Multidimensional Modeling of Atmospheric Effects and Surface Heterogeneities on Remote Sensing

The overall goal of this project is to establish a modeling capability that allows a quantitative determination of atmospheric effects on remote sensing including the effects of surface heterogeneities. This includes an improved understanding of aerosol and haze effects in connection with structural, angular, and spatial surface heterogeneities. One important objective of the research is the possible identification of intrinsic surface or canopy characteristics that might be invariant to atmospheric perturbations so that they could be used for scene identification. Conversely, an equally important objective is to find a correction algorithm for atmospheric effects in satellite-sensed surface reflectances. The technical approach is centered around a systematic model and code development effort based on existing, highly advanced computer codes that were originally developed for nuclear radiation shielding applications. Computational techniques for the numerical solution of the radiative transfer equation are adapted on the basis of the discrete-ordinates finite-element method which proved highly successful for one and two-dimensional radiative transfer problems with fully resolved angular representation of the radiation field

Computer Simulations of Cosmic Reionization

The cosmic reionization of hydrogen was the last major phase transition in the evolution of the universe, which drastically changed the ionization and thermal conditions in the cosmic gas. To the best of our knowledge today, this process was driven by the ultra-violet radiation from young, star-forming galaxies and from first quasars. We review the current observational constraints on cosmic reionization, as well as the dominant physical effects that control the ionization of intergalactic gas. We then focus on numerical modeling of this process with computer simulations. Over the past decade, significant progress has been made in solving the radiative transfer of ionizing photons from many sources through the highly inhomogeneous distribution of cosmic gas in the expanding universe. With modern simulations, we have finally converged on a general picture for the reionization process, but many unsolved problems still remain in this young and exciting field of numerical cosmology.Comment: Invited Review to appear on Advanced Science Letters (ASL), Special Issue on Computational Astrophysics, edited by Lucio Maye

Computer simulation and topological modeling of radiation effects in zircon

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Nuclear Science and Engineering, 2006.Includes bibliographical references.The purpose of this study is to understand on atomic level the structural response of zircon (ZrSiO4) to irradiation using molecular dynamics (MD) computer simulations, and to develop topological models that can describe these structural changes. Topological signatures, encoded using the concepts of primitive-rings and local clusters, were developed and used to differentiate crystalline and non-crystalline atoms in various zircon structures. Since primitive-rings and local clusters are general concepts applicable to all materials, and the algorithms to systematically identify them are well-established, topological signatures based on them are easy to implement and the method of topological signatures is applicable to all structures. The method of topological signatures is better than the Wigner-Seitz cell method, which depends on the original crystalline reference grid that is unusable in heavily damaged structures or regions; it is also better than those methods based only on local structures limited to first coordination shell, since one can decide whether or not to include ring contents of large rings into the topological signatures, effectively controlling the range of the topological signatures. The early-stage evolution of non-crystalline disorder and the subsequent recrystallization in zircon collision cascade simulations were successfully modeled by using the topological signatures to identify non-crystalline atoms. Simply using the number of displaced atoms was unable to correctly show the initial peak of structural damage followed by the subsequent annealing stage. Using the topological signatures, amorphization within a single collision cascade was observed in zircon.(cont.) In the radiation-induced amorphous zircon simulated in this study, the method of topological signatures was able to differentiate the amorphous region in the center of the simulation box and the crystalline region surrounding it. A few isolated remnant crystalline islands were identified in the amorphous region. About 5% of atoms in melted and melt-quenched structures were identified as crystalline atoms. Different amorphous zircon structures were found to be topologically different. Upon amorphization of zircon, the average ring size and the number of atoms in local cluster were found to increase. Larger average ring sizes were found in more pervasively amorphized structures. The radiation-induced amorphous structure was the least pervasively amorphized one, followed by the melt-quenched. The liquid-state amorphous structure was most pervasively amorphized and had the largest average ring size. Phase-separation of zircon into SiO2- and ZrO2-rich local regions was observed when zircon was amorphized in simulations, either thermally or by radiation. It was found in simulations using constant pressure ensembles that the zircon structure underwent abnormally huge volume swelling when it amorphized, which was attributed to the ion charges used in the potential model. Although the ion charges used in the originally chosen potential model were overall balanced, they were not balanced with regard to the phase decomposition products, and thus resulted in strong Coulombic repulsive force within locally SiO2- and ZrO2-rich regions when phase separation occurred. After the ion charges were re-balanced (and other potential parameters refitted), the volume expansion was found to be under control. The charge imbalance of SiO2 units was also found to produce unrealistically large fraction of 3-coordinated Si and shorter Si-O bond length.(cont.) The issue of charge-balance with regard to phase decomposition products applies to all complex ceramics that decompose into separate phases upon amorphization. Threshold displacement energies in zircon were systematically determined. Many special directions, such as those directed toward neighboring atoms or open spaces surrounding the PKA, were considered. Cascade detail was extensively examined, including PKA trajectory, cascade extent, time scale, thermal spike, recoil density, distribution of PKA energy among sub-lattices and number of displaced atoms. The crystallographic features of the zircon structure were found to have profound implications for collision cascades. It was found that energetic PKAs were always deflected into the open channel along the z direction. Their displacements along the longitudinal x direction were never greater than about 4 nm in our simulations. The estimation of the cascade extent assuming homogeneous media thus greatly over-predicts the PKA displacement along the longitudinal direction. The effects of PKA mass on collision cascade were studied by comparing the cascades caused by Zr and U PKAs. The U atoms were simply "super-mass" Zr atoms in this study: U-Zr, U-Si and U-O interactions were the same as Zr-Zr, Zr-Si and Zr-O interactions, respectively. It was found that heavier PKAs produced longer cascades, more structural damage, and higher temperature in thermal spike. U also traveled further along the longitudinal x direction because it was less prone to change of velocity direction. The depleted regions in the core of the cascades surrounded by a densified shell, which were found in simulations by Trachenko et al., were not found in our study. After extensive tests of recently published zircon potentials, it was found that three out of the five tested potentials yielded poor elastic constants and appear to be unfit for serious simulations. Published simulation results using these potentials should accordingly be viewed cautiously.by Yi Zhang.Ph.D

Process parameters optimization in fused deposition modeling of polyether ether ketone

Fused Deposition Modeling is increasingly used for producing high-performing, creepresistant, biocompatible, fireproof, highly-stable parts from polyether ether ketone. However, the knowledge on this process is still poor and fragmented, and the lack of relevant data inhibits many applications. In this paper, the effects of the nozzle temperature, nozzle speed and layer thickness on the properties of PEEK processed by Fused Deposition Modeling were investigated by performing indentation, tensile, Scanning Electron Microscope, Computer Tomography and Energy Dispersive X-ray Spectroscopy tests on as-built samples. The outgassing behavior was also analyzed, while the synchrotron radiation was used to characterize the structure of selected samples on a hitherto unexplored scale. The samples morphology was finally used to identify the optimal process window. The results provided new insights on the process and novel data enabling new applications

Path-tracing Monte Carlo Library for 3D Radiative Transfer in Highly Resolved Cloudy Atmospheres

Interactions between clouds and radiation are at the root of many difficulties in numerically predicting future weather and climate and in retrieving the state of the atmosphere from remote sensing observations. The large range of issues related to these interactions, and in particular to three-dimensional interactions, motivated the development of accurate radiative tools able to compute all types of radiative metrics, from monochromatic, local and directional observables, to integrated energetic quantities. In the continuity of this community effort, we propose here an open-source library for general use in Monte Carlo algorithms. This library is devoted to the acceleration of path-tracing in complex data, typically high-resolution large-domain grounds and clouds. The main algorithmic advances embedded in the library are those related to the construction and traversal of hierarchical grids accelerating the tracing of paths through heterogeneous fields in null-collision (maximum cross-section) algorithms. We show that with these hierarchical grids, the computing time is only weakly sensitivive to the refinement of the volumetric data. The library is tested with a rendering algorithm that produces synthetic images of cloud radiances. Two other examples are given as illustrations, that are respectively used to analyse the transmission of solar radiation under a cloud together with its sensitivity to an optical parameter, and to assess a parametrization of 3D radiative effects of clouds.Comment: Submitted to JAMES, revised and submitted again (this is v2

Advanced Forward Modeling and Inversion of Stokes Profiles Resulting from the Joint Action of the Hanle and Zeeman Effects

A big challenge in solar and stellar physics in the coming years will be to decipher the magnetism of the solar outer atmosphere (chromosphere and corona) along with its dynamic coupling with the magnetic fields of the underlying photosphere. To this end, it is important to develop rigorous diagnostic tools for the physical interpretation of spectropolarimetric observations in suitably chosen spectral lines. Here we present a computer program for the synthesis and inversion of Stokes profiles caused by the joint action of atomic level polarization and the Hanle and Zeeman effects in some spectral lines of diagnostic interest, such as those of the He I 10830 A and D_3 multiplets. It is based on the quantum theory of spectral line polarization, which takes into account all the relevant physical mechanisms and ingredients (optical pumping, atomic level polarization, Zeeman, Paschen-Back and Hanle effects). The influence of radiative transfer on the emergent spectral line radiation is taken into account through a suitable slab model. The user can either calculate the emergent intensity and polarization for any given magnetic field vector or infer the dynamical and magnetic properties from the observed Stokes profiles via an efficient inversion algorithm based on global optimization methods. The reliability of the forward modeling and inversion code presented here is demonstrated through several applications, which range from the inference of the magnetic field vector in solar active regions to determining whether or not it is canopy-like in quiet chromospheric regions. This user-friendly diagnostic tool called "HAZEL" (from HAnle and ZEeman Light) is offered to the astrophysical community, with the hope that it will facilitate new advances in solar and stellar physics.Comment: 62 pages, 19 figures, 3 tables. Accepted for publication in Ap

Vehicle Design Data Format and Process for a Complete TARIS and OLTARIS Radiation Analysis for Designers and Engineers

Protecting astronauts from space radiation is a complex task when it comes to modeling and simulation. This document shows what information is needed from a spacecraft designer using CAD (Computer-Assisted Design) at each phase of the design to enable the engineers to evaluate the design phase against space radiation limits to determine the suitability of the design for space flight. The current personal exposure limits are listed in NASA STD-3001. A proxy to determine the REID (Radiation Exposure Induced Death) in NASA STD-3001 is the whole body effective dose equivalent (E or effective dose). For short-term tissue effects, organ-averaged gray equivalent (G (sub T)) is used. The TARIS (Tool for the Assessment of Radiation In Space - for LaRC (Langley Research Center) engineers) and OLTARIS (On-Line TARIS) - for designers) systems are used to generate these response functions. The E can use ICRP60 or NASA Q-values. A possible space radiation design basis environment for short-term tissue effects is described and used in all analyses. A single space vehicle was designed with three astronaut configurations and two of those configurations were used in a storm shelter thickness perturbation analysis. Conversion of the data from the CAD model to input necessary for TARIS and OLTARIS is also discussed in detail with relevant examples

Current status of NLTE analysis of stellar atmospheres

Various available codes for NLTE modeling and analysis of hot star spectra are reviewed. Generalizations of standard equations of kinetic equilibrium and their consequences are discussed.Comment: in Determination of Atmospheric Parameters of B-, A-, F- and G-Type Stars, E. Niemczura et al. eds., Springer, in pres