7,147 research outputs found
Soft X-ray emission in kink-unstable coronal loops
Solar flares are associated with intense soft X-ray emission generated by the
hot flaring plasma. Kink unstable twisted flux-ropes provide a source of
magnetic energy which can be released impulsively and account for the flare
plasma heating. We compute the temporal evolution of the thermal X-ray emission
in kink-unstable coronal loops using MHD simulations and discuss the results of
with respect to solar flare observations. The model consists of a highly
twisted loop embedded in a region of uniform and untwisted coronal magnetic
field. We let the kink instability develop, compute the evolution of the plasma
properties in the loop (density, temperature) without accounting for mass
exchange with the chromosphere. We then deduce the X-ray emission properties of
the plasma during the whole flaring episode. During the initial phase of the
instability plasma heating is mostly adiabatic. Ohmic diffusion takes over as
the instability saturates, leading to strong and impulsive heating (> 20 MK),
to a quick enhancement of X-ray emission and to the hardening of the thermal
X-ray spectrum. The temperature distribution of the plasma becomes broad, with
the emission measure depending strongly on temperature. Significant emission
measures arise for plasma at temperatures T > 9 MK. The magnetic flux-rope then
relaxes progressively towards a lower energy state as it reconnects with the
background flux. The loop plasma suffers smaller sporadic heating events but
cools down conductively. The total thermal X-ray emission slowly fades away
during this phase, and the high temperature component of emission measure
distribution converges to the power-law distribution . The
amount of twist deduced directly from the X-ray emission patterns is
considerably lower than the maximum magnetic twist in the simulated flux-ropes.Comment: submitted to A&
Quantum chaos in open systems: a quantum state diffusion analysis
Except for the universe, all quantum systems are open, and according to
quantum state diffusion theory, many systems localize to wave packets in the
neighborhood of phase space points. This is due to decoherence from the
interaction with the environment, and makes the quasiclassical limit of such
systems both more realistic and simpler in many respects than the more familiar
quasiclassical limit for closed systems. A linearized version of this theory
leads to the correct classical dynamics in the macroscopic limit, even for
nonlinear and chaotic systems. We apply the theory to the forced, damped
Duffing oscillator, comparing the numerical results of the full and linearized
equations, and argue that this can be used to make explicit calculations in the
decoherent histories formalism of quantum mechanics.Comment: 18 pages standard LaTeX + 9 figures; extensively trimmed; to appear
in J. Phys.
The Low Redshift Lyman Alpha Forest in Cold Dark Matter Cosmologies
We study the physical origin of the low-redshift Lyman alpha forest in
hydrodynamic simulations of four CDM cosmologies. Our main conclusions are
insensitive to the cosmological model but depend on our assumption that the UV
background declines at low redshift. We find that the expansion of the universe
drives rapid evolution of dN/dz (the number of absorbers per unit z) at z >
1.7, but that at lower redshift the fading of the UV background counters the
influence of expansion, leading to slow evolution. At every redshift, weaker
lines come primarily from moderate fluctuations of the diffuse, unshocked IGM,
and stronger lines originate in shocked or radiatively cooled gas of higher
overdensity. However, the neutral hydrogen column density associated with
structures of fixed overdensity drops as the universe expands, so an absorber
at z = 0 is dynamically analogous to an absorber with neutral hydrogen column
density 10 to 50 times higher at z = 2-3. We find no clear distinction between
lines arising in "galaxy halos" and lines arising in larger scale structures;
however, galaxies tend to lie near the dense regions of the IGM that produce
strong Lyman alpha lines. The simulations provide a unified physical picture
that accounts for the most distinctive observed properties of the low redshift
Lyman alpha forest: (1) a sharp transition in the evolution of dN/dz at z ~
1.7, (2) stronger evolution for absorbers of higher equivalent width, (3) a
correlation of increasing Lyman alpha equivalent width with decreasing galaxy
impact parameter, and (4) a tendency for stronger lines to arise in close
proximity to galaxies while weaker lines trace more diffuse large scale
structure. (Abridged)Comment: 57 pages, 18 figures, submitted to Ap
Nonlinear position and stiffness Backstepping controller for a two Degrees of Freedom pneumatic robot
This paper presents an architecture of a 2 Degrees of Freedom pneumatic robot which can be used as a haptic interface. To improve the haptic rendering of this device, a nonlinear position and stiffness controller without force measurement based on a Backstepping synthesis is presented. Thus, the robot can follow a targeted trajectory in Cartesian position with a variable compliant behavior when disturbance forces are applied. An appropriate tuning methodology of the closed-loop stiffness and closed-loop damping of the robot is given to obtain a desired disturbance response. The models, the synthesis and the stability analysis of this controller are described in this paper. Two models are presented in this paper, the first one is an accurate simulation model which describes the mechanical behavior of the robot, the thermodynamics phenomena in the pneumatic actuators, and the servovalves characteristics. The second model is the model used to synthesize the controller. This control model is obtained by simplifying the simulation model to obtain a MIMO strict feedback form. Finally, some simulation and experimental results are given and the controller performances are discussed and compared with a classical linear impedance controller
The development of a carbon roadmap investment strategy for carbon intensive food retail industries
This work presents an approach to develop an innovative decarbonisation investment strategy framework for carbon intensive UK industries by using statistical analysis and optimisation modelling. The case study focuses on taking a representative sample of retail buildings and assesses the financial viability of installing low-carbon Combined Heat and Power units (CHPs) and Photovoltaic Solar Panels (PVs) across a portfolio of buildings. Simulation of each building are initially conducted, and the results generate a set of regression coefficients, via a multivariate adaptive regression splines (MARS), which are inputted into a Mixed Integer Linear Programming (MILP) problem. Solving the MILP yields the optimal decarbonisation investment strategy for the case study up to 2050, considering market trends such as electricity prices, gas prices and policy incentives. Results indicate the level of investment required per year, the operational and carbon savings associated, and a program for such investments. This method is reiterated for several scenarios where different parameters such as utility prices, capital costs and grid carbon factors are forecasted up to 2050 (following the Future Energy Scenarios from National Grid). This work shows how a clear mathematical framework can assist decision-makers in commercial organisations to reduce their carbon footprint cost-effectively and thus reach science-based targets
Classical Dynamics of the Quantum Harmonic Chain
The origin of classical predictability is investigated for the one
dimensional harmonic chain considered as a closed quantum mechanical system. By
comparing the properties of a family of coarse-grained descriptions of the
chain, we conclude that local coarse-grainings in this family are more useful
for prediction than nonlocal ones. A quantum mechanical system exhibits
classical behavior when the probability is high for histories having the
correlations in time implied by classical deterministic laws. But approximate
classical determinism holds only for certain coarse-grainings and then only if
the initial state of the system is suitably restricted. Coarse-grainings by the
values of the hydrodynamic variables (integrals over suitable volumes of
densities of approximately conserved quantities) define the histories usually
used in classical physics. But what distinguishes this coarse-graining from
others? This paper approaches this question by analyzing a family of
coarse-grainings for the linear harmonic chain. At one extreme in the family
the chain is divided into local groups of atoms. At the other extreme the
atoms are distributed nonlocally over the whole chain. Each coarse-graining
follows the average (center of mass) positions of the groups and ignores the
``internal'' coordinates within each group, these constituting a different
environment for each coarse-graining. We conclude that noise, decoherence, and
computational complexity favor locality over nonlocality for deterministic
predictability.Comment: 38 pages RevTeX 3.0 + 4 figures (postscript). Numerous minor
corrections. Submitted to Physical Review
On compatibility and improvement of different quantum state assignments
When Alice and Bob have different quantum knowledges or state assignments
(density operators) for one and the same specific individual system, then the
problems of compatibility and pooling arise. The so-called first
Brun-Finkelstein-Mermin (BFM) condition for compatibility is reobtained in
terms of possessed or sharp (i. e., probability one) properties. The second BFM
condition is shown to be generally invalid in an infinite-dimensional state
space. An argument leading to a procedure of improvement of one state
assifnment on account of the other and vice versa is presented.Comment: 8 page
A new 21-cm absorber identified with an galaxy
We present Giant Metrewave Radio Telescope (GMRT) observations of redshifted
21-cm absorption from the metal line absorption system towards PKS
1243-072. HI absorption is clearly detected; the absorption profile has a
velocity spread of km/s. Detection of 21-cm absorption indicates that
the absorber has an HI column density large enough to be classified as a damped
Lyman- system. Follow up ground based optical imaging and spectroscopy
allow us to identify the absorber with an galaxy at an impact
parameter of kpc from the line of sight to the QSO. The absorbing
galaxy is unusual in that it has bright emission lines. On the basis of the
optical spectrum we are unable to uniquely classify the galaxy since its
emission line ratios lie in the transition region between starburst and Seyfert
II type spectra.Comment: 5 pages, 3 figures. Accepted for publication in Astronomy &
Astrophysic
A population-based approach to background discrimination in particle physics
Background properties in experimental particle physics are typically
estimated using control samples corresponding to large numbers of events. This
can provide precise knowledge of average background distributions, but
typically does not consider the effect of fluctuations in a data set of
interest. A novel approach based on mixture model decomposition is presented as
a way to estimate the effect of fluctuations on the shapes of probability
distributions in a given data set, with a view to improving on the knowledge of
background distributions obtained from control samples. Events are treated as
heterogeneous populations comprising particles originating from different
processes, and individual particles are mapped to a process of interest on a
probabilistic basis. The proposed approach makes it possible to extract from
the data information about the effect of fluctuations that would otherwise be
lost using traditional methods based on high-statistics control samples. A
feasibility study on Monte Carlo is presented, together with a comparison with
existing techniques. Finally, the prospects for the development of tools for
intensive offline analysis of individual events at the Large Hadron Collider
are discussed.Comment: Updated according to the version published in J. Phys.: Conf. Ser.
Minor changes have been made to the text with respect to the published
article with a view to improving readabilit
Log-normal distributions of suspended particles in the open ocean
A scanning electron microscope-electron microprobe technique was used to chemically distinguish and size particles as fine as 0.2/µm on GEOSECS suspended matter filters from the open ocean…
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