8,754 research outputs found
Numerical simulations of quiet Sun magnetic fields seeded by Biermann battery
The magnetic fields of the quiet Sun cover at any time more than 90\% of its
surface and their magnetic energy budget is crucial to explain the thermal
structure of the solar atmosphere. One of the possible origins of these fields
is due to the action of local dynamo in the upper convection zone of the Sun.
Existing simulations of the local solar dynamo require an initial seed field,
and sufficiently high spatial resolution, in order to achieve the amplification
of the seed field to the observed values in the quiet Sun. Here we report an
alternative model of seeding based on the action of the Bierman battery effect.
This effect generates a magnetic field due to the local imbalances in electron
pressure in the partially ionized solar plasma. We show that the battery effect
self-consistently creates from zero an initial seed field of a strength of the
order of micro G, and together with dynamo amplification, allows the generation
of quiet Sun magnetic fields of a similar strength to those from solar
observations.Comment: To appear in Astronomy & Astrophysic
Study of the derivative expansions for the nuclear structure functions
We study the convergence of the series expansions sometimes used in the
analysis of the nuclear effects in Deep Inelastic Scattering (DIS) proccesses
induced by leptons. The recent advances in statistics and quality of the data,
in particular for neutrinos calls for a good control of the theoretical
uncertainties of the models used in the analysis. Using realistic nuclear
spectral functions which include nucleon correlations, we find that the
convergence of the derivative expansions to the full results is poor except at
very low values of
Local analysis of a new multipliers method
http://www.sciencedirect.com/science/article/B6VCT-45X2SGP-1/1/1571cb1c8b840e1e82cd33e423a0e19
Local Convergence of the Affine-Scaling Interior-Point Algorithm for Nonlinear Programming
This paper addresses the local convergence properties of the affine-scaling interior-point algorithm for nonlinear programming. The analysis of local convergence is developed in terms of parameters that control the interior-point scheme and the size of the residual of the linear system that provides the step direction. The analysis follows the classical theory for quasi-Newton methods and addresses q-linear, q-superlinear, and q-quadratic rates of convergence
Implicitly and densely discrete black-box optimization problems
This paper addresses derivative-free optimization problems where the
variables lie implicitly in an unknown discrete closed set. The evaluation of the
objective function follows a projection onto the discrete set, which is assumed dense
rather than sparse. Such a mathematical setting is a rough representation of what
is common in many real-life applications where, despite the continuous nature of
the underlying models, a number of practical issues dictate rounding of values or
projection to nearby feasible figures.
We discuss a definition of minimization for these implicitly discrete problems and
outline a direct search algorithm framework for its solution. The main asymptotic
properties of the algorithm are analyzed and numerically illustrated.FCT POCI/MAT/59442/2004, PTDC/MAT/64838/200
Implicitly and densely discrete black-box optimization problems
This paper addresses derivative-free optimization problems where the
variables lie implicitly in an unknown discrete closed set. The evaluation of the
objective function follows a projection onto the discrete set, which is assumed dense
rather than sparse. Such a mathematical setting is a rough representation of what
is common in many real-life applications where, despite the continuous nature of
the underlying models, a number of practical issues dictate rounding of values or
projection to nearby feasible figures.
We discuss a definition of minimization for these implicitly discrete problems and
outline a direct search algorithm framework for its solution. The main asymptotic
properties of the algorithm are analyzed and numerically illustrated.FCT POCI/MAT/59442/2004, PTDC/MAT/64838/200
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