Modulation of galactic cosmic ray anisotropy in heliomagnetosphere: Influence of cosmic ray scattering on sidereal daily variation     

S. Yasue  I. Morishita  K. Nagashima 《Planetary and Space Science》1985,33(9):1057-1068

In previous papers, the present authors have shown that the galactic anisotropy is modulated due to cosmic ray orbital deflection in the heliomagnetosphere, and that the sidereal time daily variations of galactic origin can be expressed using the basic vectors, which have been obtained by calculating trajectories of cosmic rays in a model magnetosphere having Parker's Archimedian spiral structure with a flat or a wavy neutral sheet. In the present paper, the magnetic irregularities superposed on the Parker's spiral field have been taken into account, which cause the scattering of cosmic rays and disturb their orbits. We examined the fluctuations of asymptotic directions calculating their orbits by the Monte-Carlo simulation, based on the theory of the multiple scattering process. It is shown that the dispersion of the projected deviation angle is determined mainly by the scattering mean free path and by the structure of the order magnetic field, e.g. the polarity state of the heliomagnetosphere and the extent of the neutral sheet. We investigated also the influence of the fluctuations of asymptotic directions on the sidereal daily variation. It is found that, under some conditions, the scattering causes only the attenuation of the amplitude of the basic vector, and does not change its phase. The attenuation is negligibly small at high rigidities larger than ~ 1000 GV, but becomes more serious with decreasing rigidity. The rigidity dependence curve of the attenuation rate was calculated for various cases. A simple and approximate method is also presented for the derivation of those curves for any value of the magnitude of the mean free path and for various model magnetospheres. It is noted, however, that the lower limiting rigidity below which the present method is not applicable is relatively high in the Positive polarity state.  相似文献   

Dynamical response to supernova-induced gas removal in two-component spherical galaxies     

Masahiro Nagashima &#;  Yuzuru Yoshii 《Monthly notices of the Royal Astronomical Society》2003,340(2):509-518

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Possible associations of daytime fireballs and minor planets     

Ichiro Hasegawa 《Earth, Moon, and Planets》1996,72(1-3):311-316

Chinese and Japanese historical records of daytime fireballs, and world-wide daytime meteorite falls in the catalogue have been investigated. Among them, there are 253 and 104 records of great daytime fireballs in China and in Japan respectively, and 506 meteorite falls in the daytime are recorded in the Catalogue of Meteorites (1985). The same trends of seasonal and daily variations in the flux of daytime fireballs are clearly seen in both Chinese and Japanese records, and then the distributions of the daytime fireballs seem to suggest the association with meteorites and near-earth minor planets rather than with comets. Possible relations with minor planets, such as (1566) Icarus, (3671) Dionysius, (4450) Pan, (4486) Mithra and others are suggested.  相似文献   

Oxygen isotopic compositions of Allende Type C CAIs: Evidence for isotopic exchange during nebular melting and asteroidal metamorphism     

A.N. Krot  M. Chaussidon  H. Yurimoto  N. Sakamoto  K. Nagashima  I.D. Hutcheon  G.J. MacPherson 《Geochimica et cosmochimica acta》2008,72(10):2534-2555

In situ oxygen isotopic measurements of primary and secondary minerals in Type C CAIs from the Allende CV3 chondrite reveal that the pattern of relative enrichments and depletions of ¹⁶O in the primary minerals within each individual CAI are similar to the patterns observed in Types A and B CAIs from the same meteorite. Spinel is consistently the most ¹⁶O-rich (Δ¹⁷O = −25‰ to −15‰), followed by Al,Ti-dioside (Δ¹⁷O = −20‰ to −5‰) and anorthite (Δ¹⁷O = −15‰ to 0‰). Melilite is the most ¹⁶O-depleted primary mineral (Δ¹⁷O = −5‰ to −3‰). We conclude that the original melting event that formed Type C CAIs occurred in a ¹⁶O-rich (Δ¹⁷O  −20‰) nebular gas and they subsequently experienced oxygen isotopic exchange in a ¹⁶O-poor reservoir. At least three of these (ABC, TS26F1 and 93) experienced remelting at the time and place where chondrules were forming, trapping and partially assimilating ¹⁶O-poor chondrule fragments. The observation that the pyroxene is ¹⁶O-rich relative to the feldspar, even though the feldspar preceded it in the igneous crystallization sequence, disproves the class of CAI isotopic exchange models in which partial melting of a ¹⁶O-rich solid in a ¹⁶O-poor gas is followed by slow crystallization in that gas. For the typical (not associated with chondrule materials) Type C CAIs as well for as the Types A and B CAIs, the exchange that produced internal isotopic heterogeneity within each CAI must have occurred largely in the solid state. The secondary phases grossular, monticellite and forsterite commonly have similar oxygen isotopic compositions to the melilite and anorthite they replace, but in one case (CAI 160) grossular is ¹⁶O-enriched (Δ¹⁷O = −10‰ to −6‰) relative to melilite (Δ¹⁷O = −5‰ to −3‰), meaning that the melilite and anorthite must have exchanged its oxygen subsequent to secondary alteration. This isotopic exchange in melilite and anorthite likely occurred on the CV parent asteroid, possibly during fluid-assisted thermal metamorphism.  相似文献   

Difference in the prevailing periods of internal tides between Sagami and Suruga Bays: Numerical experiments     

Atsushi Ohwaki  Masaji Matsuyama  Hideki Nagashima 《Journal of Oceanography》1994,50(4):449-463

Current measurements in the surface layer in Sagami and Suruga Bays showed existence of significant tidal currents which are considered to be mainly due to internal tides (Inaba, 1982; Ohwaki,ea al., 1991). In addition, the prevailing period of the tidal currents is semidiurnal in Sagami Bay, but diurnal in Suruga Bay. To explain this difference in the prevailing, periods, numerical experiments were carried out using a two layer model. The internal tides are generated on the Izu Ridge outside the two bays. The semidiurnal internal tide propagates into Sagami Bay having characteristics of an internal inertia-gravity wave, while it propagates into Suruga Bay having characteristics of either an internal inertia-gravity wave or an internal Kelvin wave. The diurnal internal tide behaves only as an internal Kelvin wave, because the diurnal period is longer than the inertia period. Thus, the diurnal internal tide generated on the Izu Ridge can be propagated into Suruga Bay, while it cannot propagate into the inner region of Sagami Bay, though it is trapped around Oshima Island, which is located at the mouth of Sagami Bay. The difference in the propagation characteristics between the semidiurnal and diurnal internal tides can give a mechanism to explain the difference in the prevailing periods of the internal tides between Sagami and Suruga Bays.  相似文献   

Numerical experiments on double-diffusive intrusions in the ocean and their relation to laboratory experiments     

J. Yoshida  H. Nagashima 《Progress in Oceanography》2003,56(3-4):549

Numerical experiments on double-diffusive intrusions are reviewed briefly. Though the number of studies is very limited at present, they have undoubtedly an advantage that a heat–salt system can be studied without undesired heat loss from the boundaries.Several possibilities for future numerical experiments are summarized.  相似文献   

Convection induced by cooling at one side wall in two-dimensional non-rotating fluid —applicability to the deep Pacific circulation—     

Ichiro Ishikawa  Shigeaki Aoki  Ryo Furue  Nobuo Suginohara 《Journal of Oceanography》1996,52(5):617-632

In recognition that similarity in the density balance leads to resemblance in circulation between the two-dimensional non-rotating and three-dimensional rotating systems which have similar density stratification, we investigate convection induced by cooling at one side wall and heating at the sea surface by using a two-dimensional non-rotating model as idealized representation for the deep Pacific circulation. In the model, various vertical profiles are taken for the side wall cooling, which are assumed to correspond to the density structure of the Anatarctic Circumpolar Current. In a small diffusivity range, two important features are found to be robust against change in the vertical profile of the side wall cooling. One is that the density stratification is horizontally almost uniform. The other is that the balance in the density equation between the vertical advection and the vertical diffusion holds in the interior. Consequently, the vertical density balance, together with the equation of continuity, determines the circulation pattern for the prescribed vertical profile of the side wall cooling. The multi-layered meridional flow, which is expected to exist in the deep Pacific, is shown to form for certain vertical profiles of the side wall cooling.  相似文献   

Target strength of the lanternfish, Stenobrachius leucopsarus (family Myctophidae), a fish without an airbladder, measured in the Bering Sea   总被引：1，自引：0，他引：1  

Yasuma  Hiroki; Takao  Yoshimi; Sawada  Kouichi; Miyashita  Kazushi; Aoki  Ichiro 《ICES Journal of Marine Science》2006,63(4):683-692

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Simulation of Formation and Spreading of Salinity Minimum Associated with NPIW Using a High-Resolution Model   总被引：1，自引：2，他引：1  

Hiroshi Ishizaki  Ichiro Ishikawa 《Journal of Oceanography》2004,60(2):463-485

A series of numerical experiments were conducted with a high-resolution (eddy-permitting) North Pacific model to simulate the formation and spreading of the salinity minimum associated with the North Pacific Intermediate Water (NPIW). It was found that two factors are required to simulate a realistic configuration of the salinity minimum: a realistic wind stress field and small-scale disturbances. The NCEP reanalyzed wind stress data lead to better results than the Hellerman and Rosenstein wind stress data, due to the closer location of the simulated Oyashio and Kuroshio at the western boundary. Small-scale disturbances formed by relaxing computational diffusivity included in the advection scheme promote the large-scale isopycnal mixing between the Oyashio and Kuroshio waters, simulating a realistic configuration of the salinity minimum. A detailed analysis of the Oyashio water transport was carried out on the final three-year data of the experiment with reduced computational diffusivity. Simulated transport of the Kuroshio Extension in the intermediate layer is generally smaller than the observed value, while those of the Oyashio and the flow at the subarctic front are comparable to the observed levels. In the Oyashio-Kuroshio interfrontal zone the zonally integrated southward transport of the Oyashio water (140–155°E) is borne by the eddy activity, though the time-mean flow reveals the existence of a coastal Oyashio intrusion. In the eastern part (155°E–180°) the zonally integrated transport of the Oyashio water indicates a southward peak at the southern edge of the Kuroshio Extension, which corresponds to the branching of the recirculating flow from the Kuroshio Extension. This revised version was published online in July 2006 with corrections to the Cover Date.  相似文献   

Swimming behaviour and target strength of isada krill (Euphausia pacifica)   总被引：1，自引：0，他引：1  

Miyashita  Kazushi; Aoki  Ichiro; Inagaki  Tadashi 《ICES Journal of Marine Science》1996,53(2):303-308

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