Radiative transfer

About: Radiative transfer is a research topic. Over the lifetime, 43287 publications have been published within this topic receiving 1176539 citations.

Radiative Processes, Spectral States and Variability of Black-Hole Binaries

Abstract: We review radiative processes responsible for X-ray emission in hard (low) and soft (high) spectral states of black-hole binaries. The main process in the hard state appears to be scattering of blackbody photons from a cold disk by thermal electrons in a hot inner flow, and any contribution from nonthermal synchrotron emission is at most small. In the soft states, blackbody disk emission dominates energetically, and its high-energy tail is due to scattering by hybrid, thermal/nonthermal electrons, probably in active regions above the disk surface. State transitions appear to correspond to a variable inner radius of the cold disk driven by changes of the accretion rate. The existence of two accretion solutions, hot and cold, in a range of the accretion rate leads to hysteresis in low-mass X-ray binaries.

Finite Volume Method for Radiative Heat Transfer Using Unstructured Meshes

Abstract: The e nite volume method has been shown to accurately predict radiative heat transfer in absorbing, emitting, and scattering media. However, computations have for the most part been restricted to structured, body-e tted meshes. In this paper a conservative cell-based numerical scheme is devised for computing radiative heat transfer using meshes composed of arbitrary unstructured polyhedra. The method is shown to be robust and accurate through comparisons with published solutions.

Effect of metal films on the photoluminescence and electroluminescence of conjugated polymers

Abstract: We report the modification of photoluminescence (PL) and electroluminescence (EL) from conjugated polymers due to the proximity of metal films. The presence of a metal film alters the radiative decay rate of an emitter via interference effects, and also opens up an efficient nonradiative decay channel via energy transfer to the metal film. We show that these effects lead to substantial changes in the PL and EL quantum efficiencies and the emission spectra of the polymers studied here [cyano derivatives of poly($p$-phenylenevinylene), PPV] as a function of the distance of the emitting dipoles from the metal film. We have measured the PL quantum efficiency directly using an integrating sphere, and found its distance dependence to be in good agreement with earlier theoretical predictions. Using the spectral dependence of the emission, we have been able to investigate the effect of interference on the radiative rate as a function of the wavelength and the distance between the emitter and the mirror. We compare our results with simulations of the radiative power of an oscillating dipole in a similar system. From our results we can determine the orientation of the dipoles in the polymer film, and the branching ratio that gives the fraction of absorbed photons leading to singlet excitons. We propose design rules for light-emitting diodes (LED's) and photovoltaic cells that optimize the effects of the metal film. By making optimum use of above effects we have substantially increased the EL quantum efficiencies of PPV/cyano-PPV double-layer LED's.

Three-dimensional Stereoscopic Analysis of Solar Active Region Loops. I. SOHO/EIT Observations at Temperatures of (1.0-1.5) × 106 K

Abstract: The three-dimensional structure of solar active region NOAA 7986 observed on 1996 August 30 with the Extreme-Ultraviolet Imaging Telescope (EIT) on board the Solar and Heliospheric Observatory (SOHO) is analyzed. We develop a new method of dynamic stereoscopy to reconstruct the three-dimensional geometry of dynamically changing loops, which allows us to determine the orientation of the mean loop plane with respect to the line of sight, a prerequisite to correct properly for projection effects in three-dimensional loop models. With this method and the filter-ratio technique applied to EIT 171 and 195 A images we determine the three-dimensional coordinates [x(s), y(s), z(s)], the loop width w(s), the electron density ne(s), and the electron temperature Te(s) as a function of the loop length s for 30 loop segments. Fitting the loop densities with an exponential density model ne(h) we find that the mean of inferred scale height temperatures, Tλe=1.22 ± 0.23 MK, matches closely that of EIT filter-ratio temperatures, TEITe=1.21 ± 0.06 MK. We conclude that these cool and rather large-scale loops (with heights of h≈30-225 Mm) are in hydrostatic equilibrium. Most of the loops show no significant thickness variation w(s), but we measure for most of them a positive temperature gradient (dT/ds>0) across the first scale height above the footpoint. Based on these temperature gradients we find that the conductive loss rate is about 2 orders of magnitude smaller than the radiative loss rate, which is in strong contrast to hot active region loops seen in soft X-rays. We infer a mean radiative loss time of τrad≈40 minutes at the loop base. Because thermal conduction is negligible in these cool EUV loops, they are not in steady state, and radiative loss has entirely to be balanced by the heating function. A statistical heating model with recurrent heating events distributed along the entire loop can explain the observed temperature gradients if the mean recurrence time is 10 minutes. We computed also a potential field model (from SOHO/MDI magnetograms) and found a reasonable match with the traced EIT loops. With the magnetic field model we determined also the height dependence of the magnetic field B(h), the plasma parameter β(h), and the Alfven velocity vA(h). No correlation was found between the heating rate requirement EH0 and the magnetic field Bfoot at the loop footpoints.

Identification of gaseous SO2 and new upper limits for other gases on Io

Abstract: The identification of gaseous sulfur dioxide on Io by Voyager 1 is reported, and preliminary upper limits for other atmospheric gases are presented. Averaged spectra taken by the Voyager IRIS experiment in the range of 1,000 to 1,200/cm are interpreted as containing three fundamental sulfur dioxide bands, with intensities most nearly corresponding to an atmospheric model with a sulfur dioxide abundance of 0.2 cm atm. Upper limits for COS, CS2, SO3, H2S, CO2, O3, N2O, H2O, CH4, NH3 and HC1, not detected in the spectra, were calculated on the basis of the radiative transfer equation for temperatures of 130 and 250 K; a depletion of hydrogen, carbon and nitrogen is noted. It is suggested that a SO2 outgassing from a cooling sulfur extrusion is the major source of the observed atmospheric SO2.