76,135 research outputs found
Proximity Effects in Radiative Transfer
Though the dependence of near-field radiative transfer on the gap between two
planar objects is well understood, that between curved objects is still
unclear. We show, based on the analysis of the surface polariton mediated
radiative transfer between two spheres of equal radii and minimum gap ,
that the near--field radiative transfer scales as as
and as for larger values of up to the far--field limit. We
propose a modified form of the proximity approximation to predict near--field
radiative transfer between curved objects from simulations of radiative
transfer between planar surfaces.Comment: 5 journal pages, 4 figure
Radiative transfer models
The purpose of this work was to assist with the development of analytical techniques for the interpretation of infrared observations. We have done the following: (1) helped to develop models for continuum absorption calculations for water vapor in the far infrared spectral region; (2) worked on models for pressure-induced absorption for O2 and N2 and their comparison with available observations; and (3) developed preliminary studies of non-local thermal equilibrium effects in the upper stratosphere and mesosphere for infrared gases. These new techniques were employed for analysis of balloon-borne far infrared data by a group at the Harvard-Smithsonian Center for Astrophysics. The empirical continuum absorption model for water vapor in the far infrared spectral region and the pressure-induced N2 absorption model were found to give satisfactory results in the retrieval of the mixing ratios of a number of stratospheric trace constituents from balloon-borne far infrared observations
General Relativistic Radiative Transfer
We present a general method to calculate radiative transfer including
scattering in the continuum as well as in lines in spherically symmetric
systems that are influenced by the effects of general relativity (GR). We
utilize a comoving wavelength ansatz that allows to resolve spectral lines
throughout the atmosphere. The used numerical solution is an operator splitting
(OS) technique that uses a characteristic formal solution. The bending of
photon paths and the wavelength shifts due to the effects of GR are fully taken
into account, as is the treatment of image generation in a curved spacetime. We
describe the algorithm we use and demonstrate the effects of GR on the
radiative transport of a two level atom line in a neutron star like atmosphere
for various combinations of continuous and line scattering coefficients. In
addition, we present grey continuum models and discuss the effects of different
scattering albedos on the emergent spectra and the determination of effective
temperatures and radii of neutron star atmospheres
Optical depth in polarised Monte Carlo radiative transfer
Context: The Monte Carlo method is the most widely used method to solve radiative transfer problems in astronomy, especially in a fully general 3D geometry. A crucial concept in any Monte Carlo radiative transfer code is the random generation of the next interaction location. In polarised Monte Carlo radiative transfer with aligned non-spherical grains, the nature of dichroism complicates the concept of optical depth.
Aims: We investigate, in detail, the relation between optical depth and the optical properties and density of the attenuating medium in polarised Monte Carlo radiative transfer codes that take dichroic extinction into account.
Methods: Based on solutions for the radiative transfer equation, we discuss the optical depth scale in polarised radiative transfer with spheroidal grains. We compare the dichroic optical depth to the extinction and total optical depth scale.
Results: In a dichroic medium, the optical depth is not equal to the usual extinction optical depth, nor to the total optical depth. For representative values of the optical properties of dust grains, the dichroic optical depth can differ from the extinction or total optical depth by several tens of percent. A closed expression for the dichroic optical depth cannot be given, but it can be derived efficiently through an algorithm that is based on the analytical result corresponding to elongated grains with a uniform grain alignment.
Conclusions: Optical depth is more complex in dichroic media than in systems without dichroic attenuation, and this complexity needs to be considered when generating random free path lengths in Monte Carlo radiative transfer simulations. There is no benefit in using approximations instead of the dichroic optical depth
Radiative transfer dynamo effect
Magnetic fields in rotating and radiating astrophysical plasma can be
produced due to a radiative interaction between plasma layers moving relative
to each other. The efficiency of current drive, and with it the associated
dynamo effect, is considered in a number of limits. It is shown here, however,
that predictions for these generated magnetic fields can be significantly
higher when kinetic effects, previously neglected, are taken into account.Comment: 6 pages, 4 figures, published in Physical Review
Monte Carlo Radiative Transfer
I outline methods for calculating the solution of Monte Carlo Radiative
Transfer (MCRT) in scattering, absorption and emission processes of dust and
gas, including polarization. I provide a bibliography of relevant papers on
methods with astrophysical applications.Comment: To appear in the Chandra Centennial issue of the Bulletin of the
Astronomical Society of India, volume 39 (2011), eds D.J. Saikia and Virginia
Trimble; 27 pages, 1 figur
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