38,369 research outputs found
Suppression of parallel transport in turbulent magnetized plasmas and its impact on the non-thermal and thermal aspects of solar flares
The transport of the energy contained in electrons, both thermal and suprathermal, in solar flares plays a key role in our understanding of many aspects of the flare phenomenon, from the spatial distribution of hard X-ray emission to global energetics. Motivated by recent RHESSI observations that point to the existence of a mechanism that confines electrons to the coronal parts of flare loops more effectively than Coulomb collisions, we here consider the impact of pitch-angle scattering off turbulent magnetic fluctuations on the parallel transport of electrons in flaring coronal loops. It is shown that the presence of such a scattering mechanism in addition to Coulomb collisional scattering can significantly reduce the parallel thermal and electrical conductivities relative to their collisional values. We provide illustrative expressions for the resulting thermoelectric coefficients that relate the thermal flux and electrical current density to the temperature gradient and the applied electric field. We then evaluate the effect of these modified transport coefficients on the flare coronal temperature that can be attained, on the post-impulsive-phase cooling of heated coronal plasma, and on the importance of the beam-neutralizing return current on both ambient heating and the energy loss rate of accelerated electrons. We also discuss the possible ways in which anomalous transport processes have an impact on the required overall energy associated with accelerated electrons in solar flares
The role of diffusion in the transport of energetic electrons during solar flares
The transport of the energy contained in suprathermal electrons in solar
flares plays a key role in our understanding of many aspects of flare physics,
from the spatial distributions of hard X-ray emission and energy deposition in
the ambient atmosphere to global energetics. Historically the transport of
these particles has been largely treated through a deterministic approach, in
which first-order secular energy loss to electrons in the ambient target is
treated as the dominant effect, with second-order diffusive terms (in both
energy and angle) being generally either treated as a small correction or even
neglected. We here critically analyze this approach, and we show that spatial
diffusion through pitch-angle scattering necessarily plays a very significant
role in the transport of electrons. We further show that a satisfactory
treatment of the diffusion process requires consideration of non-local effects,
so that the electron flux depends not just on the local gradient of the
electron distribution function but on the value of this gradient within an
extended region encompassing a significant fraction of a mean free path. Our
analysis applies generally to pitch-angle scattering by a variety of
mechanisms, from Coulomb collisions to turbulent scattering. We further show
that the spatial transport of electrons along the magnetic field of a flaring
loop can be modeled rather effectively as a Continuous Time Random Walk with
velocity-dependent probability distribution functions of jump sizes and
occurrences, both of which can be expressed in terms of the scattering mean
free path.Comment: 11 pages, to be published in Astrophysical Journa
Lie Algebras and Suppression of Decoherence in Open Quantum Systems
Since there are many examples in which no decoherence-free subsystems exist
(among them all cases where the error generators act irreducibly on the system
Hilbert space), it is of interest to search for novel mechanisms which suppress
decoherence in these more general cases. Drawing on recent work
(quant-ph/0502153) we present three results which indicate decoherence
suppression without the need for noiseless subsystems. There is a certain
trade-off; our results do not necessarily apply to an arbitrary initial density
matrix, or for completely generic noise parameters. On the other hand, our
computational methods are novel and the result--suppression of decoherence in
the error-algebra approach without noiseless subsystems--is an interesting new
direction.Comment: 7 page
Difficult to Show Properties and Utility Maximizing Brokers
This article is the winner of the Real Estate and the Internet manuscript prize (sponsored by PricewaterhouseCoopers) presented at the American Real Estate Society Annual Meeting. Brokers have long believed that difficult to show properties sell at lower prices and take longer to sell. Where difficult to show properties are defined as those properties that present extraordinary difficulties for a broker in arranging or showing the listing to a particular buyer. Buyers’ recent access to online real estate applications may make the cost of avoiding these properties prohibitive to brokers. Employing a hedonic pricing model and duration modeling techniques, this study finds that property price and marketing time are not significantly affected for these properties. The results suggest that brokers possess limited market power.
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Sipping Fuel and Saving Lives: Increasing Fuel Economy without Sacrificing Safety
Demonstrates how new fuel-efficiency technologies make it possible, and advisable, to significantly increase the fuel economy of motor vehicles without compromising their safety
Zeno and anti-Zeno effects for quantum Brownian motion
In this paper we investigate the occurrence of the Zeno and anti-Zeno effects
for quantum Brownian motion. We single out the parameters of both the system
and the reservoir governing the crossover between Zeno and anti-Zeno dynamics.
We demonstrate that, for high reservoir temperatures, the short time behaviour
of environment induced decoherence is the ultimate responsible for the
occurrence of either the Zeno or the anti-Zeno effect. Finally we suggest a way
to manipulate the decay rate of the system and to observe a controlled
continuous passage from decay suppression to decay acceleration using
engineered reservoirs in the trapped ion context .Comment: 4 pages, 1 figure. v2: Replaced with the published version. Minor
modifications in the text and titl
Critical Behavior of Light
Light is shown to exhibit critical and tricritical behavior in passive
mode-locked lasers with externally injected pulses. It is a first and unique
example of critical phenomena in a one-dimensional many body light-mode system.
The phase diagrams consist of regimes with continuous wave, driven para-pulses,
spontaneous pulses via mode condensation, and heterogeneous pulses, separated
by phase transition lines which terminate with critical or tricritical points.
Enhanced nongaussian fluctuations and collective dynamics are observed at the
critical and tricritical points, showing a mode system analog of the critical
opalescence phenomenon. The critical exponents are calculated and shown to
comply with the mean field theory, which is rigorous in the light system.Comment: RevTex, 5 pages, 3 figure
On the variation of solar flare coronal x-ray source sizes with energy
Observations with {\em RHESSI} have enabled the detailed study of the
structure of dense hard X-ray coronal sources in solar flares. The variation of
source extent with electron energy has been discussed in the context of
streaming of non-thermal particles in a one-dimensional cold-target model, and
the results used to constrain both the physical extent of, and density within,
the electron acceleration region. Here we extend this investigation to a more
physically realistic model of electron transport that takes into account the
finite temperature of the ambient plasma, the initial pitch-angle distribution
of the accelerated electrons, and the effects of collisional pitch-angle
scattering. The finite temperature results in the thermal diffusion of
electrons, that leads to the observationally-inferred value of the acceleration
region volume being an overestimate of its true value. The different directions
of the electron trajectories, a consequence of both the non-zero injection
pitch-angle and scattering within the target, cause the projected propagation
distance parallel to the guiding magnetic field to be reduced, so that a
one-dimensional interpretation can overestimate the actual density by a factor
of up to . The implications of these results for the determination of
acceleration region properties (specific acceleration rate, filling factor,
etc.) are discussed.Comment: 45 pages, 9 figures, accepted for publication in Ap
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