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111.
To investigate temporal and spatial evolution of global geomagnetic field variations from high-latitude to the equator during geomagnetic storms, we analyzed ground geomagnetic field disturbances from high latitudes to the magnetic equator. The daytime ionospheric equivalent current during the storm main phase showed that twin-vortex ionospheric currents driven by the Region 1 field-aligned currents (R1 FACs) are intensified significantly and expand to the low-latitude region of-30~ magnetic latitude. Centers of the currents were located around 70~ and 65~ in the morning and afternoon, respectively. Corresponding to intensification of the R1 FACs, an enhancement of the eastward/westward equatorial electrojet occurred at the daytime/nighttime dip equator. This signature suggests that the enhanced convection electric field penetrates to both the daytime and nighttime equa- tor. During the recovery phase, the daytime equivalent current showed that two new pairs of twin vortices, which are different from two-cell ionospheric currents driven by the R1 FACs, appear in the polar cap and mid latitude. The former led to enhanced north- ward Bz (NBZ) FACs driven by lobe reconnection tailward of the cusps, owing to the northward interplanetary magnetic field (IMF). The latter was generated by enhanced Region 2 field-aligned currents (R2 FACs). Associated with these magnetic field variations in the mid-latitudes and polar cap, the equatorial magnetic field variation showed a strongly negative signature, produced by the westward equatorial electrojet current caused by the dusk-to-dawn electric field.  相似文献   
112.
Curved twin I-girder bridges (CTIGBs) have low torsional stiffness that makes them vulnerable to dynamic loads.This study investigates the effects of bottom bracings on the torsional dynamic characteri...  相似文献   
113.
Numerical experiments were performed to investigate the effects of eddies generated in deep water formation processes on an abyssal circulation in a closed bowl-shaped basin. Two sets of experiments were performed. One set was eddy-restricted experiments in which only a volume-driven (upwelling-driven) circulation was simulated and the other set was eddy-permitted experiments in which both a volume-driven circulation and an eddy-driven circulation were simulated. In the two layer experiment where the lower layer water is formed, a mean along-slope current is formed in the lower layer for both the eddy-restricted and eddy-permitted experiments. The direction of the current was not unique in the eddy-restricted experiment, but it was cyclonic in the eddy-permitted experiment. In the three layer experiments where water of the intermediate layer is formed, the mean along-slope current in the lowest layer is negligibly small in the eddy-restricted experiment, while it is large and cyclonic in the eddy-permitted experiment. The driving forcings of the eddy-driven circulation are quantified in terms of eddy fluxes of relative vorticity (Reynolds stress) and layer thickness (bolus velocity). These terms increase as the volume of the newly formed water increases, but they do not change greatly with the slope height. The magnitude of these terms changes with the slope width, but the sum of these terms does not vary greatly. As a result, the intensity of the eddy-driven circulation depends primarily on the volume of newly formed water. These dependences of eddy fluxes were interpreted using downgradient diffusion of potential vorticity.  相似文献   
114.
The effect of ocean acidification, caused by the increase in pCO2 in seawater, on phytoplankton population and on related organic nitrogen production was experimentally examined by use of a natural coastal microbial population. pCO2 and pH were controlled by aeration with air in which pCO2 was at the current level (control), for which ambient air was used, and with air in which pCO2 was ??800?? and ??1200?? ppm, in 500-L culture vessels. The experiment was continued for 15?days after addition of the inorganic nutrients such as nitrate, phosphate, and silicate. During most of the experimental period, a minor increase in phytoplankton biomass was noted, probably because of low irradiance, an increase in phytoplankton biomass was observed at the end of the experiment. Flow cytometric and microscopic observations revealed that this increase was because of Chrysochromulina sp. (Haptophyceae). The growth of Chrysochromulina sp. was most obvious in the control vessel, and tended to be obscured by increasing pCO2 (decrease in pH), indicating the possibility that ocean acidification inhibits the growth of specific phytoplankton groups, for example Chrysochromulina sp. Production of particulate organic nitrogen (PON), determined by the 15N tracer method, also diminished under acidified conditions compared with that at the current level.  相似文献   
115.
We investigated the size fraction and pigment-derived class compositions of phytoplankton within the euphotic zone of the Antarctic marginal ice zone between 63.3°S and 66.5°S along the 140°E meridian on two consecutive cruises in the late austral summer and early austral autumn of 2003. We observed significant temporal and spatial variations in phytoplankton size and taxonomic composition, although chlorophyll a concentrations were generally below 1 μg l−1 during both periods. Microphytoplankton (>20 μm), mainly diatoms, were prominent in the euphotic zone in the southernmost area around 66.5°S during late summer. In the rest of the study area during both cruises, the phytoplankton community was dominated by pico- and nano-sized populations (<20 μm) throughout the euphotic zone. The small-size populations mostly consisted of diatoms and haptophytes, although chlorophytes were dominant in extremely cold water (−1.5°C) below the overlying warm water around 65.5°S during late summer. From late summer to early autumn, chlorophytes declined in abundance, probably due to increasing temperature within the euphotic zone (−1 to 0°C). These pico- and nano-phytoplankton-dominated populations were often accompanied by relatively high concentrations of ammonium, suggesting the active regeneration of nutrients within the small-size plankton community.  相似文献   
116.
The South Kitakami Massif is one of the oldest geological domains in Japan having Silurian strata with acidic pyroclastic rocks and Ordovician–Silurian granodiorite–tonalite basement, suggesting that it was matured enough to develop acidic volcanisms in the Silurian period. On the northern and western margin of the South Kitakami Massif, an Ordovician arc ophiolite (Hayachine–Miyamori Ophiolite) and high‐pressure and low‐temperature metamorphic rocks (Motai metamorphic rocks) exhumed sometime in the Ordovician–Devonian periods are distributed. Chronological, geological, and petrochemical studies on the Hayachine–Miyamori Ophiolite, Motai metamorphic rocks, and other early Paleozoic geological units of the South Kitakami Massif are reviewed for reconstruction of the South Kitakami arc system during Ordovician to Devonian times with supplementary new data. The reconstruction suggests a change in the convergence polarity from eastward‐ to westward‐dipping subduction sometime before the Late Devonian period. The Hayachine–Miyamori Ophiolite was developed above the eastward‐dipping subduction through three distinctive stages. Two separate stages of overriding plate extension inducing decompressional melting with minor involvement of slab‐derived fluid occurred before and after a stage of melting under strong influence of slab‐derived fluids. The first overriding plate extension took place in the back‐arc side forming a back‐arc basin. The second one took place immediately before the ophiolite exhumation and near the fore‐arc region. We postulate that the second decompressional melting was triggered by slab breakoff, which was preceded by slab rollback inducing trench‐parallel wedge mantle flow and non‐steady fluid and heat transport leaving exceptionally hydrous residual mantle. The formation history of the Hayachine–Miyamori Ophiolite implies that weaker plate coupling may provide preferential conditions for exhumation of very hydrous mantle. Very hydrous peridotites involved in arc magmatism have not yet been discovered except for in the Cambrian–Ordovician periods, suggesting its implications for global geodynamics, such as the thermal state and water circulation in the mantle.  相似文献   
117.
1600 K和20 GP温压条件下的顽火辉石电导率   总被引:2,自引:1,他引:1       下载免费PDF全文
在温度750~1600 K和压力10~20 GPa条件下,借助于Kawai-5000多面顶砧高温高压设备,就位测量了(Mg0.9Fe0.1) SiO3 顽火辉石的电导率.实验结果显示,顽火辉石的电导率在高温区以小极化子机制为主,在低温区以质子导电为主,因为实验后的样品中有一定的水含量.另外,X射线衍射实验表明压力(20 GPa)诱发了顽火辉石向林伍德石的相变,这是我们首次在顽火辉石的电导率实验中观测到林伍德石含水相变,而且含水林伍德石的电导率与已有的实验结果相当一致.  相似文献   
118.
Two silicate-rich dust layers were found in the Dome Fuji ice core in East Antarctica, at Marine Isotope Stages 12 and 13. Morphologies, textures, and chemical compositions of constituent particles reveal that they are high-temperature melting products and are of extraterrestrial origin. Because similar layers were found ~ 2000 km east of Dome Fuji, at EPICA (European Project for Ice Coring in Antarctica)-Dome C, particles must have rained down over a wide area 434 and 481 ka. The strewn fields occurred over an area of at least 3 × 106 km2. Chemical compositions of constituent phases and oxygen isotopic composition of olivines suggest that the upper dust layer was produced by a high-temperature interaction between silicate-rich melt and water vapor due to an impact explosion or an aerial burst of a chondritic meteoroid on the inland East Antarctic ice sheet. An estimated total mass of the impactor, on the basis of particle flux and distribution area, is at least 3 × 109 kg. A possible parent material of the lower dust layer is a fragment of friable primitive asteroid or comet. A hypervelocity impact of asteroidal/cometary material on the upper atmosphere and an explosion might have produced aggregates of sub-μm to μm-sized spherules. Total mass of the parent material of the lower layer must exceed 1 × 109 kg. The two extraterrestrial horizons, each a few millimeters in thickness, represent regional or global meteoritic events not identified previously in the Southern Hemisphere.  相似文献   
119.
Recently,effects of Earth's curvature and radial heterogeneity on coseismic deformations are often investigated based on the 2004 Sumatra earthquake.However,such effects are strongly related to earthquake types.As a low dip angle event,the 2004 Sumatra earthquake is not a good seismic case for such a topic since the effects for moderate dip angle events are much bigger.In this study,the half-space and spherical dislocation theories are used,respectively,to calculate coseismic displacements caused by the 2008 Wenchuan earthquake and the 2004 Sumatra earthquake.Effects of Earth's curvature and stratification are investigated through the discrepancies of results calculated using the two dislocation theories.Results show that the effects of Earth's curvature and stratification for the 2008 Wenchuan earthquake are much larger than those for the 2004 Sumatra earthquake.Ignoring the effects will cause errors up to 100%-200% in far field displacements for a moderate dip angle event like the 2008 Wenchuan earthquake.Such great effects are much bigger than those conclusions of previous studies.Besides,comparison with observations verifies that spherical dislocation theories yield better results than half-space ones in far fields.  相似文献   
120.
The estimation of strength of shaking at a site from the initial P-wave portion of ground motion is the key problems for shortening the alert time of the earthquake Early Warning (EEW). The most of the techniques proposed for the purpose utilize (a) ground motion models based on the estimated magnitude and hypocentral distance, or (b) the interim proxies, such as initial vertical displacement P d . We suggest the instrumental Japan Meteorological Agency (JMA) intensity (JMAI) as a characteristic for fast estimation of damage potential in the EEW systems. We investigated the scaling relations between JMAI measured using the whole earthquake recordings (overall intensity) and using particular time intervals of various duration (2.0–8.0 s) starting from the P-wave arrival (preliminary intensity). The dataset included 3,660 records (K-NET and the KiK-net networks) from 55 events (M W 4.1–7.4) occurred in 1999–2008 in Japan. We showed that the time interval of 4–5 s from the P-wave arrival can be used for reliable estimations of the overall intensity with the average standard error of about 0.5 JMA units. The uncertainty in the prediction may be reduced by consideration of local site conditions or by development of the station-specific models.  相似文献   
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