487 research outputs found
Mechanisms for MHD Poynting flux generation in simulations of solar photospheric magneto-convection
We investigate the generation mechanisms of MHD Poynting flux in the
magnetised solar photosphere. Using radiative MHD modelling of the solar
photosphere with initial magnetic configurations that differ in their field
strength and geometry, we show the presence of two different mechanisms for MHD
Poynting flux generation in simulations of solar photospheric
magneto-convection. The weaker mechanism is connected to vertical transport of
weak horizontal magnetic fields in the convectively stable layers of the upper
photosphere, while the stronger is the production of Poynting flux in strongly
magnetised intergranular lanes experiencing horizontal vortex motions. These
mechanisms may be responsible for the energy transport from the solar
convection zone to the higher layers of the solar atmosphere.Comment: 5 pages, 5 figures, accepted for ApJ
Near-surface stellar magneto-convection: simulations for the Sun and a metal-poor solar analog
We present 2D local box simulations of near-surface radiative
magneto-convection with prescribed magnetic flux, carried out with the MHD
version of the CO5BOLD code for the Sun and a solar-like star with a metal-poor
chemical composition (metal abundances reduced by a factor 100, [M/H]=-2). The
resulting magneto-hydrodynamical models can be used to study the influence of
the metallicity on the properties of magnetized stellar atmospheres. A
preliminary analysis indicates that the horizontal magnetic field component
tends to be significantly stronger in the optically thin layers of metal-poor
stellar atmospheres.Comment: Proc. IAU Symposium 259, Cosmic Magnetic Fields: from Planets, to
Stars and Galaxies, K.G. Strassmeier, A.G. Kosovichev and J.E. Beckman, eds.
(2009) p.23
The intensity contrast of solar granulation: comparing Hinode SP results with MHD simulations
The contrast of granulation is an important quantity characterizing solar
surface convection. We compare the intensity contrast at 630 nm, observed using
the Spectro-Polarimeter (SP) aboard the Hinode satellite, with the 3D radiative
MHD simulations of V{\"o}gler & Sch{\"u}ssler (2007). A synthetic image from
the simulation is degraded using a theoretical point-spread function of the
optical system, and by considering other important effects. The telescope
aperture and the obscuration by the secondary mirror and its attachment spider,
reduce the simulated contrast from 14.4 % to 8.5 %. A slight effective defocus
of the instrument brings the simulated contrast down to 7.5 %, close to the
observed value of 7.0 %. A proper consideration of the effects of the optical
system and a slight defocus, lead to sufficient degradation of the synthetic
image from the MHD simulation, such that the contrast reaches almost the
observed value. The remaining small discrepancy can be ascribed to straylight
and slight imperfections of the instrument, which are difficult to model.
Hence, Hinode SP data are consistent with a granulation contrast which is
predicted by 3D radiation MHD simulations.Comment: 5 pages, 4 figures, to be published in A&
Explanation of the activity sensitivity of Mn I 5394.7 \AA
There is a long-standing controversy concerning the reason why the Mn I
5394.7 A line in the solar irradiance spectrum brightens more at larger
activity than most other photospheric lines. The claim that this activity
sensitivity is caused by spectral interlocking to chromospheric emission in Mg
II h & k is disputed.
Classical one-dimensional modeling is used for demonstration; modern
three-dimensional MHD simulation for verification and analysis.
The Mn I 5394.7 A line thanks its unusual sensitivity to solar activity to
its hyperfine structure. This overrides the thermal and granular Doppler
smearing through which the other, narrower, photospheric lines lose such
sensitivity. We take the nearby Fe I 5395.2 A line as example of the latter and
analyze the formation of both lines in detail to demonstrate and explain
granular Doppler brightening. We show that this affects all narrow lines.
Neither the chromosphere nor Mg II h & k play a role, nor is it correct to
describe the activity sensitivity of Mn I 5394.7 A through plage models with
outward increasing temperature contrast.
The Mn I 5394.7 A line represents a proxy diagnostic of strong-field magnetic
concentrations in the deep solar photosphere comparable to the G band and the
blue wing of H-alpha, but not a better one than these. The Mn I lines are more
promising as diagnostic of weak fields in high-resolution Stokes polarimetry.Comment: 12 pages, 8 figures, accepted by A&
A Model-View-Controller Extension for Pervasive Multi-Client User Interfaces
This paper addresses the implementation of pervasive Java Web applications using a development approach that is based on the ModelâViewâController (MVC) design pattern. We combine the MVC methodology with a hierarchical task-based state transition model in order to achieve the distinction between the task state and the view state of an application. More precisely, we propose to add a device-independent TaskStateBean and a device-specific ViewStateBean for each task state as an extension to the J2EE Service to Worker design pattern. Furthermore, we suggest representing the task state and view state transitionmodels as finite state automata in two sets of XML files. This paper shows that the distinction between an applicationâs task state and view state is both intuitive and facilitates several, otherwise complex, tasks, such as changing devices 'on the fly'
Convex geometry of finite exchangeable laws and de Finetti style representation with universal correlated corrections
We present a novel analogue for finite exchangeable sequences of the de
Finetti, Hewitt and Savage theorem and investigate its implications for
multi-marginal optimal transport (MMOT) and Bayesian statistics. If
is a finitely exchangeable sequence of random variables
taking values in some Polish space , we show that the law of the
first components has a representation of the form
\mu_k=\int_{{\mathcal P}_{\frac{1}{N}}(X)} F_{N,k}(\lambda) \, \mbox{d}
\alpha(\lambda)
for some probability measure on the set of -quantized
probability measures on and certain universal polynomials . The
latter consist of a leading term and a finite, exponentially decaying series of
correlated corrections of order (). The
are precisely the extremal such laws, expressed via an explicit polynomial
formula in terms of their one-point marginals . Applications include
novel approximations of MMOT via polynomial convexification and the
identification of the remainder which is estimated in the celebrated error
bound of Diaconis-Freedman between finite and infinite exchangeable laws
Vortices in simulations of solar surface convection
We report on the occurrence of small-scale vortices in simulations of the
convective solar surface. Using an eigenanalysis of the velocity gradient
tensor, we find the subset of high vorticity regions in which the plasma is
swirling. The swirling regions form an unsteady, tangled network of filaments
in the turbulent downflow lanes. Near-surface vertical vortices are underdense
and cause a local depression of the optical surface. They are potentially
observable as bright points in the dark intergranular lanes. Vortex features
typically exist for a few minutes, during which they are moved and twisted by
the motion of the ambient plasma. The bigger vortices found in the simulations
are possibly, but not necessarily, related to observations of granular-scale
spiraling pathlines in "cork animations" or feature tracking.Comment: 11 pages, 13 figures, accepted for publication in A&A, complementary
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