Polarization of Light Scattered by Solar System Bodies and the Aggregate Model of Dust Particles

The present study considers the dependence of characteristics of light scattering by aggregate particles on the refractive index, size, and number of spherical particles composing the aggregate, as well as on the structure and porosity of the cluster. The parameters were varied in sufficiently wide ranges to let a coherent picture of the polarimetric properties of relatively small aggregate particles emerge (the size parameter of the aggregate is less than 10). It was shown that, in the framework of the aggregate model, the behavior of polarization phase curves observed for both comets and regolith surfaces can be explained. The modeling carried out confirms that the sizes of the cometary dust particles are larger than the wavelength. However, the grains forming the cometary dust particles or the regolith (or details of the particle surface) have a size less than 0.3–0.5 m. This agrees with estimates obtained by other methods. The determining role in the formation of the polarization phase curve is played by the structure of the external layer of the clusters. The appearance of the negative branch of polarization and its shape substantially depend on the effectiveness of the interference of multiply scattered waves and on the interaction in the near field at these phase angles. Interference and interaction in the near field in turn are determined by the sizes of elementary scatterers and the structure of the ensemble. If the number of constituent particles in the aggregate is larger than several tens, its role in the formation of the negative branch of polarization is minor, while the influence on the polarization maximum position is rather substantial. The polarimetric data alone cannot provide a unique estimate of the refractive index: the brightness measurements must be invoked as well. For a more complete quantitative interpretation of the observations, the scattering matrix of aggregates comparable in size to or larger than the wavelength must be calculated in the short- and long-wavelength ranges, which still encounters serious theoretical and technical difficulties. Moreover, in order to obtain unique results, it is obvious that the spectral range of observations must be extended and that other types of measurements, such as spectroscopic ones, must also be used.     

Light scattering by fluffy Mg-Fe-SiO and C mixtures as cometary analogs (PROGRA experiment)

Cometary particles mainly consist of silicates and carbon compounds; they seem to be fluffy aggregates of tiny grains, as found in some IDPs. The linear polarization of the scattered light is an efficient method to characterize their physical properties. Laboratory simulations of light scattering by cometary analog particles help to disentangle different physical parameters by comparison with observational data. We present here polarization laboratory results with nine samples levitating particles: five samples of vapor-condensed magnesiosilica, one ferrosilica smoke, a mixture of magnesio-ferrosilica smokes, one mixture of ferrosilica with carbon and one mixture of magnesio-ferrosilica with carbon. The phase curves are bell-shaped with a maximum polarization at a phase range of (80°-100°). A shallow negative branch can be present at phase angles smaller than 20°. The different characteristics of the phase curves are discussed considering the size and the structure of the constituent grains and the size of the particles. For the five magnesiosilica samples, the maximum in polarization is in the 40% range (close to cometary values), and no wavelength dependence is detected; the negative branch, whose presence seems to be linked to the presence of large aggregates of fine silica (SiO₂) grains, does not always exist. For the ferrosilica smoke, the maximum in polarization is about 30% in red light (632.8 nm) and 40% in green light (543.5 nm); the negative branch occurs for phase angles smaller than 20°. For the two mixtures with carbon black, the polarization spectral gradient is positive, as expected for cometary analog particles. Finally, the phase curves obtained for agglomerates of magnesio-ferrosilica and carbon (expected to be the main components of cometary particles) are comparable to those obtained by remote observations of dust in cometary comae.     

Polarization of Cosmic Microwave Backgdound Scattered by Moving Flat Protoobjects

The work is devoted to the investigations of possible observational manifestations of protoobjects related to the dark ages epoch (10 < z < 1000), before formation of self-luminous galaxies and stars. These objects can distort the cosmic microwave background. Formation of these objects is described in the pancake theory and in the model of hierarchic clustering. According to these theories we may consider these protoobjects as flat layers. We consider both Thomson (with Rayleigh phase matrix) and resonance (for complete frequency redistribution) scattering of cosmic microwave background radiation by a moving flat layer. The resulting anisotropy and polarization of cosmic microwave radiation are calculated for a wide range of layer optical thickness (from an optically thin layer to an optically thick one). Analytical solutions are also obtained for the case of an optically thin layer and are compared with the numerical ones.     

A Negative Branch of Polarization for Comets and Atmosphereless Celestial Bodies and the Light Scattering by Aggregate Particles

Optical observations of comets and atmosphereless celestial bodies show that a change of sign of the linear polarization of scattered light from negative to positive at phase angles less than 20° is typical of the cometary coma, as well as of the regolith of Mercury, the Moon, planetary satellites, and asteroids. To explain a negative branch of polarization, this research suggests a unified approach to the treatment of cometary-dust particles and regolith grains as aggregate forms. A composite structure of aggregate particles resulting in the interaction of composing structural elements (monomers) in the light-scattering process is responsible for the negative polarization at small phase angles, if the monomer sizes are comparable to the wavelength. The characteristics of single scattering of light calculated for aggregates of this kind turned out to be close to the properties observed for cometary dust. Unlike the cometary coma, the regolith is an optically semi-infinite medium, where the interaction between particles is significant. To find the reflectance characteristics of regolith, the radiative-transfer equation should be solved for a regolith layer. In this case, the interaction between scatterers can be modeled to a certain extent by representing the regolith grains as aggregate structures consisting of several or many elements. Although real regolith grains are much larger than the particles considered here, laboratory measurements have shown that it is precisely the surface irregularities comparable to the wavelength that cause a negative branch of polarization. The main observed features of the phase and spectral dependence of the linear polarization of light scattered from comets and atmosphereless celestial bodies, which are due to the difference of the elementary scatterers in composition, size, and structure, can be successfully explained using the aggregate model of particles.     

Prospects for Estimating the Properties of a Loose Surface from the Phase Profiles of Polarization and Intensity of the Scattered Light

It has been shown that the model of a scattering medium composed of clusters located in the far zones of each other allows some properties of regolith-like surfaces to be quantitatively estimated from the phase dependences of intensity and polarization measured in the backscattering domain. From the polarization profiles, the sizes of particles, the structure and porosity of the medium, and a portion of the surface area covered with a disperse material can be determined. At the same time, the intensity profiles of the scattered light weakly depend on the sizes and structure of particles; they are mainly controlled by the concentration of scatterers in the medium and the shadow-hiding contribution at small phase angles. Since the latter effect is beyond the considered model, a good agreement between the model and the measured intensity cannot be achieved. Nevertheless, if a portion of the surface that participates in coherent backscattering has been found from the phase profile of polarization, the present model makes it possible to determine the relative contribution of the shadow-hiding effect to the brightness surge measured at zero phase angle. This, in turn, may allow the roughness of the scattering surface to be estimated. The model contains no free parameters, but there is currently no possibility to verify it comprehensively by the data obtained in laboratory measurements of the samples with thoroughly controlled characteristics, because such measurements are rare for a wide range of the properties of particles in a medium, their packing density, and phase angles.

     

Preliminary Remediation of Scattered Light in NEAR MSI Images

During a failed Eros orbit insertion maneuver on 20 December 1998, more than 28 kg of hydrazine were expended by attitude control jets on the NEAR Shoemaker spacecraft. Deposition of burn products on the outer optic of the multispectral imager, or MSI, resulted in a wavelength-dependent degradation of the system point-spread function (PSF). The scattered light is progressively worse in the shortest and longest wavelength filters, especially at 450 and 1050 nm. The degraded PSF was characterized using numerous images of Canopus acquired subsequent to the anomaly. There is no evidence for temporal change in the PSF since the burn abort incident. A fast Fourier transform-based image restoration method using the optimal filter recovers most of the spatial resolution of the original images while preserving radiometric accuracy for the 550- to 1000-nm images. This procedure has enabled nearly unimpeded monochrome imaging of asteroid morphology and select 5-color measurements at a scale of ∼5 pixels.     

Scattering of Light by Composite Particles in a Planetary Surface

It has been proposed that composite particles containing internal scatterers may provide the explanation for the fact that most photometric studies of planetary surfaces based on Hapke's model of bidirectional reflectance have found the planetary particles to exhibit moderately backscattering phase functions. However, an implicit assumption made in this explanation is that the scattering by composite particles containing multiple internal inclusions in a planetary surface can still be adequately computed using standard radiative transfer theory assuming the composite particles to be the fundamental individual scatterers even though such particles are necessarily in close proximity to each other. In this paper, this assumption is explored by examining the effects of close packing on the light scattering by spherical particles containing isotropic internal scatterers using a Monte Carlo routine. As expected, classical radiative transfer (assuming a random distribution of scattering particles) coupled with the assumption that the composite particle is the fundamental scatterer provides a good approximation in the high porosity limit. However, even for porosities as high as 90% the effects of close packing are clearly seen with the radiative transfer calculation underestimating the scattering by ∼10% at high incidence, emission, and phase angles. As the porosity is lowered further, the discrepancy becomes more severe and can reach 50% or more. In contrast, assuming the individual scatterer properties in the radiative transfer calculation leads to a substantial overestimate of the scattering even for porosities as low as 27.5%. This suggests that parameters derived using the classical radiative transfer theory will yield results intermediate between those of the composite as a whole and those of the internal scatterers. Thus, one should exercise caution in interpreting the results of models based on classical radiative transfer theory in terms of the physical properties of the surface particles and, where possible, the bidirectional reflectance of densely packed composite particles should be computed using more accurate methods such as the stochastic radiative transfer theory.     

Cassini图像中受散射光影响的7颗土星近环内卫星的天体测量

卡西尼号探测器携带了光学成像科学子系统(ISS: Imaging Science Subsystem), 在2004--2017年间拍摄了一些土星内卫星的图像. 部分图像中土星内卫星非常靠近土星环, 观测对象受土星环散射光影响导致测量精度差, 甚至无法测量. 由此提出一种适用于这类图像的背景消除算法, 可对受土星光环散射光影响的土星内卫星进行测量. 归算了7颗近环内卫星(土卫十(Janus)、土卫十一(Epimetheus)、土卫十五(Atlas)、土卫十六(Prometheus)、土卫十七(Pandora)、土卫三十二(Methone)和土卫四十九(Anthe))的70张ISS图像, 并与不消除散射光的方法进行了比较. 结果显示, 该方法至少可以提高43%的精度. 和喷气推进实验室的土星卫星历表\lk SAT415相比, 测量得到的赤经和赤纬方向的残差均值分别为0.72km和2.26km, 标准差分别为10.99km和11.36km.     

Polarization Light Curves and Spectra of AM-Her Polars

Some polarization spectra and light curves are being generated to explore the properties of AM-Her(culis) shocks for which the cyclotron emission is self-consistently calculated. Here, we quantify several of the relationships between the polarization and the properties of the magnetic field for magnetic white dwarf oscillating radiative accretion shocks. We discuss the properties of the polarization for time scales ≫ tosc, the oscillation period of the radiative shocks. The time-averaged polarization curves are calculated for a polar cap composed of independently oscillating flux tubes. We choose our funnels to cover the polar cap subtending an angle of 18° from the polar axis. This revised version was published online in July 2006 with corrections to the Cover Date.     

The Central Regions of M82 – Scattered Light and Extinction

Photographic photometric measurements of 2 arcsec resolution of the central regions of M82 are presented in the three colours U, B, V. The observed light is decomposed into two parts: stellar light and light from the central source scattered by dust. The variation of the reddening of both components is derived. The dust distribution pattern is different near the spatial centre of M82 and farther out. The equator of the dust complex obscuring the central source is inclined to the overall symmetry plane of M82. The outer dust clouds responsible for the obscuration of the visible stellar light are probably arranged in a spiral-like fashion; the “plane” containing these clouds is strongly warped and distrubed. The light of the bright knots A and C consists to a considerable part of scattered light from the central source nearly behind them. H_α light and scattered continuum light is strongly correlated in M82, pointing to an ionisation of the gas by the source of this continuum in addition to the presence of scattered H_α.     

Characteristics of Light Scattering at Small Phase Angles by Surfaces Consisting of Spherical Particles

We present the results of measurements of the phase dependences of brightness and of the polarization and depolarization characteristics for surfaces consisting of spherical glass particles in the phase-angle range from 0.1° to 5.0°. The measurements are performed using the laboratory photometer/polarimeter of Kharkov Astronomical Observatory and the photometer of Jet Propulsion Laboratory. An optically thick layer of transparent-glass spheres with mean size of about 57 m and refractive index of 1.44 exhibits a strong opposition effect due to single scattering. The contribution of interparticle scattering is nearly independent of the phase angle. At an angle of 0.4°, the spheres exhibit a glory ring that manifests itself in the phase behavior of all characteristics investigated. Small details are seen on the curves when a monochromatic radiation source is used for measurements. Their occurrence is confirmed by calculations based on the Mie theory. The unusual behavior of the phase dependences of reflectivity, degree of polarization, and color index for layers composed of spherical particles can be used to search for sites of possible deposits of spherical glass (or ice) particles in regoliths of atmosphereless celestial bodies.     

Light curves of Asteroids (IV)

Light curves, based on photoelectric observations made in 1965 and 1978, are given for 11 asteroids. For 7 of them, periods have been determined (TABLE 1). Our period for (16) Psyche is different from some previous determinations.     

Light Scattering by Aggregate Particles Comparable in Size to Wavelength: Application to Cometary Dust

Many naturally occurring particles (including, most likely, cometary dust) have an aggregate structure. We study the scattering properties of polydisperse independent aggregate particles (clusters) comparable in size to visible wavelengths. The sizes of the monomers constituting a cluster play a significant role in forming the angular dependences of intensity and linear polarization of the scattered light. Irregularly structured aggregates composed of a moderate number of spheres (<50) with size parameters 1.3–1.65 exhibit properties typical of cometary dust particles: a slight increase in backscattering intensity, a negative polarization at small phase angles, an inversion phase angle close to the observed one, an increase in brightness, and a linear polarization with increasing wavelength. In this case, the imaginary part of the refractive index for particles can increase with decreasing wavelength in the visible spectral range, which is typical of silicates with an admixture of iron or organic material. The spectral dependence of extinction efficiency for aggregates is less steep than that for equivalent spherical particles, and its maximum is shifted to larger size parameters. Therefore, when analyzing extinction measurements, the scatterer shape must be taken into account to avoid underestimation of the scattering-particle sizes.     

Polarization of the Cosmic Microwave Background: the Milan Polarimeter Experiment

We describe a Two Channel Correlation Polarimeter built by the Milan Radio Group for searching polarization of the Cosmic Microwave Background. The system sensitivity is sufficient to detect linearly polarized signals or set firm upper limits at levels of few μK. The program of observations and some preliminary results are presented.     

Light scattering by morphologically complex objects and opposition effects (a review)

Over the last decade, considerable progress has been achieved in the theory of light scattering by morphologically complex objects, which extends the potential of correct interpretation of photometric and polarimetric observations. This especially concerns the backscattering domain, where the opposition effects in brightness and polarization are observed. Although the equations of radiative transfer and weak localization (coherent backscattering) are rigorously valid only for sparse media, the results of exact computer solutions of the Maxwell equations for a macroscopic volume filled with randomly positioned particles show that their application area can be wider. In particular, the observations can be correctly interpreted if the packing density of particles in the medium reaches 20–30%. The recently suggested approximate solution of the coherent backscattering problem allowed interesting effects in the spectra of Saturn’s satellites to be explained. In the densely packed media, the effects that are impossible in the sparse media and caused by the near-field contribution can be observed. To calculate the characteristics of radiation reflected by such a medium, it is not sufficient to solve the radiative transfer and weak localization equations, even if they are written in a form without the far-zone limitations. Nowadays, the influence of the interaction of particles in the near field can be analyzed only for the restricted ensembles of particles. It shows that the substantial increase of the packing density essentially changes the phase functions of intensity and polarization in the backscattering domain. This allows the packing density of particles in the medium and their absorbing properties to be estimated from the shape of the phase curves measured. However, the task of quantitative interpretation of the measurements of radiation reflected by a densely packed medium, in terms of sizes of particles, their refractive index, and packing density, still remains unsolved.     

散射光对准确CCD平场的影响--修改某些反光镜挡光筒的必要性

继前文之后，进一步说明，晨昏蒙影平场与夜天露光的差别主要由于存在散射光．像Grundahl和Sorensen那样，上海天文台1．56米和南京大学63厘米反光镜的针孔像也已获得，直观地显示了散射光的存在，1．56米和63厘米反光镜都有标准设计的挡光筒，所有标准设计的反光镜都只有两个挡光筒，分别置于主镜和付镜前面．为了在得出准确平场时CCD不受散射光影响，这样的挡光筒是不够的．这是所有标准设计的档光筒共同的弱点．为了得到更准确的平场改正，修改所有类似的反光镜主镜挡光筒是极其重要的。     

The Imaging Vector Magnetograph at Haleakala –: III. Effects of Instrumental Scattered Light on Stokes Spectra

The scattering of light over the field of view of a solar spectropolarimeter affects all Stokes parameters. The magnetic field vector inferred from the Stokes spectra then has systematic error. The reason is that scattering affects polarized radiation as well as unpolarized. Accurate correction of the Stokes spectra from the Imaging Vector Magnetograph (IVM) of the Mees Solar Observatory illustrates the problem and the solutions.     

Polarimetric and Photometric Phase Effects Observed on Transneptunian Object (29981) 1999 TD10

The Kuiper-Belt Object (29981) 1999 TD₁₀, classified as a Scattered-Disk Object, has been observed at three different phase angles with the ESO 8.2-m VLT and FORS 1 instrument in polarimetric mode in November and December 2003. These observations have been used to compute the Stokes parameter q, which represents the linear polarization degree. We have also used the previously published photometric observations to improve the R-band phase function. The main conclusions are as follows: (i) a negative linear polarization degree decreasing with phase angle α up to, at least, α=3°, (ii) for α=3°, (iii) a possible color effect between the R and V band, the polarization degree being more negative in R. The R-band polarimetric observations can be explained by the coherent-backscattering mechanism and fitted by a two-component Rayleigh-scatterer model for a spherical small body. The rotation period of 15.382±0.001 h published by Mueller et al. (2004, Icarus 171, 506–515) and Choi et al. (2003, Icarus 165, 101–111) is confirmed. The R-band phase curve provides H=8.35±0.02 and G=−0.25±0.022 parameters with the IAU H–G formalism.Based on observations obtained at the Cerro Paranal observatory of the European Southern Observatory (ESO) in Chile.     

Color imaging of Mars by the High Resolution Imaging Science Experiment (HiRISE)

HiRISE has been producing a large number of scientifically useful color products of Mars and other planetary objects. The three broad spectral bands, coupled with the highly sensitive 14 bit detectors and time delay integration, enable detection of subtle color differences. The very high spatial resolution of HiRISE can augment the mineralogic interpretations based on multispectral (THEMIS) and hyperspectral datasets (TES, OMEGA and CRISM) and thereby enable detailed geologic and stratigraphic interpretations at meter scales. In addition to providing some examples of color images and their interpretation, we describe the processing techniques used to produce them and note some of the minor artifacts in the output. We also provide an example of how HiRISE color products can be effectively used to expand mineral and lithologic mapping provided by CRISM data products that are backed by other spectral datasets. The utility of high quality color data for understanding geologic processes on Mars has been one of the major successes of HiRISE.