16,713 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&
Symmetry Aspects in Nonrelativistic Multi-Scalar Field Models and Application to a Coupled Two-Species Dilute Bose Gas
We discuss unusual aspects of symmetry that can happen due to entropic
effects in the context of multi-scalar field theories at finite temperature. We
present their consequences, in special, for the case of nonrelativistic models
of hard core spheres. We show that for nonrelativistic models phenomena like
inverse symmetry breaking and symmetry non-restoration cannot take place, but a
reentrant phase at high temperatures is shown to be possible for some region of
parameters. We then develop a model of interest in studies of Bose-Einstein
condensation in dilute atomic gases and discuss about its phase transition
patterns. In this application to a Bose-Einstein condensation model, however,
no reentrant phases are found.Comment: 8 pages, 1 eps figure, IOP style. Based on a talk given by R. O.
Ramos at the QFEXT05 workshop, Barcelona, Spain, September 5-9, 2005. One
reference was update
Thermal and non-thermal emission from reconnecting twisted coronal loops
Twisted magnetic fields should be ubiquitous in flare-producing active
regions where the magnetic fields are strongly non-potential. It has been shown
that reconnection in helical magnetic coronal loops results in plasma heating
and particle acceleration distributed within a large volume, including the
lower coronal and chromospheric sections of the loops. This scenario can be an
alternative to the standard flare model, where particles are accelerated only
in a small volume located in the upper corona. We use a combination of MHD
simulations and test-particle methods, which describe the development of kink
instability and magnetic reconnection in twisted coronal loops using resistive
compressible MHD, and incorporate atmospheric stratification and large-scale
loop curvature. The resulting distributions of hot plasma let us estimate
thermal X-ray emission intensities. The electric and magnetic fields obtained
are used to calculate electron trajectories using the guiding-centre
approximation. These trajectories combined with the MHD plasma density
distributions let us deduce synthetic HXR bremsstrahlung intensities. Our
simulations emphasise that the geometry of the emission patterns produced by
hot plasma in flaring twisted coronal loops can differ from the actual geometry
of the underlying magnetic fields. The twist angles revealed by the emission
threads (SXR) are consistently lower than the field-line twist present at the
onset of the kink-instability. HXR emission due to the interaction of energetic
electrons with the stratified background are concentrated at the loop
foot-points in these simulations, even though the electrons are accelerated
everywhere within the coronal volume of the loop. The maximum of HXR emission
consistently precedes that of SXR emission, with the HXR light-curve being
approximately proportional to the temporal derivative of the SXR light-curve.Comment: (accepted for publication on A&A
Inverse Symmetry Breaking in Multi-Scalar Field Theories
We review how the phenomena of inverse symmetry breaking (and symmetry
nonrestoration) may arise in the context of relativistic as well as
nonrelativistic multi-scalar field theories. We discuss how the consideration
of thermal effects on the couplings produce different transition patterns for
both theories. For the relativistic case, these effects allow the appearance of
inverse symmetry breaking (and symmetry nonrestoration) at arbitrarily large
temperatures. On the other hand, the same phenomena are suppressed in the
nonrelativistic case, which is relevant for condensed matter physics. In this
case, symmetry nonrestoration does not happen while inverse symmetry is allowed
only to be followed by symmetry restoration characterizing a reentrant phase.
The aim of this paper is to give more insight concerning the, qualitatively
correct, results obtained by using one loop perturbation theory in the
evaluation of thermal masses and couplings.Comment: 7 pages, 3 figures, talk given at the workshop on Quantum Fields
Under the Influence of External Conditions, QFEXT05, Barcelona, sep-200
Comments on the Quantum Potential Approach to a Class of Quantum Cosmological Models
In this comment we bring attention to the fact that when we apply the
ontological interpretation of quantum mechanics, we must be sure to use it in
the coordinate representation. This is particularly important when canonical
tranformations that mix momenta and coordinates are present. This implies that
some of the results obtained by A. B\l aut and J. Kowalski-Glikman are
incorrect.Comment: 7 pages, LaTe
Recording from two neurons: second order stimulus reconstruction from spike trains and population coding
We study the reconstruction of visual stimuli from spike trains, recording
simultaneously from the two H1 neurons located in the lobula plate of the fly
Chrysomya megacephala. The fly views two types of stimuli, corresponding to
rotational and translational displacements. If the reconstructed stimulus is to
be represented by a Volterra series and correlations between spikes are to be
taken into account, first order expansions are insufficient and we have to go
to second order, at least. In this case higher order correlation functions have
to be manipulated, whose size may become prohibitively large. We therefore
develop a Gaussian-like representation for fourth order correlation functions,
which works exceedingly well in the case of the fly. The reconstructions using
this Gaussian-like representation are very similar to the reconstructions using
the experimental correlation functions. The overall contribution to rotational
stimulus reconstruction of the second order kernels - measured by a chi-squared
averaged over the whole experiment - is only about 8% of the first order
contribution. Yet if we introduce an instant-dependent chi-square to measure
the contribution of second order kernels at special events, we observe an up to
100% improvement. As may be expected, for translational stimuli the
reconstructions are rather poor. The Gaussian-like representation could be a
valuable aid in population coding with large number of neurons
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