Structure of tidal mixing at the Hayasui Straits     

Hideo Nakajima 《Journal of Oceanography》1975,31(6):227-234

Based on the observation at the Hayasui Straits, actual conditions of the tidal mixing, of two water masses separated by the strait, are investigated for the time scale shorter than the the tidal period. Analyses show that the tidal mixing of water masses occurs by the disintegration of mutually penetrating small masses of water, which have been produced by the reciprocating tidal motion of the water masses with some topographical effect, especially at the slack water as large tongues. The mutual penetration occurs rather horizontally, and some of these turbulent smaller masses of water keep their own properties for a rather long period of the order of three hours, without mixing even in the violent shear flows. Initial horizontal scale of these turbulent masses is estimated as of the order of three kilometers. These actual conditions reveal themselves as the macroscopic diffusion coefficient.  相似文献   

Community structures of coral reefs around Peninsular Malaysia     

Tatsuki Toda  Tomoko Okashita  Takeshi Maekawa  Bin Abdul Adzis Kee Alfian  Mohd Kushairi Mohd Rajuddin  Ryota Nakajima  Wenxi Chen  Kunio T. Takahashi  Bin Haji Ross Othman  Makoto Terazaki 《Journal of Oceanography》2007,63(1):113-123

Coral community structures at eleven fringing reef sites were investigated along the coast of Peninsular Malaysia. Estimated coverage of coral communities is examined by applying quantitative digital image analysis to the line transect method. Four coral community types were characterized by dominant genera and lifeforms: Acropora branching community, Montipora-Acropora community, Porites massive community, and Heliopora community. Live coral coverage in all study sites ranged from 17.9% to 68.6%. Most reefs were in “fair” condition while some reefs were shown to be in “poor” condition. Coral community structures among the study sites were divided into the west coast of Peninsular Malaysia with a dominant Porites massive lifeform, and the east coast with a variety of lifeform categories of Montipora and Acropora. Physical effects such as the monsoon wind regime and sedimentation are likely to influence the formation of dominant coral communities around Peninsular Malaysia.  相似文献   

Parallel Visualization of Large-scale Unstructured Geophysical Data for the Earth Simulator     

Li Chen  Issei Fujishiro  Kengo Nakajima 《Pure and Applied Geophysics》2004,161(11-12):2245-2263

— A scalable and high-performance parallel visualization subsystem has been developed in GeoFEM for the Earth Simulator. As part of the Earth Simulator project in Japan, the proposed subsystem is effective for the visualization of large-scale geoscientific data, and can be concurrent with computation subsystems on the same high-performance parallel computer. Moreover, several parallel visualization methods are developed for large unstructured datasets, covering scalar, vector and tensor fields. Furthermore, a number of strategies are adopted in order to improve the parallel performance of the proposed subsystem for the Earth simulator, including the three-level hybrid parallelization and dynamic load balancing. Good visualization images and high parallel performance have been obtained on a Hitachi SR8000 for large unstructured geoscientific data sets, and thus demonstrating the feasibility and effectiveness of the proposed method for use on the Earth Simulator.  相似文献   

Along-arc lateral variation of Rb−Sr and K−Ar ages of Cretaceous granitic rocks in Southwest Japan     

Takashi Nakajima  Teruo Shirahase  Ken Shibata 《Contributions to Mineralogy and Petrology》1990,104(4):381-389

Cretaceous granitic rocks were emplaced over a distance of 700 km along arc in Southwest Japan. Rb–Sr and K–Ar ages of a major group of these granitic rocks, with ilmenite series ore mineralogy, were examined. Rb–Sr whole rock ages of 92.8±4.0 Ma and Rb–Sr and K–Ar biotite ages of 80–88 Ma were obtained on one group of these granitic rocks from Kamo-Sera area of central Hiroshima Prefecture. The K–Ar ages of various minerals, combined with the Rb–Sr whole-rock age, give a smooth cooling curve, which suggests a 5 to 10 Ma time-lag between intrusion and cooling at 300° C for the Cretaceous granitic rocks. The Rb–Sr whole-rock and Rb–Sr/K–Ar biotite ages of these granitic rocks become younger eastward along the Southwest Japan arc, and the time-lag between the two systems remains constant at 5 to 10 Ma over the entire area. The along-arc age variation does not support the genetical relationship of the Cretaceous granitoids with steady-state subduction. The Cretaceous granitic province at the eastern margin of Eurasian continent was, at least partly, formed by an episodic event such as ridge subduction.  相似文献   

Microwave Neutral Line Associated Source and a Current Sheet     

A. M. Uralov  V. V. Grechnev  G. V. Rudenko  I. G. Rudenko  H. Nakajima 《Solar physics》2008,249(2):315-335

Neutral Line associated Sources (NLSs) are quasi-stationary microwave sources projected onto vicinities of the neutral line of the photospheric magnetic field. NLSs are often precursors of powerful flares, but their nature is unclear. We endeavor to reveal the structure of an NLS and to analyze a physical connection between such a source with a site of energy release in the corona above NOAA 10488 (October/November 2003). Evolution of this AR includes emergence and collision of two bipolar magnetic structures, rise of the main magnetic separator, and the appearance of an NLS underneath. The NLS appears at a contact site of colliding sunspots, whose relative motion goes on, resulting in a large shear along a tangent. Then the nascent NLS becomes the main source of microwave fluctuations in the AR. The NLS emission at 17 GHz is dominated by either footpoints or the top of a loop-like structure, an NLS loop, which connects two colliding sunspots. During a considerable amount of time, the emission dominates over that footpoint of the NLS loop, where the magnetic field is stronger. At that time, the NLS resembles a usual sunspot–associated radio source, whose brightness center is displaced towards the periphery of a sunspot. Microwave emission of an X2.7 flare is mainly concentrated in an ascending flare loop, initially coinciding with the NLS loop. The top of this loop is located at the base of a non-uniform bar-like structure visible in soft X-rays and at 34 GHz at the flare onset. We reveal i) upward lengthening of this bar before the flare onset, ii) the motion of the top of an apparently ascending flare loop along the axis of this bar, and iii) a non-thermal microwave source, whose descent along the bar was associated with the launching of a coronal ejection. We connect the bar with a probable position of a nearly vertical diffusion region, a site of maximal energy release inside an extended pre-flare current sheet. The top of the NLS loop is located at the bottom of this region. A combination of the NLS loop and diffusion region constitutes the skeleton of a quasi-stationary microwave NLS.  相似文献   

Appearance pattern of tooth germs with developmental process inMylopharyngodon piceus     

Yue Pei-qi  Nakajima Tsuneo 《中国海洋湖沼学报》1995,13(2):155-161

The pharyngeal dental formula ofMylopharyngodon piceus is 4–5 as a rule, and the dentition is asymmetrical. It is difficult to identify each tooth in the larval dentition. In this paper the appearance pattern of tooth germ with development process in this fish is described in detail. The formation pattern of the left dentition is contrasted with that of the right one. In the developmental process, the left pharyngeal dentition lacks teeth at position An3. Thus the left dentition is D-type as designated by Nakajima (1984), while the right one is A-type. This project was funded by the International Cooperation of Japan-China and the National Natural Science Foundation of China.  相似文献   

A flare-associated thermal burst in the mm-wave region     

K. Akabane  H. Nakajima  K. Ohki  F. Moriyama  T. Miyaji 《Solar physics》1973,33(2):431-437

A mm-wave thermal burst has been observed at 73 GHz. The simultaneous observation at 17 GHz revealed that this mm-wave burst was quite a different component from the non-thermal burst co-existing at a cm-wavelength range. Since the source of this burst seemed to be opaque or nearly opaque, the temperature became several tens of thousands degrees. Considering also the similarity between time profiles of the 73 GHz intensity and the H light curve, it is concluded that this mm-wave burst is situated very close to the H flare region.  相似文献   

Differences of observed characteristics between impulsive bursts and post-burst increases     

Keizo Kai  Takeo Kosugi  Hiroshi Nakajima 《Solar physics》1982,75(1-2):331-339

The change of source characteristics during the transition from the impulsive phase to the post-burst phase is investigated for cm bursts on a statistical basis. The results are the following: (1) The sudden decrease of the circular polarization degree is found almost invariably at the transition; typically from 20–30% down to a few percent. (2) Some bursts show remarkable source expansions in the post-burst phase. There are no cases in which impulsive bursts have larger source size than the associated post-burst increases. (3) Type III bursts which are indicative of non-thermal phenomena are associated with the impulsive phase but not with the post-burst phase. Implications of these observed results are discussed.  相似文献   

Formation of Distinct Granitic Magma Batches by Partial Melting of Hybrid Lower Crust in the Izu Arc Collision Zone, Central Japan   总被引：1，自引：0，他引：1  

Saito  Satoshi; Arima  Makoto; Nakajima  Takashi; Misawa  Keiji; Kimura  Jun-Ichi 《Journal of Petrology》2007,48(9):1761-1791

The Miocene Kofu Granitic Complex (KGC) occurs in the Izu CollisionZone where the Izu–Bonin–Mariana (IBM) arc has beencolliding with the Honshu arc since the middle Miocene. TheKGC includes rocks ranging in compositions from biotite-bearinggranite (the Shosenkyo and Mizugaki plutons), and hornblende–biotite-bearinggranodiorite, tonalite, quartz-diorite, and granite (the Shiodaira,Sanpo, Hirose and Sasago plutons), to hornblende-bearing tonaliteand trondhjemite (the Ashigawa–Tonogi pluton), indicatingthat it was constructed from multiple intrusions of magma withdifferent bulk chemistry. The Sr-isotopic compositions correctedto sensitive high-resolution ion microprobe (SHRIMP) zirconages (SrI) suggest that the primary magmas of each pluton wereformed by anatexis of mixed lower crustal sources involvingboth juvenile basalt of the IBM arc and Shimanto sedimentaryrocks of the Honshu arc. After the primary magmas had formed,the individual plutons evolved by crystal fractionation processeswithout significant crustal assimilation or additional mantlecontribution. SHRIMP zircon U–Pb ages in the KGC rangefrom 16·8 to 10·6 Ma and overlap the resumptionof magmatic activity in the IBM and Honshu arcs at c. 17 Maand the onset of IBM arc–Honshu arc collision at c. 15Ma. The age of the granite plutons is closely related to theepisodic activity of arc magmatism and distinct granitic magmabatches could be formed by lower crustal anatexis induced byintrusion of underplated mantle-derived arc magmas. Based onpressures determined with the Al-in-hornblende geobarometer,the KGC magmas intruded into the middle crust. Thus, the KGCcould represent an example of the middle-crust layer indicatedthroughout the IBM arc by 6·0–6·5 km/s seismicvelocities. This granitic middle-crust layer acted buoyantlyduring the IBM arc–Honshu arc collision, leading to accretionof buoyant IBM arc middle crust to the Honshu arc. KEY WORDS: arc–arc collision; crustal anatexis; granite; Izu–Bonin–Mariana (IBM) arc; Izu Collision Zone  相似文献   

Spinel group minerals in LL3.00-6 chondrites: Indicators of nebular and parent body processes     

M. Kimura  H. Nakajima  M.K. Weisberg 《Geochimica et cosmochimica acta》2006,70(22):5634-5650

We carried out a systematic study of spinel group minerals in LL3.00-3.9 and LL4-6 chondrites. With increasing petrologic type, the size and abundance of spinel increase. The compositions of spinel group minerals in type 3 chondrites depend on the occurrence; Mg-Al-rich spinel occurs mainly in chondrules. Some chromite occurs in chondrules and matrix, and nearly pure chromite is exclusively encountered in the matrix. The occurrence of nearly pure chromite and the wide compositional variations distinguish spinel group minerals in types 3.00-3.3 from those in the other types. Spinel group minerals in types 3.5-3.9 show a narrower range of compositions, and those in types 4-6 are homogeneous. The changes in composition and abundance of spinel in type 3 chondrites are most likely due to thermal metamorphism. Therefore, the chemistry of spinel group minerals could be used as a sensitive indicator of metamorphic conditions, not only for type 3-6, but also 3.00-3.9. They can be applied to identify the most primitive (least metamorphosed) chondrites. The bulk compositions of spinel-bearing chondrules and the textural setting of the spinel indicate that most spinel group minerals crystallized directly from chondrule melts. However, some spinel grains, especially those enclosed in olivine phenocrysts, can not be explained by in situ crystallization in the chondrule. We interpret these spinel grains to be relic phases that survived chondrule melting. This is supported by the oxygen isotopic composition of a spinel grain, which has significantly lighter oxygen than the coexisting olivine. The oxygen isotopic composition of this spinel is similar to those of Al-rich chondrules. Our discovery of relic spinel in chondrules is an indication of the complexities in the early solar nebular processes that ranged from formation of refractory inclusion, through Al-rich chondrule, to ferromagnesian chondrules, and attests to the recycling of earlier formed materials into the precursors of later formed materials. The characteristic features of spinel group minerals are not only sensitive to thermal metamorphism, but also shed light on chondrule formation processes.  相似文献