149,891 research outputs found
Particle-in-cell and weak turbulence simulations of plasma emission
The plasma emission process, which is the mechanism for solar type II and
type III radio bursts phenomena, is studied by means of particle-in-cell and
weak turbulence simulation methods. By plasma emission, it is meant as a loose
description of a series of processes, starting from the solar flare associated
electron beam exciting Langmuir and ion-acoustic turbulence, and subsequent
partial conversion of beam energy into the radiation energy by nonlinear
processes. Particle-in-cell (PIC) simulation is rigorous but the method is
computationally intense, and it is difficult to diagnose the results. Numerical
solution of equations of weak turbulence (WT) theory, termed WT simulation, on
the other hand, is efficient and naturally lends itself to diagnostics since
various terms in the equation can be turned on or off. Nevertheless, WT theory
is based upon a number of assumptions. It is, therefore, desirable to compare
the two methods, which is carried out for the first time in the present paper
with numerical solutions of the complete set of equations of the WT theory and
with two-dimensional electromagnetic PIC simulation. Upon making quantitative
comparisons it is found that WT theory is largely valid, although some
discrepancies are also found. The present study also indicates that it requires
large computational resources in order to accurately simulate the radiation
emission processes, especially for low electron beam speeds. Findings from the
present paper thus imply that both methods may be useful for the study of solar
radio emissions as they are complementary.Comment: 21 pages, 9 figure
Hardness of Graph Pricing through Generalized Max-Dicut
The Graph Pricing problem is among the fundamental problems whose
approximability is not well-understood. While there is a simple combinatorial
1/4-approximation algorithm, the best hardness result remains at 1/2 assuming
the Unique Games Conjecture (UGC). We show that it is NP-hard to approximate
within a factor better than 1/4 under the UGC, so that the simple combinatorial
algorithm might be the best possible. We also prove that for any , there exists such that the integrality gap of
-rounds of the Sherali-Adams hierarchy of linear programming for
Graph Pricing is at most 1/2 + .
This work is based on the effort to view the Graph Pricing problem as a
Constraint Satisfaction Problem (CSP) simpler than the standard and complicated
formulation. We propose the problem called Generalized Max-Dicut(), which
has a domain size for every . Generalized Max-Dicut(1) is
well-known Max-Dicut. There is an approximation-preserving reduction from
Generalized Max-Dicut on directed acyclic graphs (DAGs) to Graph Pricing, and
both our results are achieved through this reduction. Besides its connection to
Graph Pricing, the hardness of Generalized Max-Dicut is interesting in its own
right since in most arity two CSPs studied in the literature, SDP-based
algorithms perform better than LP-based or combinatorial algorithms --- for
this arity two CSP, a simple combinatorial algorithm does the best.Comment: 28 page
SMA observations of the proto brown dwarf candidate SSTB213 J041757
Context. The previously identified source SSTB213 J041757 is a proto brown
dwarf candidate in Taurus, which has two possible components A and B. It was
found that component B is probably a class 0/I proto brown dwarf associated
with an extended envelope.
Aims. Studying molecular outflows from young brown dwarfs provides important
insight into brown dwarf formation mechanisms, particularly brown dwarfs at the
earliest stages such as class 0, I. We therefore conducted a search for
molecular outflows from SSTB213 J041757.
Methods. We observed SSTB213 J041757 with the Submillimeter Array to search
for CO molecular outflow emission from the source.
Results. Our CO maps do not show any outflow emission from the proto brown
dwarf candidate.
Conclusions. The non-detection implies that the molecular outflows from the
source are weak; deeper observations are therefore needed to probe the outflows
from the source.Comment: 7 pages, 4 figures, accepted for publication in A&
PowerAqua: fishing the semantic web
The Semantic Web (SW) offers an opportunity to develop novel, sophisticated forms of question answering (QA). Specifically, the availability of distributed semantic markup on a large scale opens the way to QA systems which can make use of such semantic information to provide precise, formally derived answers to questions. At the same time the distributed, heterogeneous, large-scale nature of the semantic information introduces significant challenges. In this paper we describe the design of a QA system, PowerAqua, designed to exploit semantic markup on the web to provide answers to questions posed in natural language. PowerAqua does not assume that the user has any prior information about the semantic resources. The system takes as input a natural language query, translates it into a set of logical queries, which are then answered by consulting and aggregating information derived from multiple heterogeneous semantic sources
Scaling of Reaction Zones in the A+B->0 Diffusion-Limited Reaction
We study reaction zones in three different versions of the A+B->0 system. For
a steady state formed by opposing currents of A and B particles we derive
scaling behavior via renormalization group analysis. By use of a previously
developed analogy, these results are extended to the time-dependent case of an
initially segregated system. We also consider an initially mixed system, which
forms reaction zones for dimension d<4. In this case an extension of the
steady-state analogy gives scaling results characterized by new exponents.Comment: 4 pages, REVTeX 3.0 with epsf, 2 uuencoded postscript figures
appended, OUTP-94-33
Charge Oscillations in Debye-Hueckel Theory
The recent generalized Debye-Hueckel (GDH) theory is applied to the
calculation of the charge-charge correlation function G_{ZZ}(r). The resulting
expression satisfies both (i) the charge neutrality condition and (ii) the
Stillinger-Lovett second-moment condition for all T and rho_N, the overall ion
density, and (iii) exhibits charge oscillations for densities above a "Kirkwood
line" in the (rho_N,T) plane. This corrects the normally assumed DH
correlations, and, when combined with the GDH analysis of the density
correlations, leaves the GDH theory as the only complete description of ionic
correlation functions, as judged by (i)-(iii), (iv) exact low-density (rho_N,T)
variation, and (v) reasonable behavior near criticality.Comment: 6 pages, EuroPhys.sty (now available on archive), 1 eps figur
Large scale emergent properties of an autocatalytic reaction-diffusion model subject to noise
The non-equilibrium dynamic fluctuations of a stochastic version of the
Gray-Scott (GS) model are studied analytically in leading order in perturbation
theory by means of the dynamic renormalization group. There is an attracting
stable fixed point at one-loop order, and the asymptotic scaling of the
correlation functions is predicted for both spatial and temporally correlated
noise sources. New effective three-body reaction terms, not present in the
original GS model, are induced by the combined interplay of the fluctuations
and nonlinearities.Comment: 13 pages, 2 figure
Collapse Dynamics of a Homopolymer: Theory and Simulation
We present a scaling theory describing the collapse of a homopolymer chain in
poor solvent. At time t after the beginning of the collapse, the original
Gaussian chain of length N is streamlined to form N/g segments of length R(t),
each containing g ~ t monomers. These segments are statistical quantities
representing cylinders of length R ~ t^{1/2} and diameter d ~ t^{1/4}, but
structured out of stretched arrays of spherical globules. This prescription
incorporates the capillary instability. We compare the time-dependent structure
factor derived for our theory with that obtained from ultra-large-scale
molecular dynamics simulation with explicit solvent. This is the first time
such a detailed comparison of theoretical and simulation predictions of
collapsing chain structure has been attempted. The favorable agreement between
the theoretical and computed structure factors supports the picture of the
coarse-graining process during polymer collapse.Comment: 4 pages, 3 figure
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