On the X-ray image of the Crab nebula: comparison with Chandra observations     

Shinpei Shibata  Haruhiko Tomatsuri  Makiko Shimanuki  Kazuyuki Saito  Koji Mori 《Monthly notices of the Royal Astronomical Society》2003,346(3):841-845

An axisymmetric model for the Crab nebula is constructed to examine the flow dynamics in the nebula. The model is based on that of Kennel & Coroniti, although we assume that the kinetic-energy-dominant wind is confined to an equatorial region. The evolution of the distribution function of the electron–positron plasma flowing out in the nebula is calculated. Given viewing angles, we reproduce an image of the nebula and compare it with the Chandra observation.
The reproduced image is not ring-like, but is rather 'lip-shaped'. It is found that the assumption of a toroidal field does not reproduce the Chandra image. We must assume that there is a disordered magnetic field with an amplitude as large as the mean toroidal field. In addition, the brightness contrast between the front and back sides of the ring cannot be reproduced if we assume that the magnetization parameter σ is as small as ∼10⁻³. The brightness profile along the semimajor axis of the torus is also examined. The non-dissipative, ideal-magnetohydrodynamic approximation in the nebula appears to break down.
We speculate that if the magnetic energy is released by some process that produces a turbulent field in the nebula flow and causes heating and acceleration – for example, by magnetic reconnection – then the present difficulties may be resolved (i.e. we can reproduce a ring image and a higher brightness contrast). Thus, the magnetization parameter σ can be larger than previously expected.  相似文献   

Tourist preferences for agricultural landscapes: a case study of terraced paddy fields in Noto Peninsula,Japan   总被引：2，自引：0，他引：2  

Bixia Chen  Zhenmian Qiu  Koji Nakamura 《山地科学学报》2016,13(10):1880-1892

Many agricultural landscapes have been formed through the interaction and co-evolution of nature and people, and represent the living landscapes shaped over time through intensive and continuous human cultivation. Terraced paddy fields are being re-evaluated to take into account the multiple functions they fill beyond only rice cultivation, particularly their contribution to the national biodiversity strategy of Japan. Since the 1990s, terraced paddy fields have been considered a representative cultural landscape of Japan and, at the same time, multi-stakeholder conservation activities have been conducted throughout Japan to reverse the increasing abandonment of terraces. Shiroyone Senmaida is an outstanding cultural landscape and a major tourist attraction in Noto Peninsula, Ishikawa Prefecture, which was designated through an initiative by the Food and Agriculture Organization (FAO) as a Globally Important Agricultural Heritage Systems (GIAHS) pilot site in 2011. It is important to clarify tourist preference for terraced paddy field landscapes to contribute to future policy making toward improved agricultural landscape conservation. A key finding of this study is that tourists visiting to enjoy the agricultural landscape are also concerned on the sustainability of the farming methods and preferred to maintain the naturalness of the landscape. Respondents with higher educational levels and greater concern for the biological and traditional farming knowledge aspects of the rice terraces were also more inclined to favor sustainable farming practices. Tourists preferred to maintain the naturalness and rurality of the agricultural landscape, and indicated that construction of excessive tourist facilities would cause the landscape to deteriorate. It was suggested that the local community and surroundings, including rural settlements, hills, and forests, should also be conserved together with the scenic terrace.  相似文献   

Reconstructions of annual discharge and equilibrium line altitude of glaciers at Qilian Shan,northwest China,from 1978 to 2002     

Akiko Sakai  Koji Fujita  Chiyuki Narama  Jumpei Kubota  Masayoshi Nakawo  Tandong Yao 《水文研究》2010,24(19):2798-2806

Discharge was calculated from a mountainous area, including discharge from glaciers, in the Qilian Shan (Qilian Mountains) of northwest China. The studied Yingluoxia basin is 9983 km² in area, with glaciers making up 0·3% of the basin. The calculation method was based on the heat balance, requiring only daily temperature and precipitation. Calculated annual discharge from the basin corresponded well with the observed data. Calculated annual discharge from glaciers was 3·6% of the total discharge from the basin. The temporal trend of the calculated equilibrium line altitude (ELA) at the July 1st Glacier (western side of the Yingluoxia basin) was similar to that of the observed ELA. The calculated annual mass balance of glaciers within the Yingluoxia basin has a larger negative value than the other glaciers in China, as the ratio of accumulation area to the total glacier area in the Yingluoxia basin is much lower than in neighbouring basins to the west. Copyright © 2010 John Wiley & Sons, Ltd.  相似文献   

Phytoplankton growth and microzooplankton grazing in the Sea of Okhotsk during late summer of 2006     

Hongbin Liu  Koji Suzuki  Jun Nishioka  Rumi Sohrin  Takeshi Nakatsuka 《Deep Sea Research Part I: Oceanographic Research Papers》2009,56(4):561-570

The Sea of Okhotsk is one of the most productive marine basins in the world ocean and plays an important role in transport of organic carbon and iron to the western subarctic Pacific. We report the first measurements of phytoplankton growth and microzooplankton grazing rates in the Sea of Okhotsk, in late summer of 2006. The study area can be divided into two areas: nutrient-sufficient waters on the continental shelf along the east coast of Sakhalin Island and in the vicinity of Bussol Strait, and surface nutrient-depleted waters beyond the shelf break and in the vicinity of Sakhalin Bay. Phytoplankton growth rate in the studied area was strongly affected by nutrient availability, with high phytoplankton growth rate (0.55±0.14 d^?1) in the nutrient-replete region and severely depressed growth (0.03±0.05 d^?1) in the nutrient-depleted region. On the other hand, microzooplankton grazing rates in both the nutrient-replete and nutrient-depleted regions were approximately the same (0.26±0.20 d^?1 vs. 0.27±0.24 d^?1). Consequently, microzooplankton grazing consumed <50% of the phytoplankton growth in nutrient-rich waters but >3 times the phytoplankton growth in nutrient-depleted waters. Phytoplankton physiological condition as measured by the maximum photochemical quantum efficiency (F_v/F_m) of algal photosystem II (PS II) showed a general trend in agreement with the in situ growth rate of phytoplankton. In contrast to the phytoplankton community, picophytoplankton, especially the cyanobacteria Synechococcus, showed no nutrient effect on their growth, and the growth and mortality rates were well balanced, suggesting that they have a low nutrient requirement and their biomass was controlled principally by microzooplankton grazing.  相似文献   

Estimation of heat flux through the eastern Bering Strait     

Kohei Mizobata  Koji Shimada  Rebecca Woodgate  Sei-Ichi Saitoh  Jia Wang 《Journal of Oceanography》2010,66(3):405-424

We estimated the northward heat flux through the eastern channel of the Bering Strait during the ice-free seasons between 1999 and 2008. This is likely about half of the total heat flux through the strait. The net volume transport and heat flux through the eastern channel of the strait were estimated from multiple linear regression models with in-situ/satellite remotely sensed datasets and NCEP reanalysis 10 m wind. The net volume transport was well explained by the west-east slope of sea level anomaly and NNW wind component at the strait. On the heat flux, the contributions of both barotropic and baroclinic components were taken into account. Estimated volume transport and vertical profile of temperature were used to calculate northward heat flux through the eastern channel of the strait. The magnitude of the estimated heat flux is comparable to estimates from in-situ measurements. Averaged heat flux in the eastern Bering Strait between 2004 and 2007 was about 1.9 times larger than that between 2000 and 2003. Maximum heat flux occurred in 2004, and same magnitude of heat flux was estimated from 2005 to 2007. This resulted not only from the increase in northward volume transport but also anomalous warm water intrusion from the Bering Sea. Our results suggest a candidate among the important parameters controlling heat budget, which contributes to the Arctic sea ice reduction, whereas more studies are required to confirm that this mechanism is actually responsible for the interannual and longer timescale variability.  相似文献   

Observations and modeling of the ice-ocean conditions in the coastal Chukchi and Beaufort Seas     

JIN Meibing  WANG Ji  MIZOBATA Kohei  HU Haoguo  SHIMADA Koji 《海洋学报(英文版)》2008,27(3):79-87

The Chukchi and Beaufort Seas include several important hydrological features： inflow of the Pacific water, Alaska coast current （ ACC ）, the seasonal to perennial sea ice cover, and landfast ice ＇along the Alaskan coast. The dynamics of this coupled ice-ocean system is important for both regional scale oceanography and large-scale global climate change research. A mumber of moorings were deployed in the area by JAMSTEC since 1992, and the data revealed highly variable characteristics of the hydrological environment. A regional high-resolution coupled ice-ocean model of the Chukchi and Beaufort Seas was established to simulate the ice-ocean environment and unique seasonal landfast ice in the coastal Beaufort Sea. The model results reproduced the Beaufort gyre and the ACC. The depthaveraged annual mean ocean currents along the Beaufort Sea coast and shelf hreak compared well with data from four moored ADCPs, but the simulated velocity had smaller standard deviations, which indicate small-scale eddies were frequent in the region. The model resuits captured the sea,real variations of sea ice area as compared with remote sensing data, and the simulated sea ice velocity showed an ahnost stationary area along the Beaufort Sea coast that was similar to the observed landfast ice extent. It is the combined effects of the weak oceanic current near the coast, a prevailing wind with an onshore component, the opposite direction of the ocean current, and the blocking hy the coastline that make the Beaufort Sea coastal areas prone to the formation of landfast ice.  相似文献   

Determination of the freshwater origin of Coastal Oyashio Water using humic-like fluorescence in dissolved organic matter     

Yu Mizuno  Jun Nishioka  Takahiro Tanaka  Yuya Tada  Koji Suzuki  Yuta Tsuzuki  Atsuko Sugimoto  Youhei Yamashita 《Journal of Oceanography》2018,74(5):509-521

Coastal Oyashio Water (COW), defined as a water mass with a temperature lower than 2 °C and a salinity lower than 33.0, is distributed in the North Pacific Ocean off southeastern Hokkaido, Japan, from winter to spring. COW is rich in macronutrients and dissolved iron and is thus considered to affect the spring phytoplankton blooms in the Oyashio region. Although river water and sea-ice melt water have been considered freshwater end-members of COW, the contributions of these freshwater sources to COW have not been well described. In this study, the humic-like components in dissolved organic matter were first applied as a parameter to evaluate the freshwater end-members of COW in March 2015. Linear regressions with negative slopes were determined between the humic-like components and the salinity of COW. The intercepts of the regressions against the humic-like components were within the ranges of those observed for the local rivers of Hokkaido but were very different from those of sea ice. These findings suggest that river water contributed to the COW observed here as a freshwater end-member, although the contribution of sea-ice melt water to COW could not be evaluated. This novel approach also highlighted two different less-saline water masses in COW. The first was characterized by a lower temperature and relatively high levels of humic-like components, while the second was higher in temperature and had higher levels of humic-like components. It is suggested that these different characteristics are due to the contributions of water from different rivers and/or different effects of sea-ice melt water.  相似文献   

Rediscovering wood-laden debris flow studies: A perspective from Japan     

Kenta Koyanagi  Taku Yamada  Koji Ishida 《地球表面变化过程与地形》2023,48(1):104-118

Debris flow is one of the dominant processes distributing large wood (LW) within mountainous catchments. However, little has been reviewed on wood-laden debris flow (WLDF), presumably owing to limited reviewable works. This article, therefore, navigates the international readers through 40 years of WLDF studies, most of which have been published only in Japanese. Firstly, we reviewed the historical development of Japanese WLDF particularly focusing on the 1980s and the 1990s. A series of post-disaster fieldworks from the July 1982 Nagasaki flood to the July 1990 Kumamoto flood provided 32 catchment-scale wood budgeting data; empirical relationships among drainage area, dominant tree species, sediment yield, and wood loads associated with single debris flow disasters were illustrated. Secondly, the characteristics of WLDF were summarized based on relevant previous studies on the recruitment, transport, and deposition processes of LW during debris flows. Thirdly, we discussed the connectivity between those Japanese WLDF studies and international LW studies by relating/contrasting their research approaches and spatiotemporal scales. In contrast to global LW research trends, Japanese WLDF studies have almost exclusively regarded LW as hazardous materials (i.e., “driftwood” or “woody debris”) that need to be retained upstream of the inhabited areas. Those practice-oriented WLDF studies were concentrated on drainage areas of 10⁻² to 10⁰ km², representing 1–6 orders of magnitude smaller spatial scales than those generally covered by existing international LW studies. Strongly motivated by engineering requirements, “dynamic” interactions between debris flows and LW during floods have also been physically presented, mainly based on unique laboratory experiments involving steep flume (> 0.05) and mobile bed conditions. Finally, some future works for WLDF were briefly stated from practical and scientific perspectives. By “rediscovering” those WLDF studies domestically developed in Japanese debris flow channels since the 1980s, a more comprehensive understanding of LW dynamics in the river system may be achieved.  相似文献   

Ocean Tide Models Developed by Assimilating TOPEX/POSEIDON Altimeter Data into Hydrodynamical Model: A Global Model and a Regional Model around Japan   总被引：36，自引：0，他引：36  

Koji Matsumoto  Takashi Takanezawa  Masatsugu Ooe 《Journal of Oceanography》2000,56(5):567-581

A global ocean tide model (NAO.99b model) representing major 16 constituents with a spatial resolution of 0.5° has been estimated by assimilating about 5 years of TOPEX/POSEIDON altimeter data into barotropic hydrodynamical model. The new solution is characterized by reduced errors in shallow waters compared to the other two models recently developed; CSR4.0 model (improved version of Eanes and Bettadpur, 1994) and GOT99.2b model (Ray, 1999), which are demonstrated in comparison with tide gauge data and collinear residual reduction test. This property mainly benefits from fine-scale along-track tidal analysis of TOPEX/POSEIDON data. A high-resolution (1/12°) regional ocean tide model around Japan (NAO.99Jb model) by assimilating both TOPEX/POSEIDON data and 219 coastal tide gauge data is also developed. A comparison with 80 independent coastal tide gauge data shows the better performance of NAO.99Jb model in the coastal region compared with the other global models. Tidal dissipation around Japan has been investigated for M₂ and K₁ constituents by using NAO.99Jb model. The result suggests that the tidal energy is mainly dissipated by bottom friction in localized area in shallow seas; the M₂ ocean tidal energy is mainly dissipated in the Yellow Sea and the East China Sea at the mean rate of 155 GW, while the K₁ energy is mainly dissipated in the Sea of Okhotsk at the mean rate of 89 GW. TOPEX/POSEIDON data, however, detects broadly distributed surface manifestation of M₂ internal tide, which observationally suggests that the tidal energy is also dissipated by the energy conversion into baroclinic tide.  相似文献   

A derivation of the scaling law, the power law and the scaling relation for fetch-limited wind waves using the renormalization group theory     

Koji Ueno 《Journal of Oceanography》1998,54(6):641-649

A state of wind waves at a fetch is assumed to be transformed into another state of wind waves at a different fetch by the renormalization group transformation. The scaling laws for the covariance of water surface displacement and for the one-dimensional and two-dimensional spectrum and the power law for the growth relation are derived from the fact that the renormalization group transformation constitutes a semigroup. The scaling relation or the relation among the exponents of the power law is also derived, using the two assumptions that the renormalization group transformation is applicable to fetch-limited wind waves and that the saturated range exists, which implies that the directional distribution function of energy in the wave number region much larger than the peak wave number does not depend on wave number.  相似文献