Structure of magnetic fields generated by galactic dynamos     

Makoto Tosa  Masashi Chiba 《地球物理与天体物理流体动力学》2013,107(1-3):147-157

Abstract

We investigated global axisymmetric (m = 0) and non-axisymmetric (m = 1) modes of magnetic fields generated by the galactic dynamo including the α²-dynamo. The α²-dynamo is responsible for the field generation in the central region of galaxies where the shear of galactic rotation is weak (e.g. M51). The highest growth rate of m = 1 modes is always smaller than that of m = 0 modes; thus m = 1 modes of the standard galactic dynamo cannot explain the dominance of the bisymmetric fields in spiral galaxies. Radial extent of each m = 1 mode is too narrow to reproduce the observed bisymmetric structure extending over a disk.  相似文献   

26Al–26Mg chronology of high-temperature condensate hibonite in a fine-grained,Ca-Al-rich inclusion from reduced CV chondrite     

Noriyuki Kawasaki  Daiki Yamamoto  Sohei Wada  Changkun Park  Hwayoung Kim  Naoya Sakamoto  Hisayoshi Yurimoto 《Meteoritics & planetary science》2024,59(4):630-639

Al–Mg mineral isochron studies using secondary ion mass spectrometry (SIMS) have revealed the initial ²⁶Al/²⁷Al ratios, (²⁶Al/²⁷Al)₀, for individual Ca-Al-rich inclusions (CAIs) in meteorites. We find that the relative sensitivity factors of ²⁷Al/²⁴Mg ratio for SIMS analysis of hibonite, one of the major constituent minerals of CAIs, exhibit variations based on their chemical compositions. This underscores the critical need for using appropriate hibonite standards to obtain accurate Al−Mg data. We measured the Al−Mg mineral isochron for hibonite in a fine-grained CAI (FGI) from the Northwest Africa 8613 reduced CV chondrite by SIMS using synthesized hibonite standards with ²⁷Al/²⁴Mg of ~30, ~100, and ~400. The obtained mineral isochron of hibonite in the FGI yields (²⁶Al/²⁷Al)₀ of (4.73 ± 0.09) × 10⁻⁵, which is identical to that previously obtained from the mineral isochron of spinel and melilite in the same FGI (Kawasaki et al., 2020). The uncertainties of (²⁶Al/²⁷Al)₀ indicate that the constituent minerals in the FGI formed within ~0.02 Myr in the earliest solar system. The disequilibrium O-isotope distributions of the minerals in the FGI suggest that the O-isotope compositions of the nebular gas from which they condensed underwent a transitional change from ¹⁶O-rich to ¹⁶O-poor within ~0.02 Myr in the earliest solar system. Once formed, the FGI may have been removed from the forming region within ~0.02 Myr and transported to the accretion region of the parent body.  相似文献   

Determining the equation of state of the expanding Universe: inverse problem in cosmology     

Takashi Nakamura  Takeshi Chiba 《Monthly notices of the Royal Astronomical Society》1999,306(3):696-700

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Distribution of heterotrophic nanoflagellates and their importance as the bacterial consumer in a eutrophic coastal seawater   总被引：4，自引：0，他引：4  

Kimio Fukami  Noriyuki Murata  Yuuki Morio  Toshitaka Nishijima 《Journal of Oceanography》1996,52(4):399-407

Seasonal and vertical changes in abundances of bacteria and heterotrophic nanoflagellates (HNF), and HNF grazing on bacteria were investigated in a small eutrophic inlet of Uranouchi-Wan throughout the years. Bacterial densities in the surface water ranged from 1.2 to 11 (average 4.3)×10⁶ cells ml^–1 with a couple of maxima following the algal blooming. Densities of HNF ranged from 0.54 to 73 (average 16.4)×10³ cells ml^–1 in the surface, and showed almost similar fluctuation pattern to that of bacteria with a time lag of about 1 to 2 weeks. Grazing rates of HNF on bacteria obtained by FLB method were 4.78 to 16.9 (average 10.3±SD 4.8) cells HNF^–1h^–1 in the surface layer in summer, and consequent total bacterial consumption rates by HNF fluctuated from 4 to 99×10⁴ cells ml^–1h^–1. In deeper layers, however, as HNF densities and grazing rates on bacteria were low, the grazing pressure of HNF on bacteria was small. Turnover times of bacteria by HNF grazing in the surface layer were calculated as relatively constant values of 40 to 60 h, however, it decreased to as low as 6 to 7 h when the HNF activity was highest. These results indicate that bacteria grew so actively by consuming organic matter in seawater as to compensate high HNF grazing pressure, and that bacteria and HNF in the microbial loop play important roles on the turnover of substrates in coastal ecosystems.  相似文献   

Decadal-Scale Climate and Ecosystem Interactions in the North Pacific Ocean   总被引：7，自引：0，他引：7  

Arthur J. Miller  Fei Chai  Sanae Chiba  John R. Moisan  Douglas J. Neilson 《Journal of Oceanography》2004,60(1):163-188

Decadal-scale climate variations in the Pacific Ocean wield a strong influence on the oceanic ecosystem. Two dominant patterns of large-scale SST variability and one dominant pattern of large-scale thermocline variability can be explained as a forced oceanic response to large-scale changes in the Aleutian Low. The physical mechanisms that generate this decadal variability are still unclear, but stochastic atmospheric forcing of the ocean combined with atmospheric teleconnections from the tropics to the midlatitudes and some weak ocean-atmosphere feedbacks processes are the most plausible explanation. These observed physical variations organize the oceanic ecosystem response through large-scale basin-wide forcings that exert distinct local influences through many different processes. The regional ecosystem impacts of these local processes are discussed for the Tropical Pacific, the Central North Pacific, the Kuroshio-Oyashio Extension, the Bering Sea, the Gulf of Alaska, and the California Current System regions in the context of the observed decadal climate variability. The physical ocean-atmosphere system and the oceanic ecosystem interact through many different processes. These include physical forcing of the ecosystem by changes in solar fluxes, ocean temperature, horizontal current advection, vertical mixing and upwelling, freshwater fluxes, and sea ice. These also include oceanic ecosystem forcing of the climate by attenuation of solar energy by phytoplankton absorption and atmospheric aerosol production by phytoplankton DMS fluxes. A more complete understanding of the complicated feedback processes controlling decadal variability, ocean ecosystems, and biogeochemical cycling requires a concerted and organized long-term observational and modeling effort. This revised version was published online in July 2006 with corrections to the Cover Date.  相似文献   

A note on the height dependence of the skewness and the kurtosis of the vertical turbulent velocity in the neutral surface layer     

Osamu Chiba 《Boundary-Layer Meteorology》1984,29(4):313-319

Several experimental results indicate that S _w (the skewness of the vertical turbulent velocity) increases with height, and K _w (the kurtosis) decreases with height in the neutral surface layer. The measured behavior of S _w and K _w with height under neutral conditions is shown to be consistent with available models for the gradient-diffusion and the pressure-velocity correlation. In addition, an estimate of the mode of the probability density distribution turns out to be helpful in interpreting the height dependence of K _w.  相似文献   

Backscatter of microwaves from the sea surface and the modulation of spectra of short gravity waves     

Noriyuki Iwata 《Journal of Oceanography》1983,39(2):43-52

Backscatter cross-sections of microwaves from the sea surface are calculated by using the facet model and are compared withJonswap '75 experimental results. The principal features obtained are: (1) asymmetry of backscatter cross-sections between upwind and downwind directions is attributable to the modulation of the short gravity-capillary wave spectrum by a larger wave, and the non-Gaussian wave slope distribution has a tendency to cancel this effect, (2) angular spreading of the energy spectrum in the higher frequency range should have a narrower band than a simple cosine distribution, (3) the facet model itself should begin to break down at a larger incident angle than previously supposed.  相似文献   

Directional energy distribution of wind waves     

Noriyuki Iwata 《Journal of Oceanography》1971,27(3):102-108

Two-dimensional ocean wave spectrum developing under the atmospheric surface pressure fluctuations is linearly correlated with that of wind pressure itself, so that angular distribution of energy of ocean surface waves can be determined by directional properties of surface pressure fluctuations with the same frequency to the surface wave.From empirically determined spectral formula of the atmospheric surface pressure fluctuations the coefficients of Fourier series expanded around mean direction of wind are analytically integrated, from which r.m.s. angular distribution, spectral peakedness and long-crestedness are calculated, compared with previously proposed empirical formulae and observations carried out by ultrasonic current meter.  相似文献   

Large-scale ripple marks on the shelf margin of the Northern Okinawa Trough     

Hideo Kagami  Shigeaki Kubo  Noriyuki Nasu 《Journal of Oceanography》1984,40(3):230-242

Trains of large-scale ripple marks (megaripples and sand waves) were found on the Amakusa and East China Sea shelves bordering the northern Okinawa Trough. Side-scan sonar surveys were carried out in 1974 and 1976 to investigate sea-floor features lying along a proposed submarine cable line. Megaripples were found on the outer margin of the Amakusa shelf between depths of 140 and 200 m. The megaripples were especially well developed at a depth of 167 m. They were typically straight-transverse crested with asymmetrical profiles, and measured 7 to 15 m in wavelength and 0.4 to 1.4 m in waveheight. Formation of the megaripples on the Amakusa shelf is probably controlled by relatively complex oceanographic conditions. A secondary circulation associated with the Gotô-nada clock-wise Current may be responsible for formation of the ripple marks. Local vorticities generated in the coastal boundary layer as a result of curvature of the Gotô-nada Current are known to cause the complex flow pattern at the Gotô and Amakusa shelf margins. The main semidiurnal (M₂) tidal current may also interact with these fluid processes.On the East China Sea shelf, megaripples and sand waves were found between depths of 140 and 220 m. Sand waves (200 m in wavelength) were observed in seismic reflection profiles. Large-scale lunate megaripples were observed at a depth of 154 m by the side-scan sonar. They had wavelengths of 10 to 30 m and waveheights of 1 to as high as 3 m. It appears from the types and nature of distribution of the megaripples that they are responding to the present-day flow regime, and it is partly ascertained from our observations over an interval of two years that the megaripples appear to be short-term response elements compared wit hteh sand waves. We conclude that the megaripples on the East China Sea shelf are current-formed during peak typhoon flow in August to November. From their distribution, the long term path of the main flow of the Tsushima Current is inferred at the edge of the East China Sea shelf. An area of low sediment mud content (less than 20 per cent) coincides with this path giving further support to our interpretation.  相似文献   

Energy-momentum tensor and conservation laws for water waves     

Noriyuki Iwata 《Journal of Oceanography》1974,30(5):222-231

With the averaged variational principles and the energy-momentum tensor, conservation laws for wave action, mass, momentum and energy for slowly varying water waves are derived systematically. For Stokes waves this enables us to derive energy and momentum conservation laws developed by the concept of radiation stress from general energymomentum tensor, which is as yet not clearly understood. Moreover the change of the wave amplitude of solitary wave over an uneven bottom is obtained from energy conservation equation.  相似文献