136,766 research outputs found
Solar Dynamics Observatory discovers thin high temperature strands in coronal active regions
One scenario proposed to explain the million degrees solar corona is a
finely-stranded corona where each strand is heated by a rapid pulse. However,
such fine structure has neither been resolved through direct imaging
observations nor conclusively shown through indirect observations of extended
superhot plasma. Recently it has been shown that the observed difference in
appearance of cool and warm coronal loops (~1 MK, ~2-3 MK, respectively) --
warm loops appearing "fuzzier" than cool loops -- can be explained by models of
loops composed of subarcsecond strands, which are impulsively heated up to ~10
MK. That work predicts that images of hot coronal loops (>~6 MK) should again
show fine structure. Here we show that the predicted effect is indeed widely
observed in an active region with the Solar Dynamics Observatory, thus
supporting a scenario where impulsive heating of fine loop strands plays an
important role in powering the active corona.Comment: 11 pages, 4 figures, accepted for publicatio
Distillation of Entanglement between Distant Systems by Repeated Measurements on Entanglement Mediator
A recently proposed purification method, in which the Zeno-like measurements
of a subsystem can bring about a distillation of another subsystem in
interaction with the former, is utilized to yield entangled states between
distant systems. It is shown that the measurements of a two-level system
locally interacting with other two spatially separated not coupled subsystems,
can distill entangled states from the latter irrespectively of the initial
states of the two subsystems.Comment: 11 pages, 2 figures; the version accepted for publication in Phys.
Rev.
Tuning non-Markovianity by spin-dynamics control
We study the interplay between forgetful and memory-keeping evolution
enforced on a two-level system by a multi-spin environment whose elements are
coupled to local bosonic baths. Contrarily to the expectation that any
non-Markovian effect would be buried by the forgetful mechanism induced by the
spin-bath coupling, one can actually induce a full Markovian-to-non-Markovian
transition of the two-level system's dynamics, controllable by parameters such
as the mismatch between the energy of the two-level system and of the spin
environment. For a symmetric coupling, the amount of non-Markovianity
surprisingly grows with the number of decoherence channels.Comment: 7 pages, 6 figures, PRA versio
Nonlocal properties of entangled two-photon generalized binomial states in two separate cavities
We consider entangled two-photon generalized binomial states of the
electromagnetic field in two separate cavities. The nonlocal properties of this
entangled field state are analyzed by studying the electric field correlations
between the two cavities. A Bell's inequality violation is obtained using an
appropriate dichotomic cavity operator, that is in principle measurable.Comment: 5 pages, 1 figur
A variational method from the variance of energy
A variational method is studied based on the minimum of energy variance. The
method is tested on exactly soluble problems in quantum mechanics, and is shown
to be a useful tool whenever the properties of states are more relevant than
the eigenvalues. In quantum field theory the method provides a consistent
second order extension of the gaussian effective potential.Comment: 5 ps figure
Fluxon dynamics by microwave surface resistance measurements in MgB2
Field-induced variations of the microwave surface resistance, Rs(H), have
been investigated in high-density ceramic MgB2. At low temperatures, several
peculiarities of the Rs(H) curves cannot be justified in the framework of
models reported in the literature. We suggest that they are ascribable to the
unconventional vortex structure in MgB2, related to the presence of two gaps.
On the contrary, the results near Tc can be accounted for by the Coffey and
Clem model, with fluxons moving in the flux-flow regime, provided that the
anisotropy of the upper critical field is taken into due account.Comment: 6 pages, 4 figure
Effects of non-uniform interstellar magnetic field on synchrotron X-ray and inverse-Compton gamma-ray morphology of SNRs
Observations of SNRs in X-ray and gamma-ray bands promise to contribute with
important information in our understanding on the nature of galactic cosmic
rays. The analysis of SNRs images collected in different energy bands requires
the support of theoretical modeling of synchrotron and inverse Compton (IC)
emission. We develop a numerical code (REMLIGHT) to synthesize, from MHD
simulations, the synchrotron radio, X-ray and IC gamma-ray emission from SNRs
expanding in non-uniform interstellar medium (ISM) and/or non-uniform
interstellar magnetic field (ISMF). As a first application, the code is used to
investigate the effects of non-uniform ISMF on the SNR morphology in the
non-thermal X-ray and gamma-ray bands. We perform 3D MHD simulations of a
spherical SNR shock expanding through a magnetized ISM with a gradient of
ambient magnetic field strength. The model includes an approximate treatment of
upstream magnetic field amplification and the effect of shock modification due
to back reaction of accelerated cosmic rays. From the simulations, we
synthesize the synchrotron radio, X-ray and IC gamma-ray emission with
REMLIGHT, making different assumptions about the details of acceleration and
injection of relativistic electrons. A gradient of the ambient magnetic field
strength induces asymmetric morphologies in radio, X-ray and gamma-ray bands
independently from the model of electron injection if the gradient has a
component perpendicular to the line-of-sight. The degree of asymmetry of the
remnant morphology depends on the details of the electron injection and
acceleration and is different in the radio, X-ray, and gamma-ray bands. The
non-thermal X-ray morphology is the most sensitive to the gradient, showing the
highest degree of asymmetry. The IC gamma-ray emission is weakly sensitive to
the non-uniform ISMF, the degree of asymmetry of the SNR morphology being the
lowest in this band.Comment: 16 pages, 13 Figures; accepted for publication on A&A. Version with
full resolution images can be found at
http://www.astropa.unipa.it/~orlando/PREPRINTS/sorlando_15505.pd
Two-color ionization of hydrogen by short intense pulses
Photoelectron energy spectra resulting by the interaction of hydrogen with
two short pulses having carrier frequencies, respectively, in the range of the
infrared and XUV regions have been calculated. The effects of the pulse
duration and timing of the X-ray pulse on the photoelectron energy spectra are
discussed. Analysis of the spectra obtained for very long pulses show that
certain features may be explained in terms of quantum interferences in the time
domain. It is found that, depending on the duration of the X-ray pulse, ripples
in the energy spectra separated by the infrared photon energy may appear.
Moreover, the temporal shape of the low frequency radiation field may be
inferred by the breadth of the photoelectron energy spectra.Comment: 12 pages, 8 figure
Microwave response of bulk MgB2 samples of different granularity
The microwave response of three high-density bulk MgB2 samples has been
investigated in the linear and nonlinear regimes. The three samples,
characterized by different mean size of grains, have been obtained by reactive
infiltration of liquid Mg in powdered B preforms. The linear response has been
studied by measuring the microwave surface impedance; the nonlinear response by
detecting the power radiated by the sample at the second-harmonic frequency of
the driving field. Our results suggest that bulk MgB2 prepared by the liquid Mg
infiltration technique is particularly promising for manufacturing resonant
cavities operating at microwave frequencies.Comment: 4 pages, 2 embedded figures; Proceedings of 7th EUCAS Conference
(11-15 September 2005, Vienna - Austria
Moment equations in a Lotka-Volterra extended system with time correlated noise
A spatially extended Lotka-Volterra system of two competing species in the
presence of two correlated noise sources is analyzed: (i) an external
multiplicative time correlated noise, which mimics the interaction between the
system and the environment; (ii) a dichotomous stochastic process, whose jump
rate is a periodic function, which represents the interaction parameter between
the species. The moment equations for the species densities are derived in
Gaussian approximation, using a mean field approach. Within this formalism we
study the effect of the external time correlated noise on the ecosystem
dynamics. We find that the time behavior of the order moments are
independent on the multiplicative noise source. However the behavior of the
order moments is strongly affected both by the intensity and the
correlation time of the multiplicative noise. Finally we compare our results
with those obtained studying the system dynamics by a coupled map lattice
model.Comment: 12 pages, 7 figures, to appear in Acta Phys. Pol.
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