8,046 research outputs found
On the 3-D structure and dissipation of reconnection-driven flow-bursts
The structure of magnetic reconnection-driven outflows and their dissipation
are explored with large-scale, 3-D particle-in-cell (PIC) simulations. Outflow
jets resulting from 3-D reconnection with a finite length x-line form fronts as
they propagate into the downstream medium. A large pressure increase ahead of
this ``reconnection jet front'' (RJF), due to reflected and transmitted ions,
slows the front so that its velocity is well below the velocity of the ambient
ions in the core of the jet. As a result, the RJF slows and diverts the
high-speed flow into the direction perpendicular to the reconnection plane. The
consequence is that the RJF acts as a thermalization site for the ion bulk flow
and contributes significantly to the dissipation of magnetic energy during
reconnection even though the outflow jet is subsonic. This behavior has no
counterpart in 2-D reconnection. A simple analytic model predicts the front
velocity and the fraction of the ion bulk flow energy that is dissipated
Racing Multi-Objective Selection Probabilities
In the context of Noisy Multi-Objective Optimization, dealing with
uncertainties requires the decision maker to define some preferences about how
to handle them, through some statistics (e.g., mean, median) to be used to
evaluate the qualities of the solutions, and define the corresponding Pareto
set. Approximating these statistics requires repeated samplings of the
population, drastically increasing the overall computational cost. To tackle
this issue, this paper proposes to directly estimate the probability of each
individual to be selected, using some Hoeffding races to dynamically assign the
estimation budget during the selection step. The proposed racing approach is
validated against static budget approaches with NSGA-II on noisy versions of
the ZDT benchmark functions
Tensor Networks for Dimensionality Reduction and Large-Scale Optimizations. Part 2 Applications and Future Perspectives
Part 2 of this monograph builds on the introduction to tensor networks and
their operations presented in Part 1. It focuses on tensor network models for
super-compressed higher-order representation of data/parameters and related
cost functions, while providing an outline of their applications in machine
learning and data analytics. A particular emphasis is on the tensor train (TT)
and Hierarchical Tucker (HT) decompositions, and their physically meaningful
interpretations which reflect the scalability of the tensor network approach.
Through a graphical approach, we also elucidate how, by virtue of the
underlying low-rank tensor approximations and sophisticated contractions of
core tensors, tensor networks have the ability to perform distributed
computations on otherwise prohibitively large volumes of data/parameters,
thereby alleviating or even eliminating the curse of dimensionality. The
usefulness of this concept is illustrated over a number of applied areas,
including generalized regression and classification (support tensor machines,
canonical correlation analysis, higher order partial least squares),
generalized eigenvalue decomposition, Riemannian optimization, and in the
optimization of deep neural networks. Part 1 and Part 2 of this work can be
used either as stand-alone separate texts, or indeed as a conjoint
comprehensive review of the exciting field of low-rank tensor networks and
tensor decompositions.Comment: 232 page
Tensor Networks for Dimensionality Reduction and Large-Scale Optimizations. Part 2 Applications and Future Perspectives
Part 2 of this monograph builds on the introduction to tensor networks and
their operations presented in Part 1. It focuses on tensor network models for
super-compressed higher-order representation of data/parameters and related
cost functions, while providing an outline of their applications in machine
learning and data analytics. A particular emphasis is on the tensor train (TT)
and Hierarchical Tucker (HT) decompositions, and their physically meaningful
interpretations which reflect the scalability of the tensor network approach.
Through a graphical approach, we also elucidate how, by virtue of the
underlying low-rank tensor approximations and sophisticated contractions of
core tensors, tensor networks have the ability to perform distributed
computations on otherwise prohibitively large volumes of data/parameters,
thereby alleviating or even eliminating the curse of dimensionality. The
usefulness of this concept is illustrated over a number of applied areas,
including generalized regression and classification (support tensor machines,
canonical correlation analysis, higher order partial least squares),
generalized eigenvalue decomposition, Riemannian optimization, and in the
optimization of deep neural networks. Part 1 and Part 2 of this work can be
used either as stand-alone separate texts, or indeed as a conjoint
comprehensive review of the exciting field of low-rank tensor networks and
tensor decompositions.Comment: 232 page
Linear response within the projection-based renormalization method: Many-body corrections beyond the random phase approximation
The explicit evaluation of linear response coefficients for interacting
many-particle systems still poses a considerable challenge to theoreticians. In
this work we use a novel many-particle renormalization technique, the so-called
projector-based renormalization method, to show how such coefficients can
systematically be evaluated. To demonstrate the prospects and power of our
approach we consider the dynamical wave-vector dependent spin susceptibility of
the two-dimensional Hubbard model and also determine the subsequent magnetic
phase diagram close to half-filling. We show that the superior treatment of
(Coulomb) correlation and fluctuation effects within the projector-based
renormalization method significantly improves the standard random phase
approximation results.Comment: 17 pages, 7 figures, revised versio
Kinetic signatures of the region surrounding the X-line in asymmetric (magnetopause) reconnection
Kinetic particle-in-cell simulations are used to identify signatures of the
electron diffusion region (EDR) and its surroundings during asymmetric magnetic
reconnection. A "shoulder" in the sunward pointing normal electric field (EN >
0) at the reconnection magnetic field reversal is a good indicator of the EDR,
and is caused by magnetosheath electron meandering orbits in the vicinity of
the x-line. Earthward of the X-line, electrons accelerated by EN form strong
currents and crescent-shaped distribution functions in the plane perpendicular
to B. Just downstream of the X-line, parallel electric fields create
field-aligned crescent electron distribution functions. In the immediate
upstream magnetosheath, magnetic field strength, plasma density, and
perpendicular electron temperatures are lower than the asymptotic state. In the
magnetosphere inflow region, magnetosheath ions intrude resulting in an
Earthward pointing electric field and parallel heating of magnetospheric
particles. Many of the above properties persist with a guide field of at least
unity.Comment: Submitted to Geophysical Research Letter
Temperature dependent graphene suspension due to thermal Casimir interaction
Thermal effects contributing to the Casimir interaction between objects are
usually small at room temperature and they are difficult to separate from
quantum mechanical contributions at higher temperatures. We propose that the
thermal Casimir force effect can be observed for a graphene flake suspended in
a fluid between substrates at the room temperature regime. The properly chosen
materials for the substrates and fluid induce a Casimir repulsion. The balance
with the other forces, such as gravity and buoyancy, results in a stable
temperature dependent equilibrium separation. The suspended graphene is a
promising system due to its potential for observing thermal Casimir effects at
room temperature.Comment: 5 pages, 4 figures, in APL production 201
On the Cause of Supra-Arcade Downflows in Solar Flares
A model of supra-arcade downflows (SADs), dark low density regions also known
as tadpoles that propagate sunward during solar flares, is presented. It is
argued that the regions of low density are flow channels carved by
sunward-directed outflow jets from reconnection. The solar corona is
stratified, so the flare site is populated by a lower density plasma than that
in the underlying arcade. As the jets penetrate the arcade, they carve out
regions of depleted plasma density which appear as SADs. The present
interpretation differs from previous models in that reconnection is localized
in space but not in time. Reconnection is continuous in time to explain why
SADs are not filled in from behind as they would if they were caused by
isolated descending flux tubes or the wakes behind them due to temporally
bursty reconnection. Reconnection is localized in space because outflow jets in
standard two-dimensional reconnection models expand in the normal (inflow)
direction with distance from the reconnection site, which would not produce
thin SADs as seen in observations. On the contrary, outflow jets in spatially
localized three-dimensional reconnection with an out-of-plane (guide) magnetic
field expand primarily in the out-of-plane direction and remain collimated in
the normal direction, which is consistent with observed SADs being thin.
Two-dimensional proof-of-principle simulations of reconnection with an
out-of-plane (guide) magnetic field confirm the creation of SAD-like depletion
regions and the necessity of density stratification. Three-dimensional
simulations confirm that localized reconnection remains collimated.Comment: 16 pages, 5 figures, accepted to Astrophysical Journal Letters in
August, 2013. This version is the accepted versio
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