4,448 research outputs found
On the accuracy of the ALI method for solving the radiative transfer equation
We solve the integral equation describing the propagation of light in an
isothermal plane-parallel atmosphere of optical thickness , adopting a
uniform thermalization parameter . The solution given by the ALI
method, widely used in the field of stellar atmospheres modelling, is compared
to the exact solution. Graphs are given that illustrate the accuracy of the ALI
solution as a function of the parameters , and optical depth
variable .Comment: 7 pages, 11 figures, A&A, accepted 30 July 2003, minor correction
Strange Quark Contribution to the Nucleon Spin from Electroweak Elastic Scattering Data
The total contribution of strange quarks to the intrinsic spin of the nucleon
can be determined from a measurement of the strange-quark contribution to the
nucleon's elastic axial form factor. We have studied the strangeness
contribution to the elastic vector and axial form factors of the nucleon, using
elastic electroweak scattering data. Specifically, we combine elastic
and scattering cross section data from the Brookhaven E734
experiment with elastic and quasi-elastic and -He scattering
parity-violating asymmetry data from the SAMPLE, HAPPEx, G0 and PVA4
experiments. We have not only determined these form factors at individual
values of momentum-transfer (), but also have fit the -dependence of
these form factors using simple functional forms. We present the results of
these fits, along with some expectations of how our knowledge of these form
factors can be improved with data from Fermilab experiments.Comment: 3 pages, 1 figure, CIPANP 201
Digital library access for illiterate users
The problems that illiteracy poses in accessing information are gaining attention from the research community. Issues currently being explored include developing an understanding of the barriers to information acquisition experienced by different groups of illiterate information seekers; creating technology, such as software interfaces, that support illiterate users effectively; and tailoring content to increase its accessibility. We have taken a formative evaluation approach to developing and evaluating a digital library interface for illiterate users. We discuss modifications to the Greenstone platform, describe user studies and outline resulting design implications
A conjugate gradient method for the solution of the non-LTE line radiation transfer problem
This study concerns the fast and accurate solution of the line radiation
transfer problem, under non-LTE conditions. We propose and evaluate an
alternative iterative scheme to the classical ALI-Jacobi method, and to the
more recently proposed Gauss-Seidel and Successive Over-Relaxation (GS/SOR)
schemes. Our study is indeed based on the application of a preconditioned
bi-conjugate gradient method (BiCG-P). Standard tests, in 1D plane parallel
geometry and in the frame of the two-level atom model, with monochromatic
scattering, are discussed. Rates of convergence between the previously
mentioned iterative schemes are compared, as well as their respective timing
properties. The smoothing capability of the BiCG-P method is also demonstrated.Comment: Research note: 4 pages, 5 figures, accepted to A&
Observations of solar scattering polarization at high spatial resolution
The weak, turbulent magnetic fields that supposedly permeate most of the
solar photosphere are difficult to observe, because the Zeeman effect is
virtually blind to them. The Hanle effect, acting on the scattering
polarization in suitable lines, can in principle be used as a diagnostic for
these fields. However, the prediction that the majority of the weak, turbulent
field resides in intergranular lanes also poses significant challenges to
scattering polarization observations because high spatial resolution is usually
difficult to attain. We aim to measure the difference in scattering
polarization between granules and intergranules. We present the respective
center-to-limb variations, which may serve as input for future models. We
perform full Stokes filter polarimetry at different solar limb positions with
the CN band filter of the Hinode-SOT Broadband Filter Imager, which represents
the first scattering polarization observations with sufficient spatial
resolution to discern the granulation. Hinode-SOT offers unprecedented spatial
resolution in combination with high polarimetric sensitivity. The CN band is
known to have a significant scattering polarization signal, and is sensitive to
the Hanle effect. We extend the instrumental polarization calibration routine
to the observing wavelength, and correct for various systematic effects. The
scattering polarization for granules (i.e., regions brighter than the median
intensity of non-magnetic pixels) is significantly larger than for
intergranules. We derive that the intergranules (i.e., the remaining
non-magnetic pixels) exhibit (9.8 \pm 3.0)% less scattering polarization for
0.2<u<0.3, although systematic effects cannot be completely excluded. These
observations constrain MHD models in combination with (polarized) radiative
transfer in terms of CN band line formation, radiation anisotropy, and magnetic
fields.Comment: Accepted for publication in A&
2D non-LTE radiative modelling of He I spectral lines formed in solar prominences
The diagnosis of new high-resolution spectropolarimetric observations of
solar prominences made in the visible and near-infrared mainly, requires a
radiative modelling taking into account for both multi-dimensional geometry and
complex atomic models. Hereafter we contribute to the improvement of the
diagnosis based on the observation of He I multiplets, by considering 2D
non-LTE unpolarized radiation transfer, and taking also into account the atomic
fine structure of helium. It is an improvement and a direct application of the
multi-grid Gauss-Seidel/SOR iterative scheme in 2D cartesian geometry developed
by us. It allows us to compute realistic emergent intensity profiles for the He
I 10830 A and D3 multiplets, which can be directly compared to the simultaneous
and high-resolution observations made at THeMIS. A preliminary 2D multi-thread
modelling is also discussed.Comment: 6 pages, 9 figures, A&
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