Optical properties of dust aggregates. II: Angular dependence of scattered light
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The angular dependence of light scattered by dust aggregates has been investigated by means of the discrete dipole method. Two types of aggregates have been treated; BPCA and BCCA whose fractal dimensions are ∼3 and ∼2 in the limit of large size respectively, and whose number of constituent particles range from N=256 to 4096. The radii of constituent particles are 0.01 μm and are partly 0.03 μm. Two kinds of minerals are considered as the constituents; silicate and magnetite. The calculations have been carried out at the wavelength λ=0.6 μm. The angular dependence of the degree of linear polarization and the scattered intensity is sensitive to the structure as well as the chemical composition with increasing size of the aggregatesKeywords:
Discrete dipole approximation
Angular diameter
Circumstellar dust
Intensity
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Optical properties of dust aggregates of astrophysical interest have been investigated by means of the discrete dipole approximation (DDA) to clarify how differences in structure, size and chemical composition of dust aggregates affect their optical properties. Two types of dust aggregates have been treated; one is the Ballistic Particle-Cluster Aggregates (BPCA) whose fractal dimension D∼3, and the other is the Ballistic Cluster-Cluster Aggregates (BCCA) for which D∼2. Two kinds of minerals have been considered for the constituent particles; silicate and magnetite
Discrete dipole approximation
Circumstellar dust
Particle (ecology)
Extinction (optical mineralogy)
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view Abstract Citations (271) References (47) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS From Interstellar Dust to Comets: A Unification of Observational Constraints Greenberg, J. Mayo ; Hage, J. I. Abstract The interstellar dust model of comets is numerically worked out to satisfy simultaneously several basic constraints provided by observations of Comet Halley, and to derive the porosity of coma dust. The observational constraints are (1) the strengths of the 3.4 and 9.7 micron emission bands, (2) the shape of the 9.7 micron band, (3) the amount of silicates relative to organic materials, and (4) the mass distribution of the dust. The method used involves precise calculations of temperatures and the emission characteristics of porous aggregates of interstellar dust as a function of their mass, porosity, and distance to the sun and the wavelength. The results indicate that coma dust has a porosity in the range 0.93-0.975, i.e., a packing factor of 0.07 or less, consistent with independent observations of comet densities of 0.6 to 0.26 g/cu cm and meteor densities of less than 0.2 g/cu cm. Publication: The Astrophysical Journal Pub Date: September 1990 DOI: 10.1086/169191 Bibcode: 1990ApJ...361..260G Keywords: Cosmic Dust; Emission Spectra; Halley'S Comet; Infrared Spectra; Interstellar Matter; Silicates; Comet Nuclei; Planetary Evolution; Radiative Transfer; Astrophysics; COMETS; INFRARED: SPECTRA; INTERSTELLAR: GRAINS; RADIATIVE TRANSFER full text sources ADS |
Comet dust
Interstellar comet
Comet nucleus
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Discrete dipole approximation
Biological small-angle scattering
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Microwave analog experiments [e.g., Gustafson, 1996a, 1999] allow us to simulate light‐scattering properties of particles of almost any combination of composition, size, and shape. We have taken advantage of this ability and the broadband nature of the University of Florida microwave facility in a systematic study of light scattering by aggregates. We present angular dependencies of intensity and polarization in the scattering angle range 0°–165°, and in the wavelength interval 2.7–4 mm, which simulates 0.430–0.660 μm in the visual. The data are for a variety of sizes, shapes, and compositions of the constituent particles, as well as the number of particles in an aggregate and their packing. Wavelength dependencies are shown to be powerful diagnostics for the interpretation of comet dust observations and we urge observers to report color and polarimetric color. As an example, we estimate the size of constituent particles in cometary grains (0.1–1 μm). We also show that the grains evolve with the distance from the nucleus and that the reason apparently is the loss of some dark matter. Techniques and results reported here are general enough to apply to a broad range of natural and artificial aggregates.
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view Abstract Citations (996) References (31) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The Discrete-Dipole Approximation and Its Application to Interstellar Graphite Grains Draine, B. T. Abstract The discrete dipole approximation (DDA), a flexible method for computing scattering of radiation by particles of arbitrary shape, is extended to incorporate the effects of radiative reaction and to allow for possible anisotropy of the dielectric tensor of the material. Formulas are given for the evaluation of extinction, absorption, scattering, and polarization cross sections. A simple numerical algorithm based on the method of conjugate gradients is found to provide an efficient and robust method for obtaining accurate solutions to the scattering problem. The method works well for absorptive, as well as dielectric, grain materials. Two validity criteria for the DDA are presented. The DDA is then used to compute extinction cross sections for spherical graphite grains and to calculate extinction cross sections for nonspherical graphite grains with three different geometries. It is concluded that the interstellar 2175 A extinction feature could be produced by small graphite grains which should have aspect ratios not far from unity. Publication: The Astrophysical Journal Pub Date: October 1988 DOI: 10.1086/166795 Bibcode: 1988ApJ...333..848D Keywords: Computational Astrophysics; Cosmic Dust; Discrete Functions; Graphite; Interstellar Space; Absorption Cross Sections; Grain Size; Polarization (Waves); Radiative Transfer; Scattering Cross Sections; Astrophysics; INTERSTELLAR: GRAINS; POLARIZATION; RADIATIVE TRANSFER full text sources ADS |
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Extinction (optical mineralogy)
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view Abstract Citations (114) References (13) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Light Scattering by Porous Dust Particles in the Discrete-Dipole Approximation Lumme, K. ; Rahola, J. Abstract Electromagnetic scattering by irregular dust grains and aggregates has long been a challenging problem in various astronomical applications. Only the quite recent advancement in the discrete-dipole approximation (DDA) have made the problem trackable. We have applied a new iterative method, the quasi-minimal residual algorithm, to the system of linear equations arising in the DDA applications. This method is approximately 3 times faster than the standard iterative scheme, conjugate gradient, use with DDA. We apply DDA both to pseudospherical objects, of which we randomly remove different numbers of small scattering elements (dipoles) to model porous grains, and to aggregates of these grains which we believe to properly model fluffy particles. We study both the single particle intensity and linear polarization as a function of the grain size, packing density and the refractive index. Surprisingly, some results for inhomogeneous particles can be matched by the classical Mie theory using a generalized version of the effective medium theory. Some well-known photometric and polarimetric features of solar system objects seem to follow naturally from the scattering properties of fluffy particles. Publication: The Astrophysical Journal Pub Date: April 1994 DOI: 10.1086/174012 Bibcode: 1994ApJ...425..653L Keywords: Dipole Moments; Discrete Functions; Interplanetary Dust; Light Scattering; Mie Scattering; Coherent Scattering; Conjugate Gradient Method; Linear Polarization; Radiative Transfer; Astrophysics; ISM: DUST; EXTINCTION; INTERPLANETARY MEDIUM; POLARIZATION; RADIATIVE TRANSFER full text sources ADS |
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We present a method for determination of the random-orientation polarimetric scattering properties of an arbitrary, nonsymmetric cluster of spheres. The method is based on calculation of the cluster T matrix, from which the orientation-averaged scattering matrix and total cross sections can be analytically obtained. An efficient numerical method is developed for the T-matrix calculation, which is faster and requires less computer memory than the alternative approach based on matrix inversion. The method also allows calculation of the random orientation scattering properties of a cluster in a fraction of the time required for numerical quadrature. Numerical results for the random orientation scattering matrix are presented for sphere ensembles in the form of densely packed clusters and linear chains.
Matrix (chemical analysis)
Mueller calculus
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BASIC THEORY. Electromagnetic Theory. Absorption and Scattering by an Arbitrary Particle. Absorption and Scattering by a Sphere. Particles Small Compared with the Wavelength. Rayleigh--Gans Theory. Geometrical Optics. A Potpourri of Particles. OPTICAL PROPERTIES OF BULK MATTER. Classical Theories of Optical Constants. Measured Optical Properties. OPTICAL PROPERTIES OF PARTICLES. Extinction. Surface Modes in Small Particles. Angular Dependence of Scattering. A Miscellany of Applications. Appendices. References. Index.
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