The Experts below are selected from a list of 312 Experts worldwide ranked by ideXlab platform
Fernando Moreno - One of the best experts on this subject based on the ideXlab platform.
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Models of Rosetta/OSIRIS 67P dust coma Phase Function
Astronomical Journal, 2018Co-Authors: Fernando Moreno, Daniel Guirado, Olga Muñoz, Ivano Bertini, Cécilia Tubiana, Carsten Güttler, Marco Fulle, Alessandra Rotundi, Vincenzo Della Corte, Stavro IvanovskiAbstract:The Phase Function of the dust coma of comet 67P has been determined from Rosetta/OSIRIS images. This Function shows a deep minimum at Phase angles near 100°, and a strong backscattering enhancement. These two properties cannot be reproduced by regular models of cometary dust, most of them based on wavelength-sized and randomly oriented aggregate particles. We show, however, that an ensemble of oriented elongated particles of a wide variety of aspect ratios, with radii r gsim 10 μm, and whose long axes are perpendicular to the direction of the solar radiation, are capable of reproducing the observed Phase Function. These particles must be absorbing, with an imaginary part of the refractive index of about 0.1 to match the expected geometric albedo, and with porosity in the 60%–70% range.
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The Phase Function and density of the dust observed at comet 67P/Churyumov–Gerasimenko
Monthly Notices of the Royal Astronomical Society, 2018Co-Authors: Marco Fulle, Anny Chantal Levasseur-regourd, Ivano Bertini, Carsten Güttler, Vincenzo Della Corte, Florangela La Forgia, Stavro L. Ivanovski, Jérémie Lasue, Francesco Marzari, Fernando MorenoAbstract:The OSIRIS camera onboard Rosetta measured the Phase Function of both the coma dust and the nucleus. The two Functions have a very different slope versus the Phase angle. Here, we show that the nucleus Phase Function should be adopted to convert the brightness to the size of dust particles larger than 2.5 mm only. This makes the dust bursts observed close to Rosetta by OSIRIS, occurring about every hour, consistent with the fragmentation on impact with Rosetta of parent particles, whose flux agrees with the dust flux observed by GIADA. OSIRIS also measured the antisunward acceleration of the fragments, thus providing the first direct measurement of the solar radiation force acting on the dust fragments and thus of their bulk density, excluding any measurable rocket effect by the ice sublimation from the dust. The obtained particle density distribution has a peak matching the bulk density of most COSIMA particles, and represents a subset of the density distribution measured by GIADA. This implies a bias in the elemental abundances measured by COSIMA, which thus are consistent with the 67P dust mass fractions inferred by GIADA, i.e. (38 ± 8) per cent of hydrocarbons versus the (62 ± 8) per cent of sulphides and silicates.
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The scattering Phase Function of comet 67P/Churyumov-Gerasimenko coma as seen from the Rosetta/OSIRIS instrument
Monthly Notices of the Royal Astronomical Society, 2017Co-Authors: Ivano Bertini, Fernando Moreno, Cécilia Tubiana, Carsten Güttler, Marco Fulle, Florangela La Forgia, Elisa Frattin, Gabor Kovacs, Maurizio Pajola, Holger SierksAbstract:The study of dust, the most abundant material in cometary nuclei, is pivotal in understanding the original materials forming the solar system. Measuring the coma Phase Function provides a tool to investigate the nature of cometary dust. Rosetta/OSIRIS sampled the coma Phase Function of comet 67P/Churyumov-Gerasimenko, covering a large Phase angle range in a small amount of time. Twelve series were acquired in March 2015 - February 2016 for this scientific purpose. This data allowed, after straylight removal, measuring the Phase Function shape, its reddening, and Phase reddening while varying heliocentric and nucleocentric distances. Despite small dissimilarities within different series, we found a constant overall shape. The reflectance has a u-shape with minimum at intermediate Phase angles, reaching similar values at the smallest and largest Phase angle sampled. The comparison with cometary Phase Functions in literature indicates OSIRIS curves being consistent with the ones found in many other single comets. The dust has a negligible Phase reddening at α < 90°, indicating a coma dominated by single scattering. We measured a reddening of [11-14]%/100 nm between 376 nm and 744 nm. No trend with heliocentric or nucleocentric distance was found, indicating the coma doesn’t change its spectrum with time. These results are consistent with single coma grains and close-nucleus coma photometric results. Comparison with nucleus photometry indicates a different backscattering Phase Function shape and similar reddening values only at α < 30°. At larger Phase angles the nucleus becomes significantly redder than the coma.
Zhixiong Guo - One of the best experts on this subject based on the ideXlab platform.
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A New and Simple Technique to Normalize the HG Phase Function for Conserving Scattered Energy and Asymmetry Factor
Numerical Heat Transfer Part B: Fundamentals, 2014Co-Authors: Brian Hunter, Zhixiong GuoAbstract:A new, yet simple, technique is formulated for normalizing the Henyey-Greenstein (HG) Phase Function by ensuring conservation of both scattered energy and asymmetry factor simultaneously, and is analyzed for use in determining accurate radiative transfer predictions in strongly anisotropic scattering media using the discrete ordinates method (DOM). Two recently published simple normalization techniques are able to conserve either scattered energy or asymmetry factor after discretization solely by normalization of the forward-scattering HG Phase-Function value. However, normalization of only the forward-scattering term cannot conserve two quantities simultaneously. The present technique normalizes both the forward-scattering and backward-scattering terms in order to conserve both scattered energy and asymmetry factor simultaneously and maintain most of the Phase-Function shape while retaining simplicity and efficiency. Analysis of radiative transfer predictions shows that results generated using the presen...
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Phase Function normalization in the 3 d discrete ordinates solution of radiative transfer part i conservation of scattered energy and asymmetry factor
Numerical Heat Transfer Part B-fundamentals, 2012Co-Authors: Brian Hunter, Zhixiong GuoAbstract:The conditions for which conversation of scattered energy and Phase-Function asymmetry factor after discrete-ordinates methods (DOM) directional discretization for 3-D radiative transfer in anisotropic scattering media breaks down are examined. Directional discretization in anisotropic scattering media is found to alter the scattering asymmetry factor—a second-type of “false scattering.” Phase-Function normalization which conserves scattered energy alone cannot correct this problem, and conservation of the asymmetry factor is simultaneously required. A normalization technique developed by the authors, which was successfully tested in 2-D asymmetric cylindrical-coordinate radiative transfer analysis, is intensively examined and validated with benchmark problems in 3-D Cartesian coordinates. In Part I of this study, the degree of anisotropy for which normalization is necessary to conserve these inherent quantities is presented for various Phase-Function approximations and discrete quadrature sets.
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Phase-Function normalization for accurate analysis of ultrafast collimated radiative transfer
Applied optics, 2012Co-Authors: Brian Hunter, Zhixiong GuoAbstract:The scattering of radiation from collimated irradiation is accurately treated via normalization of Phase Function. This approach is applicable to any numerical method with directional discretization. In this study it is applied to the transient discrete-ordinates method for ultrafast collimated radiative transfer analysis in turbid media. A technique recently developed by the authors, which conserves a Phase-Function asymmetry factor as well as scattered energy for the Henyey–Greenstein Phase Function in steady-state diffuse radiative transfer analysis, is applied to the general Legendre scattering Phase Function in ultrafast collimated radiative transfer. Heat flux profiles in a model tissue cylinder are generated for various Phase Functions and compared to those generated when normalization of the collimated Phase Function is neglected. Energy deposition in the medium is also investigated. Lack of conservation of scattered energy and the asymmetry factor for the collimated scattering Phase Function causes overpredictions in both heat flux and energy deposition for highly anisotropic scattering media. In addition, a discussion is presented to clarify the time-dependent formulation of divergence of radiative heat flux.
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Phase-Function Normalization in the 3-D Discrete-Ordinates Solution of Radiative Transfer—PART I: Conservation of Scattered Energy and Asymmetry Factor
Numerical Heat Transfer Part B: Fundamentals, 2012Co-Authors: Brian Hunter, Zhixiong GuoAbstract:The conditions for which conversation of scattered energy and Phase-Function asymmetry factor after discrete-ordinates methods (DOM) directional discretization for 3-D radiative transfer in anisotropic scattering media breaks down are examined. Directional discretization in anisotropic scattering media is found to alter the scattering asymmetry factor—a second-type of “false scattering.” Phase-Function normalization which conserves scattered energy alone cannot correct this problem, and conservation of the asymmetry factor is simultaneously required. A normalization technique developed by the authors, which was successfully tested in 2-D asymmetric cylindrical-coordinate radiative transfer analysis, is intensively examined and validated with benchmark problems in 3-D Cartesian coordinates. In Part I of this study, the degree of anisotropy for which normalization is necessary to conserve these inherent quantities is presented for various Phase-Function approximations and discrete quadrature sets.
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Conservation of asymmetry factor in Phase Function discretization for radiative transfer analysis in anisotropic scattering media
International Journal of Heat and Mass Transfer, 2011Co-Authors: Brian Hunter, Zhixiong GuoAbstract:A new Phase Function normalization technique is developed for use with anisotropic scattering media and is applied to the conventional discrete-ordinates method. The new approach is shown to ensure conservation of both scattered energy and Phase Function asymmetry factor after directional discretization when considering the Henyey–Greenstein Phase Function approximation. Results show the necessity of conservation of the asymmetry factor as well as of the scattered energy. Lack of either conservation can lead to false results for radiation analysis in highly anisotropic media. Wall flux profiles predicted by the normalized DOM in a highly anisotropic scattering cylinder are compared with FVM and isotropic scaling profiles. The effect of scattering albedo and optical thickness is examined. For the tested benchmark problem, it is found that heat flux profiles generated with the present normalization approach conform more accurately to both FVM and isotropic scaling profiles than when the previous normalization techniques are implemented.
Ivano Bertini - One of the best experts on this subject based on the ideXlab platform.
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Models of Rosetta/OSIRIS 67P dust coma Phase Function
Astronomical Journal, 2018Co-Authors: Fernando Moreno, Daniel Guirado, Olga Muñoz, Ivano Bertini, Cécilia Tubiana, Carsten Güttler, Marco Fulle, Alessandra Rotundi, Vincenzo Della Corte, Stavro IvanovskiAbstract:The Phase Function of the dust coma of comet 67P has been determined from Rosetta/OSIRIS images. This Function shows a deep minimum at Phase angles near 100°, and a strong backscattering enhancement. These two properties cannot be reproduced by regular models of cometary dust, most of them based on wavelength-sized and randomly oriented aggregate particles. We show, however, that an ensemble of oriented elongated particles of a wide variety of aspect ratios, with radii r gsim 10 μm, and whose long axes are perpendicular to the direction of the solar radiation, are capable of reproducing the observed Phase Function. These particles must be absorbing, with an imaginary part of the refractive index of about 0.1 to match the expected geometric albedo, and with porosity in the 60%–70% range.
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The Phase Function and density of the dust observed at comet 67P/Churyumov–Gerasimenko
Monthly Notices of the Royal Astronomical Society, 2018Co-Authors: Marco Fulle, Anny Chantal Levasseur-regourd, Ivano Bertini, Carsten Güttler, Vincenzo Della Corte, Florangela La Forgia, Stavro L. Ivanovski, Jérémie Lasue, Francesco Marzari, Fernando MorenoAbstract:The OSIRIS camera onboard Rosetta measured the Phase Function of both the coma dust and the nucleus. The two Functions have a very different slope versus the Phase angle. Here, we show that the nucleus Phase Function should be adopted to convert the brightness to the size of dust particles larger than 2.5 mm only. This makes the dust bursts observed close to Rosetta by OSIRIS, occurring about every hour, consistent with the fragmentation on impact with Rosetta of parent particles, whose flux agrees with the dust flux observed by GIADA. OSIRIS also measured the antisunward acceleration of the fragments, thus providing the first direct measurement of the solar radiation force acting on the dust fragments and thus of their bulk density, excluding any measurable rocket effect by the ice sublimation from the dust. The obtained particle density distribution has a peak matching the bulk density of most COSIMA particles, and represents a subset of the density distribution measured by GIADA. This implies a bias in the elemental abundances measured by COSIMA, which thus are consistent with the 67P dust mass fractions inferred by GIADA, i.e. (38 ± 8) per cent of hydrocarbons versus the (62 ± 8) per cent of sulphides and silicates.
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The scattering Phase Function of comet 67P/Churyumov-Gerasimenko coma as seen from the Rosetta/OSIRIS instrument
Monthly Notices of the Royal Astronomical Society, 2017Co-Authors: Ivano Bertini, Fernando Moreno, Cécilia Tubiana, Carsten Güttler, Marco Fulle, Florangela La Forgia, Elisa Frattin, Gabor Kovacs, Maurizio Pajola, Holger SierksAbstract:The study of dust, the most abundant material in cometary nuclei, is pivotal in understanding the original materials forming the solar system. Measuring the coma Phase Function provides a tool to investigate the nature of cometary dust. Rosetta/OSIRIS sampled the coma Phase Function of comet 67P/Churyumov-Gerasimenko, covering a large Phase angle range in a small amount of time. Twelve series were acquired in March 2015 - February 2016 for this scientific purpose. This data allowed, after straylight removal, measuring the Phase Function shape, its reddening, and Phase reddening while varying heliocentric and nucleocentric distances. Despite small dissimilarities within different series, we found a constant overall shape. The reflectance has a u-shape with minimum at intermediate Phase angles, reaching similar values at the smallest and largest Phase angle sampled. The comparison with cometary Phase Functions in literature indicates OSIRIS curves being consistent with the ones found in many other single comets. The dust has a negligible Phase reddening at α < 90°, indicating a coma dominated by single scattering. We measured a reddening of [11-14]%/100 nm between 376 nm and 744 nm. No trend with heliocentric or nucleocentric distance was found, indicating the coma doesn’t change its spectrum with time. These results are consistent with single coma grains and close-nucleus coma photometric results. Comparison with nucleus photometry indicates a different backscattering Phase Function shape and similar reddening values only at α < 30°. At larger Phase angles the nucleus becomes significantly redder than the coma.
Marco Fulle - One of the best experts on this subject based on the ideXlab platform.
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Models of Rosetta/OSIRIS 67P dust coma Phase Function
Astronomical Journal, 2018Co-Authors: Fernando Moreno, Daniel Guirado, Olga Muñoz, Ivano Bertini, Cécilia Tubiana, Carsten Güttler, Marco Fulle, Alessandra Rotundi, Vincenzo Della Corte, Stavro IvanovskiAbstract:The Phase Function of the dust coma of comet 67P has been determined from Rosetta/OSIRIS images. This Function shows a deep minimum at Phase angles near 100°, and a strong backscattering enhancement. These two properties cannot be reproduced by regular models of cometary dust, most of them based on wavelength-sized and randomly oriented aggregate particles. We show, however, that an ensemble of oriented elongated particles of a wide variety of aspect ratios, with radii r gsim 10 μm, and whose long axes are perpendicular to the direction of the solar radiation, are capable of reproducing the observed Phase Function. These particles must be absorbing, with an imaginary part of the refractive index of about 0.1 to match the expected geometric albedo, and with porosity in the 60%–70% range.
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The Phase Function and density of the dust observed at comet 67P/Churyumov–Gerasimenko
Monthly Notices of the Royal Astronomical Society, 2018Co-Authors: Marco Fulle, Anny Chantal Levasseur-regourd, Ivano Bertini, Carsten Güttler, Vincenzo Della Corte, Florangela La Forgia, Stavro L. Ivanovski, Jérémie Lasue, Francesco Marzari, Fernando MorenoAbstract:The OSIRIS camera onboard Rosetta measured the Phase Function of both the coma dust and the nucleus. The two Functions have a very different slope versus the Phase angle. Here, we show that the nucleus Phase Function should be adopted to convert the brightness to the size of dust particles larger than 2.5 mm only. This makes the dust bursts observed close to Rosetta by OSIRIS, occurring about every hour, consistent with the fragmentation on impact with Rosetta of parent particles, whose flux agrees with the dust flux observed by GIADA. OSIRIS also measured the antisunward acceleration of the fragments, thus providing the first direct measurement of the solar radiation force acting on the dust fragments and thus of their bulk density, excluding any measurable rocket effect by the ice sublimation from the dust. The obtained particle density distribution has a peak matching the bulk density of most COSIMA particles, and represents a subset of the density distribution measured by GIADA. This implies a bias in the elemental abundances measured by COSIMA, which thus are consistent with the 67P dust mass fractions inferred by GIADA, i.e. (38 ± 8) per cent of hydrocarbons versus the (62 ± 8) per cent of sulphides and silicates.
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The scattering Phase Function of comet 67P/Churyumov-Gerasimenko coma as seen from the Rosetta/OSIRIS instrument
Monthly Notices of the Royal Astronomical Society, 2017Co-Authors: Ivano Bertini, Fernando Moreno, Cécilia Tubiana, Carsten Güttler, Marco Fulle, Florangela La Forgia, Elisa Frattin, Gabor Kovacs, Maurizio Pajola, Holger SierksAbstract:The study of dust, the most abundant material in cometary nuclei, is pivotal in understanding the original materials forming the solar system. Measuring the coma Phase Function provides a tool to investigate the nature of cometary dust. Rosetta/OSIRIS sampled the coma Phase Function of comet 67P/Churyumov-Gerasimenko, covering a large Phase angle range in a small amount of time. Twelve series were acquired in March 2015 - February 2016 for this scientific purpose. This data allowed, after straylight removal, measuring the Phase Function shape, its reddening, and Phase reddening while varying heliocentric and nucleocentric distances. Despite small dissimilarities within different series, we found a constant overall shape. The reflectance has a u-shape with minimum at intermediate Phase angles, reaching similar values at the smallest and largest Phase angle sampled. The comparison with cometary Phase Functions in literature indicates OSIRIS curves being consistent with the ones found in many other single comets. The dust has a negligible Phase reddening at α < 90°, indicating a coma dominated by single scattering. We measured a reddening of [11-14]%/100 nm between 376 nm and 744 nm. No trend with heliocentric or nucleocentric distance was found, indicating the coma doesn’t change its spectrum with time. These results are consistent with single coma grains and close-nucleus coma photometric results. Comparison with nucleus photometry indicates a different backscattering Phase Function shape and similar reddening values only at α < 30°. At larger Phase angles the nucleus becomes significantly redder than the coma.
Annette Kienle - One of the best experts on this subject based on the ideXlab platform.
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Goniometer for determination of the spectrally resolved scattering Phase Function of suspended particles
The Review of scientific instruments, 2019Co-Authors: S. Nothelfer, Florian Foschum, Annette KienleAbstract:An apparatus for direct determination of the spectral resolved scattering Phase Function of suspensions and emulsions is described. The system consists of a polychromatic xenon lamp as a light source and a spectrometer as a detector. Together with a stepper motor, the system enables spectrally and angularly resolved measurements in the range of 450 nm–950 nm and 10°–170°, respectively. A model for light propagation inside the cuvette in the regime of single scattering, which also takes the spectral dimension into account, was developed. A postprocessing algorithm applying the model and an extrapolation to the measured angular data allows a direct determination of the complete scattering Phase Function. By comparing measurements on polystyrene microspheres with Mie theory, the concept of the presented instrument was validated. Finally, the method was used to determine the scattering Phase Function of different types and brands of soybean oil emulsions such as Intralipid or Lipovenos. The measured scattering Phase Functions were then used to calculate the corresponding Legendre moments to an order of 20 and for wavelengths between 450 nm and 750 nm (available online), which besides the anisotropy factor also allow the determination of higher order factors such as gamma.An apparatus for direct determination of the spectral resolved scattering Phase Function of suspensions and emulsions is described. The system consists of a polychromatic xenon lamp as a light source and a spectrometer as a detector. Together with a stepper motor, the system enables spectrally and angularly resolved measurements in the range of 450 nm–950 nm and 10°–170°, respectively. A model for light propagation inside the cuvette in the regime of single scattering, which also takes the spectral dimension into account, was developed. A postprocessing algorithm applying the model and an extrapolation to the measured angular data allows a direct determination of the complete scattering Phase Function. By comparing measurements on polystyrene microspheres with Mie theory, the concept of the presented instrument was validated. Finally, the method was used to determine the scattering Phase Function of different types and brands of soybean oil emulsions such as Intralipid or Lipovenos. The measured scatterin...
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Optimized goniometer for determination of the scattering Phase Function of suspended particles: simulations and measurements.
Journal of biomedical optics, 2013Co-Authors: Florian Foschum, Annette KienleAbstract:We present simulations and measurements with an optimized goniometer for determination of the scattering Phase Function of suspended particles. We applied the Monte Carlo method, using a radially layered cylindrical geometry and mismatched boundary conditions, in order to investigate the influence of reflections caused by the interfaces of the glass cuvette and the scatterer concentration on the accurate determination of the scattering Phase Function. Based on these simulations we built an apparatus which allows direct measurement of the Phase Function from ϑ=7 deg to ϑ=172 deg without any need for correction algorithms. Goniometric measurements on polystyrene and SiO2 spheres proved this concept. Using the validated goniometer, we measured the Phase Function of yeast cells, demonstrating the improvement of the new system compared to standard goniometers. Furthermore, the scattering Phase Function of different fat emulsions, like Intralipid, was determined precisely.
