Molecular cloud regulated star formation in galaxies     

C. M. Booth  Tom Theuns  Takashi Okamoto 《Monthly notices of the Royal Astronomical Society》2007,376(4):1588-1610

  相似文献   

Textures of syntaxial quartz veins synthesized by hydrothermal experiments     

Atsushi Okamoto  Kotaro Sekine 《Journal of Structural Geology》2011,33(12):1764-1775

Syntaxial quartz veins were synthesized by hydrothermal flow-through experiments using rock blocks (metachert, sandstone, and granite) containing slits. Based on analyses of vein textures using birefringence imaging microscopy, we identified two stages of crystal growth. During stage 1, quartz grain growth occurs without an increase in grain width. During stage 2, quartz grains develop facets and grow preferentially parallel to the c-axis orientation, and the aspect ratio of quartz grains shifts toward ～2.9. Competitive growth occurs significantly at stage 2, and the transition from stage 1–2 occurs at a critical distance from the vein wall, being approximately equal to the host-rock grain size. Crystal growth in the slits produce various textures controlled by the ratio of slit aperture (H) to host-rock grain size (d). In high H/d cracks, elongate–blocky texture develops by grain impingement during stage 2, whereas in low H/d cracks, crystals that bridge the crack form without competitive growth by grain impingement at stage 1. Heterogeneous structures of fracture porosity are produced during syntaxial vein formation, due to the anisotropy in the growth rate of quartz. Such “incompletely sealed” cracks may act as important fluid pathways and as weak planes in the upper crust.  相似文献   

Soil radon in winter months under snowpack in Hokkaido, Japan     

Ryoko Fujiyoshi  Masanori Okabayashi  Yousuke Sakuta  Kazumasa Okamoto  Takashi Sumiyoshi  Ivan Kobal  Janja Vaupotič 《Environmental Earth Sciences》2013,70(3):1159-1167

Soil radon (²²²Rn) has been monitored during winter months under cool-temperate deciduous stands of different surface geology in Tomakomai and in Sapporo, Hokkaido, Japan. Radon level was lower in Tomakomai of immature soil of porous volcanic ash emitted from an active volcano (Mt. Tarumae), compared with those in Sapporo of alluvial sediments. In Tomakomai, mean value of the ²²²Rn activity concentration was higher in winter (570 Bq m^?3) than in summer (350 Bq m^?3) at a depth of 1 m, which is consistent with the results in cold and dry winter reported in the literature. In contrast, soil radon decreasing with decreasing soil temperature from mid-September (5.0 kBq m^?3) remained low (2.6 kBq m^?3) under persistent snow in Sapporo, which had already been observed in the same location. Measurements of the activity concentrations of ²²²Rn in snow and in snow air as well as in soil air indicate that the small amount of ²²²Rn is released from the ground surface to the overlying snowpack with a ²²²Rn flux density of 0.4 mBq m^?2 s^?1 under thick snow cover in Sapporo.  相似文献   

Relativistic magnetohydrodynamic winds of finite temperature     

C. F. Kennel  F. S. Fujimura  I. Okamoto 《地球物理与天体物理流体动力学》2013,107(3-4):147-222

Abstract

The singular differential equations for finite temperature relativistic magnetohydrodynamic (MHD) winds integrate to two algebraic equations when the source magnetic field is a monopole. This simplification enables an extensive characterization of the asymptotic wind solutions in terms of source parameters. We will consider only the critical solutions-those that pass smoothly through both an intermediate (Alfvenic) and a fast MHD critical point and expand to zero pressure at infinite radial distance from the source. Because the constants of motion must be specified to extremely high accuracy, the critical solutions cannot be found analytically. Synopsis of many numerical solutions reveals a uniform parametric characterization of the asymptotic wind in terms of one combination of source parameters, Z, the mean source particle energy divided by mc²[sgrave]^½, where [sgrave] is a generalization of Michel's (1969) cold relativistic wind strength parameter. Cool winds, with Z<1, behave asymptotically much as Michel's cold wind minimum torque solution; Z1 hot winds have quite different, but simply characterized, asymptotic solutions. Thus, the strength of magnetized relativistic outflows can depend critically upon the temperature of the source.  相似文献   

Inundation flow velocity of tsunami on land     

Hideo Matsutomi  Kensuke Okamoto 《Island Arc》2010,19(3):443-457

An estimation of tsunami inundation flow velocity is one of the most challenging issues among tsunami research. Based on field data of inundation depth and inundation flow velocity u estimated using Bernoulli's theorem and inundation depth, fundamental characteristics of the relationship between inundation flow velocity and inundation depth are examined. Fundamental characteristics of the velocity coefficient where g is gravitational acceleration, h_f and h_r are inundation depths at the front and the back of structures such as a rectangular building with vertical walls, respectively) implicitly included in the relationship are examined through hydraulic experiments. As a result, C_v = 0.6 is recommended as its simple and practical value. It is confirmed through these examinations that the Froude number, defined by where , ranges 0.7–2.0, and when C_v = 0.6 is adopted this Froude number ranges 0.42–1.2. By using the relationship and C_v = 0.6, two simple and practical relationships are presented for two cases where inundation flow velocity exerts the largest or the smallest fluid force on structures. These relationships can be used to roughly grasp the practical side of tsunami damage, and estimate fluid force acting on individual structures, moving velocity and collision force of floating objects and sediment transport such as boulder and sand. Fundamental characteristics of the waterline (tsunami trace) distribution around/on the typical object model (square pillar, corn and column) are also examined through steady flow experiments, and it is confirmed that the maximum and the minimum values of h_f/h₀ in the full type model of the square pillar are almost the same as those of h_f/h_r obtained by field surveys where h₀ is uniform flow depth. It is also confirmed that h_r ? h₀ when the Froude number, defined by where u₀ is uniform flow velocity, is much less than 1.0. Using a newly defined velocity coefficient, tsunami inundation flow velocity on land can be estimated practically and would be useful for checking proposed sediment transport models that are now being developed by tsunami geologists.  相似文献   

Annual and Diurnal Variation of Meteor Rates by the Forward-Scatter Radio Observation     

Sadao Okamoto  Kimio Maegawa 《Earth, Moon, and Planets》2008,103(1-2):65-68

Forward-scatter radio meteor observations have been made at Japan since 1996 using inexpensive and low-end equipment. The activity of some major meteor showers and the seasonal variability of sporadic meteors in 2006 are presented.  相似文献   

Fast and slow timescales in the tropical low-cloud response to increasing CO2 in two climate models     

Masahiro Watanabe  Hideo Shiogama  Masakazu Yoshimori  Tomoo Ogura  Tokuta Yokohata  Hajime Okamoto  Seita Emori  Masahide Kimoto 《Climate Dynamics》2012,39(7-8):1627-1641

To obtain physical insights into the response and feedback of low clouds (C _l ) to global warming, ensemble 4?×?CO₂ experiments were carried out with two climate models, the Model for Interdisciplinary Research on Climate (MIROC) versions 3.2 and 5. For quadrupling CO₂, tropical-mean C _l decreases, and hence, acts as positive feedback in MIROC3, whereas it increases and serves as negative feedback in MIROC5. Three time scales of tropical-mean C _l change were identified—an initial adjustment without change in the global-mean surface air temperature, a slow response emerging after 10–20?years, and a fast response in between. The two models share common features for the former two changes in which C _l decreases. The slow response reflects the variability of C _l associated with the El Ni?o-Southern Oscillation in the control integration, and may therefore be constrained by observations. However, the fast response is opposite in the two models and dominates the total response of C _l . Its sign is determined by a subtle residual of the C _l increase and decrease over the ascending and subsidence regions, respectively. The regional C _l increase is consistent with a more frequent occurrence of a stable condition, and vice versa, as measured by lower-tropospheric stability (LTS). The above frequency change in LTS is similarly found in six other climate models despite a large difference in both the mean and the changes in the low-cloud fraction for a given LTS. This suggests that the response of the thermodynamic constraint for C _l to increasing CO₂ concentrations is a robust part of the climate change.  相似文献   

Influence of the greenhouse effect on yields of wheat,soybean and corn in the United States for different energy scenarios     

K. Okamoto  T. Ogiwara  T. Yoshizumi  Y. Watanabe 《Climatic change》1991,18(4):397-424

The increase of atmospheric carbon dioxide has positive effects on agricultural productivity (photosynthesis stimulation), but in some regions it has negative effects (drought due to the temperature rise) as well. The central part of the United States in summer is predicted to be one of such regions, where the influence of the CO₂ increase should be assessed considering both the effects. Such calculations have been made for spring wheat, soybean and corn in a series of papers, a summary of which is presented here. Since the CO₂ emission rate depends on fossil fuel consumption, energy scenarios with different fossil fuel consumption are assumed. Positive effects of CO₂ are expressed by a model which simulates actual data. In the absence of an appropriate model negative effects are assumed to be proportional to the temperature rise, which is shown to be unexpectedly good. The difference between C3 (soybean and wheat) and C4 (corn) plants is also considered. Changes of their yields in the next century are calculated. Results show that in this region (probably up to 42–45° N) in summer an unlimited increase of atmospheric CO₂ is not desirable for the above three crops even if positive effects of CO₂ are taken into account. This work is not intended to give prediction of future crop production, but to show illustrative examples for the above argument. Thus assumptions are made so as to overestimate positive effects and underestimate negative effects, but results show that even in such cases an unlimited increase of CO₂ is not necessarily desirable for the specified regions.All inquiries about this paper should be made to K. Okamoto.  相似文献   

Radiation Pressure and the Poynting-Robertson Effect for Fluffy Dust Particles     

Hiroshi KimuraHajime Okamoto  Tadashi Mukai 《Icarus》2002,157(2):349-361

A correct understanding of the dynamical effect of solar radiation exerted on fluffy dust particles can be achieved with assistance of a light scattering theory as well as the equation of motion. We reformulate the equation of motion so that the radiation pressure and the Poynting-Robertson effect on fluffy grains are given in both radial and nonradial directions from the center of the Sun. This allows numerical estimates of these radiation forces on fluffy dust aggregates in the framework of the discrete dipole approximation, in which the first term of the scattering coefficients in Mie theory determines the polarizability of homogeneous spheres forming the aggregates.The nonsphericity in shape turns out to play a key role in the dynamical evolution of dust particles, while its consequence depends on the rotation rate and axis of the grains. Unless a fluffy dust particle rapidly revolves on its randomly oriented axis, the nonradial radiation forces may prevent, apart from the orbital eccentricity and semimajor axis, the orbital inclination of the particle from being preserved in orbit around the Sun. However, a change in the inclination is most probably controlled by the Lorentz force as a consequence of the interaction between electric charges on the grains and the solar magnetic field. Although rapidly and randomly rotating grains spiral into the Sun under the Poynting-Robertson effect in spite of their shapes and structures, fluffy grains drift inward on time scales longer at submicrometer sizes and shorter at much larger sizes than spherical grains of the same sizes. Numerical calculations reveal that the dynamical lifetimes of fluffy particles are determined by the material composition of the grains rather than by their morphological structures and sizes. The Poynting-Robertson effect alone is nevertheless insufficient for giving a satisfactory estimate of lifetimes for fluffy dust grains since their large ratios of cross section to mass would reduce the lifetimes by enhancing the collisional probabilities. We also show that the radiation pressure on a dust particle varies with the orbital velocity of the particle but that this effect is negligibly small for dust grains in the Solar System.  相似文献   

Southward extrusion of eclogite-bearing mafic–ultramafic bodies in the Sanbagawa belt, central Shikoku, Japan     

Hiroshi Yamamoto  Kazuaki Okamoto  Yoshiyuki Kaneko  Masaru Terabayashi 《Tectonophysics》2004,387(1-4):151-168

Several mafic rock masses, which have experienced eclogite facies metamorphism, are distributed in flat-lying non-eclogitic schists in an intermediate structural level (thermal core) of the Sanbagawa belt. The largest, Iratsu mass, and an associated peridotite, the Higashi-Akaishi mass, extend E–W for about 8 km, and N–S for about 3 km, and are surrounded by pelitic, basic and quartz schists. The Iratsu mass consists of metabasites of gabbroic and basaltic origin, with intercalations of ultramafic rocks, felsic gneiss, quartz schist and metacarbonate. The Iratsu mass can be divided into two layers along a WNW-trending metacarbonate layer. The Higashi-Akaishi mass consists of peridotite with intercalations of garnet clinopyroxenite. It is situated beneath the western half of the Iratsu mass, and their mutual boundary dips gently or steeply to the N or NE. These masses underwent eclogite, and subsequent epidote-amphibolite facies metamorphism as has been reported elsewhere. The Iratsu–Higashi-Akaishi masses and the surrounding rocks underwent ductile deformation under epidote-amphibolite facies (or lower P–T) metamorphic conditions. Their foliation generally trends WNW and dips moderately to the NNE, and the mineral lineation mostly plunges to the N and NE. In non-eclogitic schists surrounding the Iratsu–Higashi-Akaishi masses, the foliation generally trends WNW and dips gently or steeply to the N or S and the mineral lineation mostly plunges to the NW, N and NE. Kinematic analysis of deformation structures in outcrops and oriented samples has been performed to determine shear senses. Consistent top-to-the-north, normal fault displacements are observed in peridotite layers of the Higashi-Akaishi mass and eclogite-bearing epidote amphibolite layers of the Iratsu mass. Top-to-the-northeast or top-to-the-northwest displacements also occur in non-eclogitic pelitic–quartz schists on the northern side of the Iratsu mass. In the structural bottom of the Iratsu–Higashi-Akaishi masses and to the south, reverse fault (top-to-the-south) movements are recognized in serpentinized peridotite and non-eclogitic schists. These observations provide the following constraints on the kinematics of the rock masses: (1) northward normal displacement of Iratsu relative to Higashi-Akaishi, (2) northward normal displacement of non-eclogitic schists on the north of the Iratsu mass and (3) southward thrusting of the Iratsu–Higashi-Akaishi masses upon non-eclogitic schists in the south. The exhumation process of the Iratsu–Higashi-Akaishi masses can be explained by their southward extrusion.  相似文献