12,812 research outputs found
A New Implementation of the Magnetohydrodynamics-Relaxation Method for Nonlinear Force-Free Field Extrapolation in the Solar Corona
Magnetic field in the solar corona is usually extrapolated from photospheric
vector magnetogram using a nonlinear force-free field (NLFFF) model. NLFFF
extrapolation needs a considerable effort to be devoted for its numerical
realization. In this paper we present a new implementation of the
magnetohydrodynamics (MHD)-relaxation method for NLFFF extrapolation. The
magneto-frictional approach which is introduced for speeding the relaxation of
the MHD system is novelly realized by the spacetime conservation-element and
solution-element (CESE) scheme. A magnetic field splitting method is used to
further improve the computational accuracy. The bottom boundary condition is
prescribed by changing the transverse field incrementally to match the
magnetogram, and all other artificial boundaries of the computational box are
simply fixed. We examine the code by two types of NLFFF benchmark tests, the
Low & Lou (1990) semi-analytic force-free solutions and a more realistic
solar-like case constructed by van Ballegooijen et al. (2007). The results show
that our implementation are successful and versatile for extrapolations of
either the relatively simple cases or the rather complex cases which need
significant rebuilding of the magnetic topology, e.g., a flux rope. We also
compute a suite of metrics to quantitatively analyze the results and
demonstrate that the performance of our code in extrapolation accuracy
basically reaches the same level of the present best-performing code, e.g.,
that developed by Wiegelmann (2004).Comment: Accept by ApJ, 45 pages, 13 figure
Three-dimensional finite element modelling of stack pollutant emissions
In this paper we propose a finite element method approach formodelling the air quality in a local scale over complex terrain. The area of interest is up to tens of kilometres and it includes pollutant sources. The proposed methodology involves the generation of an adaptive tetrahedral mesh, the computation of an ambient wind field, the inclusion of the plume rise effect in the wind field, and the simulation of transport and reaction of pollutants. The methodology is used to simulate a fictitious pollution episode in La Palma island (Canary Island, Spain).Peer ReviewedPostprint (published version
An accurate calculation of the nucleon axial charge with lattice QCD
We report on a lattice QCD calculation of the nucleon axial charge, ,
using M\"{o}bius Domain-Wall fermions solved on the dynamical HISQ
ensembles after they are smeared using the gradient-flow algorithm. The
calculation is performed with three pion masses,
MeV. Three lattice spacings ( fm) are used with the
heaviest pion mass, while the coarsest two spacings are used on the middle pion
mass and only the coarsest spacing is used with the near physical pion mass. On
the MeV, fm point, a dedicated volume study is
performed with . Using a new strategy
motivated by the Feynman-Hellmann Theorem, we achieve a precise determination
of with relatively low statistics, and demonstrable control over the
excited state, continuum, infinite volume and chiral extrapolation systematic
uncertainties, the latter of which remains the dominant uncertainty. Our final
determination at 2.6\% total uncertainty is , with the
first uncertainty including statistical and systematic uncertainties from
fitting and the second including model selection systematics related to the
chiral and continuum extrapolation. The largest reduction of the second
uncertainty will come from a greater number of pion mass points as well as more
precise lattice QCD results near the physical pion mass.Comment: 17 pages + 11 pages of references and appendices. 15 figures.
Interested readers can download the Python analysis scripts and an hdf5 data
file at https://github.com/callat-qcd/project_gA_v
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