Polarization properties of odd comet 17P/Holmes

We present results of polarimetric observations of comet 17P/Holmes that in 2007 surprised the observers by its outburst that resulted in dramatically increased brightness and unusual shape of the coma. Polarimetric properties of the comet also appeared to be very peculiar. Even though the comet showed negative polarization typical for the small phase angles, its absolute values were much lower than usually observed at these phase angles and the spectral gradient of the polarization was much more pronounced than usually observed for comets (although negative that is typical for other comets too). Both these results are different from the regular trends and indicate some specifics of the comet Holmes dust during the outburst. We discuss how such unusual polarimetric properties of the comet can be explained and show how a peculiar combination of aggregated and compact particles or a composition with a unique ratio of silicates to organics can explain the observational results.     

Lumière diffusée par les particules solides éjectées du noyau de la comète 81P/Wild 2

Comet 81P/Wild 2, target of the Stardust mission, has been observed in 1997, through an imaging polarimetry technique. Intensity and polarization images, retrieved for 9 000 to 26 000 km wide fields, imply a value of the decrease of the intensity with increasing distance to the nucleus, and a decrease of the polarization in the inner coma, in agreement with previous observations of other comets. The averaged polarization values have been determined for the first time on this comet. The results suggest that it could belong to the class of comets with a low polarization at large phase angles and an absence of silicate emission features.     

Linear polarimetry of five comets

We made linear polarimetry of five comets, C/1989 X1 (Austin), 109P/Swift-Tuttle, C/1996 B2 (Hyakutake), C/1995 O1 (Hale-Bopp), and C/1996 Q1 (Tabur) simultaneously at eight wavelengths, in order to study phase angle and wavelength dependence of polarization in continuum light, and polarization behavior of molecular origin.

Polarization in continuum of Swift-Tuttle and Hyakutake were rather high i.e. similar to that of 1P/Halley, while those of Austin and Tabur were low in the positive branch. Hale-Bopp showed the highest polarization ever observed at the corresponding phase angles in the positive branch.

We also made observations scanning across the coma for Hyakutake and Hale-Bopp to investigate polarization structure. For Hale-Bopp, we found: (1) The position of maximum polarization was not at the visible center, and a red part was observed in the central region. (2) Optical properties and/or size spectrum of dust varied in a jet. On the contrary, any peculiar structure was not found for Hyakutake in spite of a much finer spatial resolution. The scanning observation of Hale-Bopp suggests that dust including its size spectrum evolves after the ejection from the nucleus, and that the optical depth in the central region plays an important role.     

Model for cometary grains to explain optical polarization

The observed optical polarizations for comets have been explained in past assuming cometary grains to be compact spheres, such that Mie theory could be applied to simulate the observed polarizations. However, recently other shapes like spheroids and then more realistic shapes like aggregates of monomers have been considered for cometary grains, to explain the observed polarizations. For this purpose T-matrix or DDA based light scattering technique was mostly used to simulate the observed polarizations. A number of authors have used T-matrix, DDA and various other techniques along with aggregate grain model to explain the polarizations of comets like 1P/Halley, C 1995/O1 Hale-Bopp, C/1990 K1 (Levy) and C/1996 B2 Hyakutake, etc. Recent STARDUST mission had suggested cometary grains to be mixtures of compact and porous aggregates. Accordingly, attempts have been made recently to reproduce the cometary polarization with mixtures of various compositions, shapes and porosity.The work presented here considers a model for cometary grains which contains (1) solid grains of pyroxene (silicate) and organic with various sizes of spheres, prolates and oblates and (2) aggregates with monomers of various sizes, with composition of pyroxene (silicate) and organic, having structures (shapes) defined by BCCA and BAM2 codes. It was found that the present model can explain the observed polarization data, especially the negative branch, for comet 1P/Halley at , more effectively as compared to other work done in past. Among the aggregates the BAM2 structure was found to play a key role, in deciding the cross-over angle and depth of negative polarization branch.     

T-matrix approach to calculating circular polarization of aggregates made of optically active materials

Optical activity is a typical property of the biological materials where left-handed amino-acids and right-handed carbohydrates dominate (so called homochirality). Observationally, optically active materials reveal themselves through the circular polarization in the light they scatter. Thus, circular polarization produced by the optically active particles can serve as a biomarker. It is known that biological (e.g. colonies of bacteria) and pre-biological (e.g. dust in comets) particles often have a complex structure that can be modeled presenting them as aggregates of small monomers. This motivated the development of the T-matrix code presented in this paper, which enables calculation of the scattering matrix - including circular polarization - of the light scattered by aggregated optically active particles. The code can be used for modeling the light scattering by biological objects (e.g. colonies of bacteria, blood cells) and for interpretation of the circular polarization produced by the cosmic dust that contains (pre)biological organic, e.g. comet dust or planetary aerosols.     

Similarity and diversity in photometric and polarimetric opposition effects of small Solar System bodies

The phase-angle dependences of brightness and polarization of light scattered by atmosphereless Solar System bodies (satellites, asteroids, planetary rings) as well as comets and zodiacal light are analysed on the basis of available ground-based and spacecraft observations. We study similarity and diversity in the photometric and polarimetric opposition effects. The similarity of the brightness and polarization phase functions for polydisperse dust media (cometary and interplanetary dust) and for atmosphereless bodies at small phase angles gives grounds to state that at least some physical properties of dust particles and light-scattering mechanisms should be similar for the two classes of objects. The aggregate structure of particles in different objects can be precisely the property that determines the photometric and polarimetric effects observed in the opposition region. Differences observed in opposition effects for different objects are likely caused by different physical properties (composition, sizes, structure, density) of the scattering particles and, therefore, by different relative contributions of the light-scattering mechanisms. However, the relationship between the photometric opposition effect, spike effect, negative polarization branch, and polarization opposition effect for different bodies is ambiguous.     

Polarisation properties of comet NEAT C/2001 Q4

Comets exhibit high (up to 25 %) amount of optical polarization when they are observed through ground based or space telescopes. These polarizations are caused due to the scattering of cometary dust. The observed linear polarisation of comets is generally a function of the wavelength of incident light (λ), the scattering angle (θ), the geometrical shape and size of the particle and the composition of dust particles in terms of the complex values of the refractive index. The scattering properties of cometary dust will help to know the nature of cometary dust. In the present work, the observed linear polarization data of Comet NEAT are studied through simulations using Ballistic Particle-Cluster Aggregate (BPCA) and Ballistic Cluster-Cluster Aggregate (BCCA). Using Superposition T-matrix code, the best-fitting values of complex refractive indices are calculated which can well fit the observed polarization data of Comet NEAT C/2001 Q4. The best fitting values of complex refractive indices coming out from the present analysis correspond to mixture of both silicates and organics.     

Accurate lidar temperatures with narrowband filters

At daytime, metal resonance lidars need narrowband spectral filtering in the receiver to measure Doppler temperatures in the mesopause region. For our K lidar equipped with a Faraday anomalous dispersion optical filter we demonstrate that the derived resonance temperatures can be shifted by 10 K because of the wavelength dependence of the filter transmission for the Rayleigh scattered light when this filter transmission is used for normalization. We show that, if all filter effects are considered correctly, temperature measurements at daytime can be as accurate as at nighttime without such filters.     

Porous irregular aggregates of sub-micron sized grains to reproduce cometary dust light scattering observations

The present study intends to interpret some of the characteristic features of the light scattered by cometary dust, such as phase angle and wavelength dependence of its polarization, through simulations using Ballistic Cluster-Cluster Aggregation (BCCA) or Ballistic Particle-Cluster Aggregation (BPCA) aggregates of up to 128 sub-micron sized grains (spherical and spheroidal with a possible size distribution) of various composition (silicates, organics, silicates core with organics mantle). The dependence of the linear polarization with the size parameter is shown to depend highly on the size and composition of the constitutive grains. Internal interactions induced by shape or orientation averaging of the grains may lessen this dependence, leading to results comparable to those observed on cometary dust for fluffy aggregates of grains with a size parameter in the 1.3–1.8 range. A size distribution of realistically shaped particles (aggregates of spheroids and larger spheroids) following a power law size distribution with a power index of -3, the smallest grains radius by 0.03– and the largest spheroids effective radius by , gives a very good fit to the Hale-Bopp observed phase curves. The best silicates–organics ratio ranges from about 50–75% organics and 25–50% silicates in volume considering the eventual presence of core-mantle grains.     

Spectral anomalies of diffracted pulsed Bessel beams in dispersive media

Starting from the Huygens–Fresnel diffraction integral, the analytical expression for the power spectrum of diffracted pulsed Bessel beams in a dispersive media is derived and used to study the spectral anomalies of pulsed Bessel beams in the far field. Numerical results are given to illustrate the dependence of the anomalous spectral behavior on the pulse parameters, aperture parameter and dispersive properties of the medium. It is shown that, near phase singularities, anomalous spectral behavior may take place. The potential applications of spectral anomalies of ultrashort pulsed beams in information encoding and information transmission are considered.     

Polarization of microwave noise source radiation in the 30 GHz to 300 GHz range

The radiation spectra of three microwave noise sources (C. P. Clare & Co., TN type series) have been compared to determine the frequency bandwidth and their polarization. A Lamellar Grating Fourier Transform spectrometer was used with a Helium cooled bolometer detector in the wavelength region of lmm to 10mm. The spectral regions were compared with the manufacturer's specifications and the results were in partial agreement. A secondary transmission region was observed to have an input current as well as a polarization dependence despite the directional output of the waveguide antenna.     

Large-amplitude dust acoustic solitons in an opposite polarity dusty plasma with generalized polarization force

Linear and nonlinear dust acoustic(DA)waves have been investigated in an opposite polarity dusty plasma comprising negatively and positively charged dust grains,Maxwellian electrons and ions,including the generalized polarization force effect.The properties of linear DA waves have been significantly altered by the dual dust polarity and polarization force.Large amplitude DA solitons have been discussed in the framework of the Sagdeev potential technique.Our results show that both rarefactive and compressive solitons can exist in such a dusty plasma.The basic features of the Sagdeev potential have been examined under the effect of the polarization force parameter R,the ratio of the charge number of the positive dust to that of the negative dust Z,and the Mach number M.The results show that these parameters play a significant role in determining the region of existence of large amplitude DA solitons.     

Distinctive field dependence of the longitudinal muon polarization for Mu* in polycrystalline silicon

A distinctive longitudinal magnetic field dependence of the muon polarization for anomalous muonium in polycrystalline semiconductor targets has been predicted. The polarization exhibits a cusp,i.e., a discontinuous jump in the slope from negative to positive. Measurements of the longitudinal polarization for polycrystalline silicon in fields up to 0.5 T, and temperatures 53 and 200 K have been made at LAMPF. A cusp in the field dependence indeed occurs at 0.345 T, in excellent agreement with the prediction. No cusp is observed at 200 K because Mu^* has been thermally ionized.     

Far-out surface science: radiation-induced surface processes in the solar system

Interplanetary space is a cosmic laboratory for surface scientists. Energetic photons, ions and electrons from the solar wind, together with galactic and extragalactic cosmic rays, constantly bombard surfaces of planets, planetary satellites, dust particles, comets and asteroids. Many of these bodies exist in ultrahigh vacuum environments, so that direct particle–surface collisions dominate the interactions. In this article, we discuss the origins of the very tenuous planetary atmospheres observed on a number of bodies, space weathering of the surface of asteroids and comets, and magnetospheric processing of the surfaces of Jupiter's icy satellites. We emphasize non-thermal processes and the important relationships between surface composition and the gas phase species observed. We also discuss what laboratory and computational modeling should be done to support the current and future space missions––e.g. the Genesis mission to recover solar wind particles, the Cassini mission to probe Saturn, the Europa Lander mission to explore the subsurface ocean hypothesis, and the Pluto/Kuiper Express to sample the outer reaches of the solar system.     

Polarization memory in single quantum dots

We measured the polarization memory of excitonic and biexcitonic optical transitions from single quantum dots at either positive, negative or neutral charge states. Positive, negative and no circular or linear polarization memory was observed for various spectral lines, under the same quasi-resonant excitation below the wetting layer bandgap. We developed a model which explains both qualitatively and quantitatively the experimentally measured polarization spectrum for all these optical transitions. We consider quite generally the loss of spin orientation of the photogenerated electron–hole pair during their relaxation towards the many-carrier ground states. Our analysis unambiguously demonstrates that while electrons maintain their initial spin polarization to a large degree, holes completely dephase.     

Comparative studies of the reflectance and degree of linear polarization of particulate surfaces and independently scattering particles

We compare measurements of the phase-angle dependencies of the intensity and degree of linear polarization of particles in air and particulate surfaces. The samples were measured at two spectral bands centered near 0.63 and 0.45 μm. The surfaces were measured with the new photometer/polarimeter at the Astronomical Institute of Kharkov National University. The scattering measurements of the particles in air were carried out with the equipment currently located at the University of Amsterdam. We study a suite of samples of natural mineral particles of different sizes all in the micrometer range, i.e. comparable with the wavelengths. The samples are characterized by a variety of particle shapes and albedos. The samples have been studied in several works and in this paper we include new SEM microphotographs of particles and spectra of powders in a wide spectral range, 0.3–50 μm, using the RELAB equipment of Brown University. We made measurements of particulate surfaces in a phase-angle range, 2–60° that is significantly wider than that of our previous studies. We confirm our earlier results that the negative polarization of the surfaces may be a remnant of the negative polarization of the single scattering by the particles that constitute the surfaces. We also find differences in the spectral behavior of the polarization degree of particles in air and particulate surfaces at large phase angles.     

All-fiber acousto-optic tunable notch filter with electronically controllable spectral profile

We demonstrate a novel all-fiber acousto-optic tunable notch filter based on coupling to cladding modes in a single-mode fiber. The device has the advantage of low loss (<0.1 dB) and low polarization dependence. By coupling input light to multiple cladding modes by use of multiple acoustic waves, we achieved an electronically controllable variable spectral profile without a significant coherent cross-talk problem.     

Cosmic dust and our origins

The small solid particles in the space between the stars provide the surfaces for the production of many simple and complex molecules. Processes involving the effects of ultraviolet irradiation of the thin (hundredth micron) mantles are shown to produce a wide range of molecules and ions also seen in comets. Some of the more complex ones inferred from laboratory experiments are expected to play an important role in the origin of life. An outline of the chemical evolution of interstellar dust as observed and as studied in the laboratory is presented. Observations of comets are shown to provide substantial evidence for their being fluffy aggregates of interstellar dust as it was in the protosolar nebula, i.e. the interstellar cloud which collapsed to form the solar system. The theory that comets may have brought the progenitors of life to the earth is summarized.     

Direct observation of antiferroquadrupolar ordering: resonant X-Ray scattering study of DyB2C2

Antiferroquadrupolar (AFQ) ordering has been conjectured in several rare-earth compounds to explain their anomalous magnetic properties. No direct evidence for AFQ ordering, however, has been reported. Using the resonant x-ray scattering technique near the Dy L(III) absorption edge, we have succeeded in observing the AFQ order parameter in DyB2C2 and analyzing the energy and polarization dependence. The much weaker coupling between the orbital degrees of freedom and the lattice in 4f electron systems than in 3d compounds makes them an ideal platform to study orbital interactions originating from electronic mechanisms.     

Polarization-controlled dispersive wave redirection in dual-core photonic crystal fiber

The complex study of the polarization-controlled supercontinuum generation in a dual-core square lattice photonic crystal fiber made of multicomponent glass was accomplished. The fiber was excited by 100-fs pulses at a 1250-nm wavelength in the anomalous dispersion region and the registered spectra exhibited soliton fission and Raman-induced self-frequency shift processes. The study also involved a detailed analysis of the infrared-to-visible light conversion exhibiting a dispersive wave origin. The special dual-core properties were investigated by separate spectral analysis of each of the two cores and the near-field profile registration. The emphasis was on the visible part of the spectrum where the input energy and polarization direction dependences were studied. The increase in the input energy allowed for the tuning of the wavelength of the visible spectral features, a further rotation of the polarization direction had an effect on the spectral dependence of the light distribution between the cores. The dual-core fiber exhibited a significant coupling performance and the spectral dependence of the visible light distribution is in good agreement with the simulated coupling length spectral characteristics. Single-core excitation in the linear regime revealed the possibility of coupling 50% of the energy to the other core and the same polarization-controlled redirection possibilities as that at the nonlinear experiments. Dual-core excitation of the fiber enhances the light redirection effect with the application potential for the polarization-controlled directional coupler accompanied by nonlinear frequency conversion.