23,931 research outputs found
Reversibility Checking for Markov Chains
In this paper, we present reversibility preserving operations on Markov chain
transition matrices. Simple row and column operations allow us to create new
reversible transition matrices and yield an easy method for checking a Markov
chain for reversibility
A model for the formation of the active region corona driven by magnetic flux emergence
We present the first model that couples the formation of the corona of a
solar active region to a model of the emergence of a sunspot pair. This allows
us to study when, where, and why active region loops form, and how they evolve.
We use a 3D radiation MHD simulation of the emergence of an active region
through the upper convection zone and the photosphere as a lower boundary for a
3D MHD coronal model. The latter accounts for the braiding of the magnetic
fieldlines, which induces currents in the corona heating up the plasma. We
synthesize the coronal emission for a direct comparison to observations.
Starting with a basically field-free atmosphere we follow the filling of the
corona with magnetic field and plasma. Numerous individually identifiable hot
coronal loops form, and reach temperatures well above 1 MK with densities
comparable to observations. The footpoints of these loops are found where small
patches of magnetic flux concentrations move into the sunspots. The loop
formation is triggered by an increase of upwards-directed Poynting flux at
their footpoints in the photosphere. In the synthesized EUV emission these
loops develop within a few minutes. The first EUV loop appears as a thin tube,
then rises and expands significantly in the horizontal direction. Later, the
spatially inhomogeneous heat input leads to a fragmented system of multiple
loops or strands in a growing envelope.Comment: 13 pages, 10 figures, accepted to publication in A&
Magnetic Jam in the Corona of the Sun
The outer solar atmosphere, the corona, contains plasma at temperatures of
more than a million K, more than 100 times hotter that solar surface. How this
gas is heated is a fundamental question tightly interwoven with the structure
of the magnetic field in the upper atmosphere. Conducting numerical experiments
based on magnetohydrodynamics we account for both the evolving
three-dimensional structure of the atmosphere and the complex interaction of
magnetic field and plasma. Together this defines the formation and evolution of
coronal loops, the basic building block prominently seen in X-rays and extreme
ultraviolet (EUV) images. The structures seen as coronal loops in the EUV can
evolve quite differently from the magnetic field. While the magnetic field
continuously expands as new magnetic flux emerges through the solar surface,
the plasma gets heated on successively emerging fieldlines creating an EUV loop
that remains roughly at the same place. For each snapshot the EUV images
outline the magnetic field, but in contrast to the traditional view, the
temporal evolution of the magnetic field and the EUV loops can be different.
Through this we show that the thermal and the magnetic evolution in the outer
atmosphere of a cool star has to be treated together, and cannot be simply
separated as done mostly so far.Comment: Final version published online on 27 April 2015, Nature Physics 12
pages and 8 figure
Nonperturbative Determination of Heavy Meson Bound States
In this paper we obtain a heavy meson bound state equation from the heavy
quark equation of motion in heavy quark effective theory (HQET) and the heavy
meson effective field theory we developed very recently. The bound state
equation is a covariant extention of the light-front bound state equation for
heavy mesons derived from light-front QCD and HQET. We determine the covariant
heavy meson wave function variationally by minimizing the binding energy
. Subsequently the other basic HQET parameters and
, and the heavy quark masses and can also be
consistently determined.Comment: 15 pages, 1 figur
Entrepreneurship through Bricolage. A Study of Displaced Entrepreneurs at Times of War and Conflict
War and conflict brings about adverse changes for those who are displaced. How do entrepreneurial individuals respond to such adversity to either set-up, or continue with their existing entrepreneurial endeavours that would improve their own livelihood or that of others who have been affected? Whilst previous studies have found local knowledge, networks and resources to be crucial in the development of ventures in the war and conflict context, alienation from mainstream society within the host location often means that to succeed, those who are displaced require alternative strategies and approaches. Through examining the entrepreneurship ventures of six internally displaced entrepreneurs in Pakistan, our study identifies that entrepreneurial individuals find different ways to adapt to the new order, with both internal and external bricolage becoming the key strategies deployed to either re-establish their previous business(es) or to develop new endeavour(s) in the host location. To compensate for lack of local knowledge, networks and resources, we found that entrepreneurs followed closely their previous paths in their bricolage attempts, relying on reconfigurations of their pre-existing competencies, as well as utilising pre-established and clandestine networks
Thermal Diagnostics with the Atmospheric Imaging Assembly onboard the Solar Dynamics Observatory: A Validated Method for Differential Emission Measure Inversions
We present a new method for performing differential emission measure (DEM)
inversions on narrow-band EUV images from the Atmospheric Imaging Assembly
(AIA) onboard the Solar Dynamics Observatory (SDO). The method yields positive
definite DEM solutions by solving a linear program. This method has been
validated against a diverse set of thermal models of varying complexity and
realism. These include (1) idealized gaussian DEM distributions, (2) 3D models
of NOAA Active Region 11158 comprising quasi-steady loop atmospheres in a
non-linear force-free field, and (3) thermodynamic models from a
fully-compressible, 3D MHD simulation of AR corona formation following magnetic
flux emergence. We then present results from the application of the method to
AIA observations of Active Region 11158, comparing the region's thermal
structure on two successive solar rotations. Additionally, we show how the DEM
inversion method can be adapted to simultaneously invert AIA and XRT data, and
how supplementing AIA data with the latter improves the inversion result. The
speed of the method allows for routine production of DEM maps, thus
facilitating science studies that require tracking of the thermal structure of
the solar corona in time and space.Comment: 21 pages, 18 figures, accepted for publication in Ap
Hadronic Production of S-wave and P-wave Charmed Beauty Mesons via Heavy Quark Fragmentation
At hadron colliders the dominant production mechanism of mesons
with large transverse momentum is due to parton fragmentation. We compute the
rates and transverse momentum spectra for production of S-wave and P-wave
mesons at the Tevatron via the direct fragmentation of the bottom
antiquark as well as the Altarelli-Parisi induced gluon fragmentation. Since
all the radially and orbitally excited mesons below the
flavor threshold will cascade into the pseudoscalar ground state through
electromagnetic and/or hadronic transitions, they all contribute to the
inclusive production of . The contributions of the excited S-wave and
P-wave states to the inclusive production of are 58 and 23\%,
respectively, and hence significant.Comment: Changes are made in the Discussio
Towards the Application of Classification Techniques to Test and Identify Faults in Multimedia Systems
The advances in computer and graphic technologies have led to the popular use of multimedia for information exchange. However, multimedia systems are difficult to test. A major reason is that these systems generally exhibit fuzziness in their temporal behaviors. The fuzziness may be caused by the existence of non-deterministic factors in their runtime environments, such as system load and network traffic. It complicates the analysis of test results. The problem is aggravated when a test involves the synchronization of different multimedia streams as well as variations in system loading.\ud
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In this paper, we conduct an empirical study on the testing and fault-identification of multimedia systems by treating the issue as a classification problem. Typical classification techniques, including Bayesian networks, k-nearest neighbor, and neural networks, are experimented with the use of X-Smiles, an open sourced multimedia authoring tool supporting the Synchronized Multimedia Integration Language (SMIL). From these experiments, we make a few interesting observations and give plausible explanations based on the geometrical properties of the test results
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