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Kōbe-shi, Japan

Kolokolova L.,University of Maryland College Park | Kimura H.,Center for Planetary Science
Astronomy and Astrophysics | Year: 2010

Context. We study how the electromagnetic interaction between the monomers in aggregates affects the polarization of cosmic dust. Aims. We aim to show that the electromagnetic interaction depends on the porosity and composition of the aggregates and contributes significantly to the spectral gradient of polarization (polarimetric color). The results may explain the observations of some comets that demonstrated atypical negative polarimetric color in the visible and also a reverse of the positive polarimetric color to the negative one in the near-infrared. Methods. We performed computer simulations of the light scattering by aggregates consisting of spheres made of a variety of materials: transparent, absorptive, and the material similar to that of the dust in comet Halley. We studied how the number of monomers covered by the electromagnetic wave at a single period (on the light path equal to one wavelength) affects their interaction by considering linear clusters of 2 and 10 monomers of radius of 0.1 μm. Results. Electromagnetic interaction between the monomers in aggregates depolarizes the light. The interaction becomes stronger if more monomers are covered by the electromagnetic wave at a single period. Thus, the porosity of aggregates influences their polarization. The electromagnetic interaction also depends on composition and is stronger for transparent materials. Conclusions. Electromagnetic interaction between the monomers in aggregates may explain why the polarimetric color of comet dust decreases as observations move from the visible to the near-infrared since a longer wavelength covers more monomers. It may also explain why some comets exhibit negative polarimetric color even in the visible; these comets may have more compact dust. Strong electromagnetic interaction resulted either from compactness or transparency of the material can explain the negative polarimetric color of interplanetary dust and debris disks and contribute to the polarization of asteroids. In general, the spectral dependence of polarization is a promising tool for studying the properties of cosmic dust particles, particularly their porosity. © ESO, 2010.

Rymer A.M.,Johns Hopkins Applied Physics Laboratory | Mitchell D.G.,Johns Hopkins Applied Physics Laboratory | Hill T.W.,Rice University | Kronberg E.A.,Max Planck Institute for Solar System Research | And 3 more authors.
Geophysical Research Letters | Year: 2013

A 2-3 day periodicity observed in Jupiter's magnetosphere (superposed on the giant planet's 9.5 h rotation rate) has been associated with a characteristic mass-loading/unloading period at Jupiter. We follow a method derived by Kronberg et al. () and find, consistent with their results, that this period is most likely to fall between 1.5 and 3.9 days. Assuming the same process operates at Saturn, we argue, based on equivalent scales at the two planets, that its period should be 4 to 6 times faster at Saturn and therefore display a period of 8 to 18 h. Applying the method of Kronberg et al. for the mass-loading source rates estimated by Smith et al. () based on data from the third and fifth Cassini-Enceladus encounters, we estimate that the expected magnetospheric refresh rate varies from 8 to 31 h, a range that includes Saturn's rotation rate of ∼10.8 h. The magnetospheric period we describe is proportional to the total mass-loading rate in the system. The period is, therefore, faster (1) for increased outgassing from Enceladus, (2) near Saturn solstice (when the highest proportion of the rings is illuminated), and (3) near solar maximum when ionization by solar photons maximizes. We do not claim to explain the few percent jitter in period derived from Saturn Kilometric Radiation with this model, nor do we address the observed difference in period observed in the north and south hemispheres. ©2013. American Geophysical Union. All Rights Reserved.

Tanaka K.K.,Hokkaido University | Yamamoto T.,Hokkaido University | Kimura H.,Center for Planetary Science
Astrophysical Journal | Year: 2010

We construct a theoretical model for low-temperature crystallization of amorphous silicate grains induced by exothermic chemical reactions. As a first step, the model is applied to the annealing experiments, in which the samples are (1) amorphous silicate grains and (2) amorphous silicate grains covered with an amorphous carbon layer. We derive the activation energies of crystallization for amorphous silicate and amorphous carbon from the analysis of the experiments. Furthermore, we apply the model to the experiment of low-temperature crystallization of an amorphous silicate core covered with an amorphous carbon layer containing reactive molecules. We clarify the conditions of low-temperature crystallization due to exothermic chemical reactions. Next, we formulate the crystallization conditions so as to be applicable to astrophysical environments. We show that the present crystallization mechanism is characterized by two quantities: the stored energy density Q in a grain and the duration of the chemical reactions τ. The crystallization conditions are given by Q>Q min and τ < τcool regardless of details of the reactions and grain structure, where τcool is the cooling timescale of the grains heated by exothermic reactions, and Q min is minimum stored energy density determined by the activation energy of crystallization. Our results suggest that silicate crystallization occurs in wider astrophysical conditions than hitherto considered. © 2010. The American Astronomical Society. All rights reserved..

Kimura H.,Kobe University | Kimura H.,Center for Planetary Science
Monthly Notices of the Royal Astronomical Society | Year: 2016

Photoelectron emission is crucial to electric charging of dust particles around main-sequence stars and gas heating in various dusty environments. An estimate of the photoelectric processes contains an ill-defined parameter called the photoelectric quantum yield, which is the total number of electrons ejected from a dust particle per absorbed photon. Here we revisit the so-called small particle effect of photoelectron emission and provide an analytical model to estimate photoelectric quantum yields of small dust particles in sizes down to nanometers. We show that the small particle effect elevates the photoelectric quantum yields of nanoparticles up to by a factor of 103 for carbon, water ice, and organics, and a factor of 102 for silicate, silicon carbide, and iron. We conclude the surface curvature of the particles is a quantity of great importance to the small particle effect, unless the particles are submicrometers in radius or larger. © 2016 The Author Published by Oxford University Press on behalf of the Royal Astronomical Society.

Suzuki A.I.,Center for Planetary Science | Nakamura A.M.,Kobe University | Kadono T.,University of Occupational and Environmental Health Japan | Wada K.,Chiba Institute of Technology | And 2 more authors.
Icarus | Year: 2013

Ejecta patterns are experimentally examined around craters formed in a layer of glass beads by vertical impacts at low velocities. The diameters of the constituent glass beads of three different targets range 53-63μm, 90-106μm, and 355-500μm. The impact velocities and ambient pressures range from a few to 240ms-1 and from 500Pa to the atmospheric pressure, respectively. Various ejecta patterns are observed around craters and are classified into two major classes based on whether they have concentric ridges or not. We propose a possible formation model for the ridges in which the wake created by a projectile as it passes through the atmosphere causes the crater rim to collapse: The model can explain the observation that the degree of collapse of the resultant crater rim depends on the impact velocity and ambient pressure. Using the ratio between the hydrodynamic drag of the airflow induced by the wake and the gravitational force of the degraded part of the rim, we calculate the critical conditions of the impact velocity and ambient pressure necessary for the wake to erode the rim. The conditions turn out to be roughly consistent with the boundary between the two morphological classes. As a result, it is possible that the projectile wake triggers the collapse of the crater rim, leading to a ground-hugging flow that settles to form the distal ridge observed in this study. This mechanism may play a role in producing ejecta morphologies on planetary bodies with atmosphere. © 2013 Elsevier Inc.

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