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Okuzumi S.,Nagoya University | Hirose S.,Japan Agency for Marin Earth Science and Technology
Astrophysical Journal | Year: 2011

Turbulence driven by magnetorotational instability (MRI) crucially affects the evolution of solid bodies in protoplanetary disks. On the other hand, small dust particles stabilize MRI by capturing ionized gas particles needed for the coupling of the gas and magnetic fields. To provide an empirical basis for modeling the coevolution of dust and MRI, we perform three-dimensional, ohmic-resistive MHD simulations of a vertically stratified shearing box with an MRI-inactive "dead zone" of various sizes and with a net vertical magnetic flux of various strengths. We find that the vertical structure of turbulence is well characterized by the vertical magnetic flux and three critical heights derived from the linear analysis of MRI in a stratified disk. In particular, the turbulent structure depends on the resistivity profile only through the critical heights and is insensitive to the details of the resistivity profile. We discover scaling relations between the amplitudes of various turbulent quantities (velocity dispersion, density fluctuation, vertical diffusion coefficient, and outflow mass flux) and vertically integrated accretion stresses. We also obtain empirical formulae for the integrated accretion stresses as a function of the vertical magnetic flux and the critical heights. These empirical relations allow us to predict the vertical turbulent structure of a protoplanetary disk for a given strength of the magnetic flux and a given resistivity profile. © 2011. The American Astronomical Society. All rights reserved. Source

Okuzumi S.,Nagoya University | Hirose S.,Japan Agency for Marin Earth Science and Technology
Astrophysical Journal Letters | Year: 2012

Turbulence driven by magnetorotational instability (MRI) affects planetesimal formation by inducing diffusion and collisional fragmentation of dust particles. We examine conditions preferred for planetesimal formation in MRI-inactive "dead zones" using an analytic dead-zone model based on our recent resistive MHD simulations. We argue that successful planetesimal formation requires not only a sufficiently large dead zone (which can be produced by tiny dust grains) but also a sufficiently small net vertical magnetic flux (NVF). Although often ignored, the latter condition is indeed important since the NVF strength determines the saturation level of turbulence in MRI-active layers. We show that direct collisional formation of icy planetesimal across the fragmentation barrier is possible when the NVF strength is lower than 10 mG (for the minimum-mass solar nebula model). Formation of rocky planetesimals via the secular gravitational instability is also possible within a similar range of the NVF strength. Our results indicate that the fate of planet formation largely depends on how the NVF is radially transported in the initial disk formation and subsequent disk accretion processes. © 2012. The American Astronomical Society. All rights reserved.. Source

Blaes O.,University of California at Santa Barbara | Krolik J.H.,Johns Hopkins University | Hirose S.,Japan Agency for Marin Earth Science and Technology | Shabaltas N.,University of California at Santa Barbara | Shabaltas N.,Cornell University
Astrophysical Journal | Year: 2011

Standard models of radiation-supported accretion disks generally assume that diffusive radiation flux is solely responsible for vertical heat transport. This requires that heat must be generated at a critical rate per unit volume if the disk is to be in hydrostatic and thermal equilibrium. This raises the question of how heat is generated and how energy is transported in MHD turbulence. By analysis of a number of radiation/MHD stratified shearing-box simulations, we show that the divergence of the diffusive radiation flux is indeed capped at the critical rate, but deep inside the disk, substantial vertical energy flux is also carried by advection of radiation. Work done by radiation pressure is a significant part of the energy budget, and much of this work is dissipated later through damping by radiative diffusion. We show how this damping can be measured in the simulations and identify its physical origins. Radiative damping accounts for as much as tens of percent of the total dissipation and is the only realistic physical mechanism for dissipation of turbulence that can actually be resolved in numerical simulations of accretion disks. Buoyancy associated with dynamo-driven, highly magnetized, nearly isobaric nonlinear slow magnetosonic fluctuations is responsible for the radiation advection flux and also explains the persistent periodic magnetic upwelling seen at all values of the radiation to gas pressure ratio. The intimate connection between radiation advection and magnetic buoyancy is the first example we know of in astrophysics in which a dynamo has direct impact on the global energetics of a system. © 2011. The American Astronomical Society. All rights reserved. Source

Noda H.,California Institute of Technology | Noda H.,Japan Agency for Marin Earth Science and Technology | Shimamoto T.,China Earthquake Administration
Journal of Structural Geology | Year: 2012

Generation of large earthquakes involves with behaviors of whole plate boundaries or faults from brittle to ductile regimes. This paper reports stability analyses of halite shear zones using a recently developed rate-and-state friction to flow law with an emphasis on the behaviors across the brittle-ductile transition. The law smoothly connects the friction law with pressure-insensitive flow law without any additional constitutive parameter. Behavior upon a velocity step is characterized by an instantaneous change in shear resistance followed by transient behavior toward a steady-state. These transient behaviors are in opposite directions between friction and flow regimes, resulting in variable transient behaviors across the brittle-ductile transition. Linear stability analyses of a spring-slider system around steady-state solutions predict pressure and temperature conditions for unstable fault motion that are consistent with experimental results. The condition for potential instability is not equal to, but includes that for rate-weakening. A nonlinear analysis at the stable-unstable boundary has revealed that a sub-critical Hopf bifurcation takes place and thus a permanently sustained oscillation around a destabilized steady-state solution does not exist although experimental results suggest it. This issue deserves further study including the investigation of the friction law and construction of a physical model for brittle-ductile transition. © 2011 Elsevier Ltd. Source

Yoshikawa S.,University of Tokyo | Okino K.,University of Tokyo | Asada M.,Japan Agency for Marin Earth Science and Technology
Marine Geology | Year: 2012

This study presents the first detailed geomorphological characterization of field-scale geological features associated with hydrothermal systems in the southern Mariana Trough, using near-bottom swath mapping data collected by the autonomous underwater vehicle (AUV) Urashima during cruise YK09-08 and dive observation data acquired by the submersible Shinkai6500 during cruise YK10-11. The motivation of this study is to examine the relationship between geomorphological characteristics and hydrothermal activity, and to examine the nature of tectonic and volcanic controls on the hydrothermal system in this area. Two of the hydrothermal sites in the study area (near 12°57'N, 143°37'E) are located on the active backarc spreading axis (the Snail and Yamanaka sites), one is located at the eastern foot of the axial high (the Archean site), and two are located on an off-axis knoll about 5. km from the spreading axis (the Pika and Urashima sites). The on-axis area is divided into tectonically dominant and volcanically dominant zones; volcanically dominant zones are characterized by mounds (height, 5-30 m; diameter, 250-320 m) cut by fissures. The Snail and Yamanaka sites are located adjacent to these fissures, and are possibly represented local activity associated with a 4th order segment-scale diking event (on the basis of comparisons with previously studied cases on the East Pacific Rise with similar on-axis geological characteristics). In contrast to the on-axis sites, the off-axis sites show no evidence of faulting. The Archean site at the foot of the axial high is characterized by a single mound (height, 50-100; diameter, 250-300 m), pronounced off-axis lava flows, and the presence of high-amplitude rugged seafloor features; the site is located at the top of the mound. Numerous ridge lines (height, mainly 2-6 m) extend radially from the top of the mound, and several chimney-like structures (up to approximately 6 m high) occur on the top and slopes of the mound. The Pika site is located on the western peak of an off-axis knoll, and the newly discovered Urashima site is located at the northern foot of the western peak of the same knoll. The western peak is characterized by bumpy seabed textures formed by numerous smaller-scale mounds and ridge lines; however, the eastern peak has a very smooth top and slope, and shows no signs of hydrothermal activity. Numerous mounds (heights, 5-75 m; diameters, 50-350 m) are developed on the comparatively gentle slope of the knoll, in contrast to the numerous ridge lines (height, mainly 1-6 m) developed on the relatively steep slopes of the knoll. On the basis of the associated geomorphological features, the three off-axis sites (Archean, Pika, and Urashima) were identified as localities created by relatively long-term large-scale hydrothermal activity, as compared with sites in the on-axis area. The sustained activity at off-axis sites appears closely related to an off-axis upwelling magma system. The three off-axis hydrothermal sites are composed mainly of breccia assemblages that probably originated from hydrothermal activity with black smoker venting. These areas are characterized by numerous ridge lines, conical mounds, and bumpy seabed texture, whereas the on-axis sites are characterized by the absence of ridge lines, and the presence of white smoker and shimmering observed on dome-shaped pillow mounds. Hence, the distribution of ridge lines, mound morphology, and bumpy seabed texture is likely to correlate with hydrothermal activity. © 2012 Elsevier B.V. Source

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