Radiative transfer

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

Viking Lander image analysis of Martian atmospheric dust

Abstract: We have reanalyzed three sets of Viking Lander 1 and 2 (VL1 and VL2) images of the Martian atmosphere to better evaluate the radiative properties of the atmospheric dust particles. The properties of interest are the first two moments of the size distribution, the single-scattering albedo, the dust single-scattering phase function, and the imaginary index of refraction. These properties provide a good definition of the influence that the atmospheric dust has on heating of the atmosphere. Our analysis represents a significant improvement over past analyses (Pollack et al. 1977, 1979) by deriving more accurate brightness closer to the sun, by carrying out more precise analyses of the data to acquire the quantities of interest, and by using a better representation of scattering by nonspherical particles. The improvements allow us to better define the diffraction peak and hence the size distribution of the particles. For a lognormal particle size distribution, the first two moments of the size distribution, weighted by the geometric cross section, are found. The geometric cross-section weighted mean radius r(sub eff) is found to be 1.85 +/- 0.3 micrometers at VL2 during northern summer when dust loading was low and 1.52 +/- 0.3 micrometers at VL1 during the first dust storm. In both cases the best cross-section weighted mean variance nu(sub eff) of the size distribution is equal to 0.5 +/- 0.2 micrometers. The changes in size distribution, and thus radiative properties, do not represent a substantial change in solar energy deposition in the atmosphere over the Pollak et al. (1977, 1979) estimates.

Radiative carrier lifetime, momentum matrix element, and hole effective mass in GaN

Abstract: By using picosecond time-resolved photoluminescence we have measured the lifetime of excess charge carriers in GaN epitaxial layers grown on sapphire at temperatures up to 300 K. The decay time turns out to be dominated by trapping processes at low excitation levels. The radiative lifetime derived from our data is dominated by free excitons at temperatures below 150 K, but also clearly shows the gradual thermal dissociation of excitons at higher temperatures. From our data, we are able to determine the free exciton binding energy and the free carrier radiative recombination coefficient. By combining these data with optical absorption data, we find the interband momentum matrix element and an estimate for the hole effective mass, which is much larger than previously thought.

Gamma-Ray Emission From Crushed Clouds in Supernova Remnants

Abstract: It is shown that the radio and gamma-ray emission observed from newly found GeV-bright supernova remnants (SNRs) can be explained by a model in which a shocked cloud and shock-accelerated cosmic rays (CRs) frozen in it are simultaneously compressed by the supernova blast wave as a result of formation of a radiative cloud shock. Simple reacceleration of pre-existing CRs is generally sufficient to power the observed gamma-ray emission through the decays of π0-mesons produced in hadronic interactions between high-energy protons (nuclei) and gas in the compressed-cloud layer. This model provides a natural account of the observed synchrotron radiation in SNRs W51C, W44, and IC 443 with flat radio spectral index, which can be ascribed to a combination of secondary and reaccelerated electrons and positrons.

Radiative states in type-II GaSb/GaAs quantum wells.

Abstract: We have studied optical properties of staggered band line-up (type-II) heterostructures based on strained GaSb sheets in a GaAs matrix. The giant valence-band offset characteristic to this heterojunction leads to an effective localization of holes in ultrathin GaSb layers. An intense photoluminescence (PL) line caused by radiative recombination of localized holes with electrons located in the nearby GaAs regions is observed. The separation of nonequilibrium electrons and holes in real space results in a dipole layer and, thus, in the formation of quantum wells for electrons in the vicinity of the GaSb layer. The luminescence maximum shifts towards higher photon energies with rising excitation density reflecting the increase in the electron quantization energy. A bimolecular recombination mechanism is revealed in PL and in time-resolved PL studies. In the case of pseudomorphic monolayer-thick GaSb layers, the radiative exciton ground state does not exist. Accordingly, small absorption coefficients and a featureless behavior of the band-to-band calorimetric absoprtion spectrum are found in the vicinity of ${\mathit{k}}_{\mathit{x},}$y=0. Remarkable enhancement of the absorption coefficient with a characteristic onset is observed for heavy holes with ${\mathit{k}}_{\mathit{x},}$yg0. Radiative states in the continuum of heavy-hole subbands are revealed also in temperature-dependent PL studies. The experimentally measured onset energies point out the importance of GaSb heavy- and light-hole mixing effects. We demonstrate intense luminescence from staggered band line-up GaSb-GaAs heterostructures up to room temperature.

The Formation of IRIS Diagnostics. I. A Quintessential Model Atom of Mg II and General Formation Properties of the Mg II h&k Lines

Abstract: NASA's Interface Region Imaging Spectrograph (IRIS) space mission will study how the solar atmosphere is energized. IRIS contains an imaging spectrograph that covers the Mg II h and k lines as well as a slit-jaw imager centered at Mg II k. Understanding the observations will require forward modeling of Mg II h and k line formation from three-dimensional (3D) radiation-MHD models. This paper is the first in a series where we undertake this forward modeling. We discuss the atomic physics pertinent to h and k line formation, present a quintessential model atom that can be used in radiative transfer computations, and discuss the effect of partial redistribution (PRD) and 3D radiative transfer on the emergent line profiles. We conclude that Mg II h and k can be modeled accurately with a four-level plus continuum Mg II model atom. Ideally radiative transfer computations should be done in 3D including PRD effects. In practice this is currently not possible. A reasonable compromise is to use one-dimensional PRD computations to model the line profile up to and including the central emission peaks, and use 3D transfer assuming complete redistribution to model the central depression.