Evgenij Zubko

Validity criteria of the discrete dipole approximation.

There are two widely accepted restrictions on the application of the discrete dipole approximation (DDA) in the study of light scattering by particles comparable to the wavelength: (1) when considering dielectric particles, the size of the cells must satisfy the condition kd|m|<0.5, where k is the wavenumber, d is the size of the cells, and m is the complex refractive index of the constituent material and (2) when considering conductive particles, the size of the cells must be small enough to reproduce sufficiently the evolution of the electromagnetic field in the skin layer. We examine both restrictions when the DDA is applied to irregularly shaped particles and show that its restrictions are not as strong as is widely accepted. For instance, when studying irregularly shaped particles averaged over orientations, even at kd|m|=1, the DDA provides highly accurate numerical results. Moreover, we show that the impact of using large constituent cells is similar to that produced by surface roughness; therefore, the replacement of the target particle by an array of large constituent cells has the same effect, qualitatively, as incorporating additional small-scale surface roughness on the particle. Such a modification of the target particle can be desirable in many practical applications of DDA when irregularly shaped particles are considered. When applying DDA to conductive, nonspherical particles, the insufficient description of the electromagnetic field in the skin layer does not lead to a violation of the Maxwell equations, although it has a visible but nonmajor influence on the light-scattering properties of the target.

COHERENT BACKSCATTERING VERIFIED NUMERICALLY FOR A FINITE VOLUME OF SPHERICAL PARTICLES

We consider electromagnetic scattering by a spherical volume sparsely and randomly populated by spherical particles of equal size and optical properties. The far-field scattering matrix of the entire volume is computed using an exact method and an approximate method. The former is a direct computer solver of the Maxwell equations called the superposition T-matrix method (STMM). The latter is a solver based on numerical Monte Carlo integration of the ladder and cyclical diagrams appearing in the microphysical theory of radiative transfer and coherent backscattering (RT-CB). The quantitative agreement between the STMM and RT-CB computations provides verification of the RT-CB theory. Prominent backscattering features exhibited by the STMM data cannot be reproduced by keeping only the ladder diagrams of RT. Our results strongly support the CB explanation of opposition brightness and polarization phenomena observed for a class of atmosphereless solar-system objects. Further research is necessary to determine the range of quantitative applicability of the RT-CB theory to densely packed particulate media.

Discrete dipole approximation simulations of scattering by particles with hierarchical structure

We use the discrete dipole approximation (DDA) method to calculate the intensity and the linear polarization degree of light scattered by agglomerated debris particles with hierarchical structure as functions of size parameter (varying from x = 2 to x = 14) and phase angle. Such structures are important, e.g., for cometary and interplanetary dust particles. Calculations for three combinations of refractive index were made, which correspond to regions of water ice, organic matter, and silicates. We examine the photometric and polarization properties of agglomerated particles with prefractal (Whitten-Sander model) and nonfractal porous structures of particle fragments formed by dipoles. We find that the aggregated particles can produce significant negative polarization at small phase angles. Increasing the packing density of dipoles and/or refractive index makes the negative polarization more prominent. The depth of the negative polarization branch depends on the type of internal structure: the negative polarization branch of particles having nonfractal structure is noticeably shallower in comparison with that of those having a prefractal structure. The negative polarization branch depth strongly depends on the imaginary part of the refractive index and increases with decreasing absorption. Polarization phase curves for agglomerated debris particles become smoother as the number of hierarchical levels increases.

Numerical Techniques for Backscattering by Random Media

Coherent backscattering of unpolarized incident light by random media of discrete scatterers leads to an opposition effect in scattered intensity (enhanced backscattering) and a surge of negative degree of linear polarization (negative polarization). We describe three computational techniques suitable for the multiple scattering problem at hand: second-order and sixth-order ray-tracing techniques for dark media, and a coherent-backscattering radiative-transfer technique for both dark and bright media. All three techniques are radiative-transfer-like approximations to the rigorous electromagnetic scattering theory. We discuss example results for miscellaneous random media of scatterers.

Interpretation of single-particle negative polarization at intermediate scattering angles

We study the interrelation of the internal field of irregular particles to the far-field scattering characteristics by modifying the internal field of dipole groups. In this paper, we concentrate on the longitudinal component, i.e., the internal-field component parallel to the incident wave vector. We use the discrete-dipole approximation to determine the internal field and switch off the longitudinal component from the dipoles that have the highest energy density above a preset cutoff value. We conclude that only a relatively small number of core dipoles, about 5% of all dipoles, contribute to the negative linear polarization at intermediate scattering angles. These core dipole groups are located at the forward part of the particles. The number of core dipoles in the group becomes greater as particle asphericity increases. We find that the interference between the scattered waves from the core dipole groups, which was studied previously for spherical particles, is preserved to a large extent for nonspherical particles.

Evaluating the carbon depletion found by the Stardust mission in Comet 81P/Wild 2

The low abundance of refractory carbonaceous material in samples collected by Stardust in comet 81P/Wild 2 coma was completely unexpected. If these results are universal to other comets, this necessitates a reformulation of current models of solar system formation. A polarimetric imaging analysis demonstrates that dust is not uniformly distributed within cometary coma, and that the circumnucleus halo region where the dust samples were collected must contain a low population of carbonaceous particles. Such regions are seen in other comets, suggesting that comet 81P/Wild 2 is not unusual and that the anomalous lack of carbon is not necessarily representative of the entire coma.

Hubble Space Telescope Pre-perihelion ACS/WFC Imaging Polarimetry of Comet Ison (c/2012 s1) at 3.81 AU

We present polarization images of Comet ISON (C/2012 S1) taken with the Hubble Space Telescope (HST) on UTC 2013 May 8 (r h = 3.81 AU, Δ = 4.34 AU), when the phase angle was α ≈ 1216. This phase angle is approximately centered in the negative polarization branch for cometary dust. The region beyond 1000 km (~0.32 arcsec ≈ 6 pixels) from the nucleus shows a negative polarization amplitude of p% ~ –1.6%. Within 1000 km of the nucleus, the polarization position angle rotates to be approximately perpendicular to the scattering plane, with an amplitude p% ~ +2.5%. Such positive polarization has been observed previously as a characteristic feature of cometary jets, and we show that Comet ISON does indeed harbor a jet-like feature. These HST observations of Comet ISON represent the first visible light, imaging polarimetry with subarcsecond spatial resolution of a Nearly Isotropic Comet beyond 3.8 AU from the Sun at a small phase angle. The observations provide an early glimpse of the properties of the cometary dust preserved in this Oort-Cloud comet.

Coherent backscattering in planetary regoliths

Atmosphereless solar-system objects exhibit two ubiquitous light-scattering phenomena at small solar phase angles (sun.object.observer angle α): first, the opposition effect in the intensity of scattered sunlight (e.g., [1]); and, second, the negative degree of linear polarization (I ┴ − I ║)/(I ┴ + I ║). Here I ║ denotes the intensity component parallel to the scattering plane defined by the Sun, the object, and the observer and I ┴ denotes the component perpendicular to that plane [2].

2 Light scattering by irregularly shaped particles with sizes comparable to the wavelength

Light scattering by single irregularly shaped particles whose sizes are comparable with wavelength plays an important role in numerous remote-sensing applications. It especially concerns applications dealing with both terrestrial and cosmic dust particles having truly irregular and random structure. The knowledge of the scattering by single irregular particles is absolutely necessary for a realistic modeling and successful interpretation of measurements of light scattering by a powder-like surface, such as, soils, sand-drift or planetary regolith. Note that, though the multiple scattering between constituent particles could significantly dominate over single scattering, it is quite evident that the one is a function of other.

Umov effect in single-scattering dust particles: effect of irregular shape

The Umov effect manifests itself as an inverse correlation between the light-scattering maximum of positive polarization Pmax and the geometric albedo A of the target. In logarithmic scales, Pmax is linearly dependent on A. This effect has been long known in the optics of particulate surfaces and, recently, it was extended for the case of single-scattering dust particles whose size is comparable to the wavelength of the incident light. In this work, we investigate the effect of irregular shape on the Umov effect in single-scattering particles. Using the discrete dipole approximation (DDA), we model light scattering by two different types of irregularly shaped particles. Despite significant differences in their morphology, both types of particles reveal remarkably similar diagrams of log(Pmax) versus log(A). Moreover, in a power-law size distribution r-n with n=2.5-3.0, the Umov diagrams in both types of particles nearly coincide. This suggests little dependence on the shape of target particles in the retrieval of their reflectance using the Umov effect.