3,572 research outputs found
Electron Acceleration by Multi-Island Coalescence
Energetic electrons of up to tens of MeV are created during explosive
phenomena in the solar corona. While many theoretical models consider magnetic
reconnection as a possible way of generating energetic electrons, the precise
roles of magnetic reconnection during acceleration and heating of electrons
still remain unclear. Here we show from 2D particle-in-cell simulations that
coalescence of magnetic islands that naturally form as a consequence of tearing
mode instability and associated magnetic reconnection leads to efficient
energization of electrons. The key process is the secondary magnetic
reconnection at the merging points, or the `anti-reconnection', which is, in a
sense, driven by the converging outflows from the initial magnetic reconnection
regions. By following the trajectories of the most energetic electrons, we
found a variety of different acceleration mechanisms but the energization at
the anti-reconnection is found to be the most important process. We discuss
possible applications to the energetic electrons observed in the solar flares.
We anticipate our results to be a starting point for more sophisticated models
of particle acceleration during the explosive energy release phenomena.Comment: 14 pages, 12 figures (degraded figure quality), 1 table. Accepted for
publication in ApJ
Bayesian Spatial Binary Regression for Label Fusion in Structural Neuroimaging
Many analyses of neuroimaging data involve studying one or more regions of
interest (ROIs) in a brain image. In order to do so, each ROI must first be
identified. Since every brain is unique, the location, size, and shape of each
ROI varies across subjects. Thus, each ROI in a brain image must either be
manually identified or (semi-) automatically delineated, a task referred to as
segmentation. Automatic segmentation often involves mapping a previously
manually segmented image to a new brain image and propagating the labels to
obtain an estimate of where each ROI is located in the new image. A more recent
approach to this problem is to propagate labels from multiple manually
segmented atlases and combine the results using a process known as label
fusion. To date, most label fusion algorithms either employ voting procedures
or impose prior structure and subsequently find the maximum a posteriori
estimator (i.e., the posterior mode) through optimization. We propose using a
fully Bayesian spatial regression model for label fusion that facilitates
direct incorporation of covariate information while making accessible the
entire posterior distribution. We discuss the implementation of our model via
Markov chain Monte Carlo and illustrate the procedure through both simulation
and application to segmentation of the hippocampus, an anatomical structure
known to be associated with Alzheimer's disease.Comment: 24 pages, 10 figure
`Island Surfing' Mechanism of Electron Acceleration During Magnetic Reconnection
One of the key unresolved problems in the study of space plasmas is to
explain the production of energetic electrons as magnetic field lines
`reconnect' and release energy in a exposive manner. Recent observations
suggest possible roles played by small scale magnetic islands in the
reconnection region, but their precise roles and the exact mechanism of
electron energization have remained unclear. Here we show that secondary
islands generated in the reconnection region are indeed efficient electron
accelerators. We found that, when electrons are trapped inside the islands,
they are energized continuously by the reconnection electric field prevalent in
the reconnection diffusion region. The size and the propagation speed of the
secondary islands are similar to those of islands observed in the magnetotail
containing energertic electrons.Comment: 5 pages, 4 figures, submitted to J. Geophys. Res
Robust heterodimensional cycles in two-parameter unfolding of homoclinic tangencies
We consider diffeomorphisms with a generic homoclinic tangency to a hyperbolic periodic point, where this point has at least one complex (non-real) central multiplier and some explicit assumptions on central multipliers are satisfied so that the dynamics near the homoclinic tangency is not effectively one-dimensional. We prove that -robust heterodimensional cycles of co-index one appear in any generic two-parameter -unfolding of such a tangency. These heterodimensional cycles also have -robust homoclinic tangencies
Glueball mass from quantized knot solitons and gauge-invariant gluon mass
We propose an approach which enables one to obtain simultaneously the
glueball mass and the gluon mass in the gauge-invariant way to shed new light
on the mass gap problem in Yang-Mills theory. First, we point out that the
Faddeev (Skyrme--Faddeev-Niemi) model can be induced through the
gauge-invariant vacuum condensate of mass dimension two from SU(2) Yang-Mills
theory. Second, we obtain the glueball mass spectrum by performing the
collective coordinate quantization of the topological knot soliton in the
Faddeev model. Third, we demonstrate that a relationship between the glueball
mass and the gluon mass is obtained, since the gauge-invariant gluon mass is
also induced from the relevant vacuum condensate. Finally, we determine
physical values of two parameters in the Faddeev model and give an estimate of
the relevant vacuum condensation in Yang-Mills theory. Our results indicate
that the Faddeev model can play the role of a low-energy effective theory of
the quantum SU(2) Yang-Mills theory.Comment: 17 pages, 2 figures, 3 tables; a version accepted for publication in
J. Phys. A: Math. Gen.; Sect. 2 and sect. 5 (old sect. 4) are modified. Sect.
4, Tables 1 and Table 3 are adde
Global Optical Control of a Quantum Spin Chain
Quantum processors which combine the long decoherence times of spin qubits
together with fast optical manipulation of excitons have recently been the
subject of several proposals. I show here that arbitrary single- and entangling
two-qubit gates can be performed in a chain of perpetually coupled spin qubits
solely by using laser pulses to excite higher lying states. It is also
demonstrated that universal quantum computing is possible even if these pulses
are applied {\it globally} to a chain; by employing a repeating pattern of four
distinct qubit units the need for individual qubit addressing is removed. Some
current experimental qubit systems would lend themselves to implementing this
idea.Comment: 5 pages, 3 figure
Phase transitions of a tethered surface model with a deficit angle term
Nambu-Goto model is investigated by using the canonical Monte Carlo
simulations on fixed connectivity surfaces of spherical topology. Three
distinct phases are found: crumpled, tubular, and smooth. The crumpled and the
tubular phases are smoothly connected, and the tubular and the smooth phases
are connected by a discontinuous transition. The surface in the tubular phase
forms an oblong and one-dimensional object similar to a one-dimensional linear
subspace in the Euclidean three-dimensional space R^3. This indicates that the
rotational symmetry inherent in the model is spontaneously broken in the
tubular phase, and it is restored in the smooth and the crumpled phases.Comment: 6 pages with 6 figure
The emergent integrated network structure of scientific research
The practice of scientific research is often thought of as individuals and
small teams striving for disciplinary advances. Yet as a whole, this endeavor
more closely resembles a complex system of natural computation, in which
information is obtained, generated, and disseminated more effectively than
would be possible by individuals acting in isolation. Currently, the structure
of this integrated and innovative landscape of scientific ideas is not well
understood. Here we use tools from network science to map the landscape of
interconnected research topics covered in the multidisciplinary journal PNAS
since 2000. We construct networks in which nodes represent topics of study and
edges give the degree to which topics occur in the same papers. The network
displays small-world architecture, with dense connectivity within scientific
clusters and sparse connectivity between clusters. Notably, clusters tend not
to align with assigned article classifications, but instead contain topics from
various disciplines. Using a temporal graph, we find that small-worldness has
increased over time, suggesting growing efficiency and integration of ideas.
Finally, we define a novel measure of interdisciplinarity, which is positively
associated with PNAS's impact factor. Broadly, this work suggests that complex
and dynamic patterns of knowledge emerge from scientific research, and that
structures reflecting intellectual integration may be beneficial for obtaining
scientific insight
THE EFFECT OF COMPRESSION TIGHTS AND DURATION OF TESTING ON CONTINUOUS JUMPING MECHANICAL POWER
INTRODUCTION: In order to improve their performance, athletes seek advancements in technology, such as clothing. Manufacturers of compression tights, advertise that their product adds support to lower extremity musculature, thus may slow the onset of fatigue. Few scientific studies, however, have been conducted to identify how advancements in apparel influence an athlete’s performance (Kraemer et al., 1996). The purpose of the present study was to identify the effects of compression tights on mechanical power for continuous jumping. Secondly, the effect of duration on the mechanical power output by using Bosco’s method (1983) over 15, 30, 45, and 60 sec time-frames, as well as the interaction between apparel and duration
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