson bracket under discrete evolution imposes
strong conditions on both allowable discretizations and Hamiltonians. These
conditions permit time discretizations for a limited class of Hamiltonians,
which does not include homogeneous cosmological models. We also present two
general classes of implicit discretizations which preserve Poisson brackets for
any Hamiltonian. Both types of discretizations generically do not preserve
first class constraint algebras. Using this observation, we show that time
discretization provides a complicated time gauge fixing for quantum gravity
models, which may be compared with the alternative procedure of gauge fixing
before discretization.Comment: 8 pages, minor changes, to appear in CQ
Real-time Assessment of Right and Left Ventricular Volumes and Function in Children Using High Spatiotemporal Resolution Spiral bSSFP with Compressed Sensing
Background: Real-time (RT) assessment of ventricular volumes and function
enables data acquisition during free-breathing. However, in children the
requirement for high spatiotemporal resolution requires accelerated imaging
techniques. In this study, we implemented a novel RT bSSFP spiral sequence
reconstructed using Compressed Sensing (CS) and validated it against the
breath-hold (BH) reference standard for assessment of ventricular volumes in
children with heart disease.
Methods: Data was acquired in 60 children. Qualitative image scoring and
evaluation of ventricular volumes was performed by 3 clinical cardiac MR
specialists. 30 cases were reassessed for intra-observer variability, and the
other 30 cases for inter-observer variability.
Results: Spiral RT images were of good quality, however qualitative scores
reflected more residual artefact than standard BH images and slightly lower
edge definition. Quantification of Left Ventricular (LV) and Right Ventricular
(RV) metrics showed excellent correlation between the techniques with narrow
limits of agreement. However, we observed small but statistically significant
overestimation of LV end-diastolic volume, underestimation of LV end-systolic
volume, as well as a small overestimation of RV stroke volume and ejection
fraction using the RT imaging technique. No difference in inter-observer or
intra-observer variability were observed between the BH and RT sequences.
Conclusions: Real-time bSSFP imaging using spiral trajectories combined with
a compressed sensing reconstruction is feasible. The main benefit is that it
can be acquired during free breathing. However, another important secondary
benefit is that a whole ventricular stack can be acquired in ~20 seconds, as
opposed to ~6 minutes for standard BH imaging. Thus, this technique holds the
potential to significantly shorten MR scan times in children
Large-scale structure in a new deep IRAS galaxy redshift survey
We present here the first results from two recently completed, fully sampled redshift surveys comprising 3703 IRAS Faint Source Survey (FSS) galaxies. An unbiased counts-in-cells analysis finds a clustering strength in broad agreement with other recent redshift surveys and at odds with the standard cold dark matter model. We combine our data with those from the QDOT and 1.2 Jy surveys, producing a single estimate of the IRAS galaxy clustering strength. We compare the data with the power spectrum derived from a mixed dark matter universe. Direct comparison of the clustering strength seen in the IRAS samples with that seen in the APM-Stromlo survey suggests b_O/b_I=1.20+/-0.05 assuming a linear, scale independent biasing. We also perform a cell by cell comparison of our FSS-z sample with galaxies from the first CfA slice, testing the viability of a linear-biasing scheme linking the two. We are able to rule out models in which the FSS-z galaxies identically trace the CfA galaxies on scales 5-20h^{-1}Mpc. On scales of 5 and 10h^{-1}Mpc no linear-biasing model can be found relating the two samples. We argue that this result is expected since the CfA sample includes more elliptical galaxies which have different clustering properties from spirals. On scales of 20h^{-1}Mpc no linear-biasing model with b_O/b_I < 1.70 is acceptable. When comparing the FSS-z galaxies to the CfA spirals, however, the two populations trace the same structures within our uncertaintie
Mechanical Metamaterials
Mechanical metamaterials are an emerging design strategy aimed at tailoring lattice structures to achieve specific properties such as negative Poisson’s ratios and guiding wave propagation. These metamaterials have received increasing attention from various application domains, including medical devices, aerospace, automobile, and infrastructure. The scope of this project is to vary a single lattice parameter and quantify its effect on the structural properties of the given 3D lattice
A note on the Cops & Robber game on graphs embedded in non-orientable surfaces
The Cops and Robber game is played on undirected finite graphs. A number of
cops and one robber are positioned on vertices and take turns in sliding along
edges. The cops win if they can catch the robber. The minimum number of cops
needed to win on a graph is called its cop number. It is known that the cop
number of a graph embedded on a surface of genus is at most ,
if is orientable (Schroeder 2004), and at most , otherwise
(Nowakowski & Schroeder 1997).
We improve the bounds for non-orientable surfaces by reduction to the
orientable case using covering spaces.
As corollaries, using Schroeder's results, we obtain the following: the
maximum cop number of graphs embeddable in the projective plane is 3; the cop
number of graphs embeddable in the Klein Bottle is at most 4, and an upper
bound is for all other .Comment: 5 pages, 1 figur
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