Magnetic pulsation Pi2 as a sensitive indicator of magnetospheric substorm     

T. Saito  K. Yumoto  Y. Koyama 《Planetary and Space Science》1976,24(11):1025-1029

The identification of substorm onset is quite important in studying the mechanism of excitation of substorm disturbances. At present, one of the most practical methods to identify accurately the onset of substorms is to use low-latitude Pi2, which is sensitively related to the plasma instability in the magnetosphere, that triggers the substorm disturbances. This method is applied to examine the onset times of three substorm events which were already defined by various methods other than the low-latitude Pi2 method. Preceding the onset times, other evident substorm onsets are clearly determined with Pi2 onsets for all the three events by examining only the H-component of rapid-run magnetogram from a single low-latitude station in the dark hemisphere. Cooperative monitoring of Pi2 at three low-latitude stations on three well-separated meridians, therefore, is really effective in detecting most substorms.  相似文献   

The arch systems,cavities and prominences in the helmet streamer observed at the solar eclipse,November 12, 1966     

Kuniji Saito  E. Tandberg-Hanssen 《Solar physics》1973,31(1):105-121

Arch systems lying above quiescent prominences in the solar corona have long drawn the attention of eclipse observers, and such formations have been investigated since the end of the last century. Almost every eclipse photograph shows one or more arches, and in most cases the arch system is accompanied by a quiescent prominence below it and a helmet streamer above it. Also, in some cases there is a dark cavity between the arch system and the prominence.On large-scale photographs obtained at the November 12, 1966 eclipse, detailed photometry has been carried out on a formation in the corona composed of a helmet streamer straddling two multiple-arch systems each with a dark cavity and a quiescent prominence. The excess of electrons in the arches and the deficiency in the cavities are evaluated. We find that the formation of a prominence requires much more material than available in the cavity before depletion. Consequently the condensation theory of coronal matter into prominences seems to have difficulties explaining the necessary amount of matter in the cases where coronal arches - delineating magnetic field lines above the cavity - may exclude inflow of material from the corona. We comment on the low velocity of solar wind in the helmet streamer.The National Center for Atmospheric Research is sponsored by the National Science Foundation.  相似文献   

Vertical distributions of large ontogenetically migrating copepods in the Oyashio region during their growing season     

Atsushi Tsuda  Hiroaki Saito  Hiromi Kasai 《Journal of Oceanography》2014,70(2):123-132

Neocalanus flemingeri, Neocalanus plumchrus, Neocalanus cristatus and Eucalanus bungii are large and dominant mesozooplankton occurring throughout the subarctic Pacific. They are an important trophic link and transporter of organic matter to the mesopelagic zone. Vertical distributions of these copepods were investigated from March to October 2000 in the Oyashio region of the western subarctic Pacific. Neocalanus plumchrus and N. flemingeri were distributed in the surface layer (0–50 m) and N. cristatus and E. bungii in the subsurface layer (50–100 m). However, when examined in detail, clear seasonal and vertical differences were observed. Neocalanus plumchrus was concentrated in the top 20 m from late April to the end of July, and N. flemingeri showed a little deeper distribution from May to July. Neocalanus cristatus showed a deeper distribution than that of grazing individuals of E. bungii from April to early July, but grazing individuals of E. bungii (C3–C6) showed a deeper distribution than that of N. cristatus from the end of July to October. Early copepodites of E. bungii were distributed much shallower than late copepodite stages and overlapped with copepodites of N. plumchrus and N. flemingeri. These results suggest that the four species of large copepods have established habitat segregation by season, vertical distribution and food resource partitioning in the Oyashio region as well as other regions of the North Pacific.  相似文献   

Burial of organic carbon in Holocene sediments of the Zhujiang (Pearl River) and Changjiang (Yangtze River) estuaries   总被引：1，自引：0，他引：1  

Shouye Yang  Min Tang  Wyss W.-S. Yim  Yongqiang Zong  Guangqing Huang  Adam D. Switzer  Yoshiki Saito 《Marine Chemistry》2011,123(1-4):1-10

The East Asian marginal seas are important sinks of terrigenous materials transported by large rivers. In this study two cores from the Changjiang (Yangtze River) and Zhujiang (Pearl River) estuaries and one core from the inner shelf off of Hong Kong were investigated to examine the burial of organic matter during the postglacial period and its possible links with paleoenvironmental changes. Based on a simple two end-member mixing model, the terrestrial organic matter supplied primarily from the Zhujiang and Changjiang dominates the estuarine areas while marine organic matter contributes more to the inner-mid shelf. The competing contributions of terrestrial and marine organic matter are responsible for the downcore variations of organic elemental compositions. The overall decreasing shifts of total organic carbon concentrations and total organic carbon to total nitrogen ratios in the Zhujiang estuarine sediments since 6.5 kaBP seems to support the notion that depth profiles of organic matter compositions deposited in the Zhujiang Estuary can aid in the reconstruction of monsoon history in the Holocene. Nevertheless, organic matter compositions in the Changjiang Estuary and inner shelf off of southeastern Hong Kong respond in a different and more complex way to freshwater discharges or precipitation changes (monsoon variability) in the catchments, owing to complex controls of deposition and preservation of organic matter in these estuarine and shelf environments. Caution is therefore needed in using organic elemental and isotopic compositions to decipher paleoenvironmental changes in East Asian continental shelves where intense river–sea interactions occur and sedimentary environments change drastically.  相似文献   

Seasonal variation of bivalve larvae on an exposed sandy beach on Kashima-nada: Tips for the sandy beach recruitment process     

Hideki Sawada  Hajime Saito  Kumiko Adachi  Haruhiko Toyohara 《Journal of Sea Research》2011,65(2):275-283

Bivalves are often the dominant macrobenthos species in exposed sandy beach environments. However, our understanding of their recruitment processes before post-settlement stages on sandy beaches with highly energetic environments is incomplete. To clarify the characteristics of the free-swimming planktonic stage that affects recruitment efficiency in sandy shore ecosystems, we investigated the temporal (weekly–biweekly) variation of bivalve planktonic larval concentration coupled with oceanographic conditions on an exposed sandy shore on the sea of Kashima-nada, Japan, from summer 2003 to autumn 2005. Larvae were observed throughout the year, but the surge of larval concentration composed of sandy beach and sessile bivalves occurred most prominently in summer, from August to September. The peak concentration of larvae during this season was more than 1000 times higher than in other seasons. The larval concentration was positively correlated with water temperature and northward wind velocity and negatively correlated with each of the nutrient concentrations. On the other hand, chlorophyll a concentration and salinity seemed to have little effect on the larval concentration. Based on this fundamental knowledge, further investigations about planktonic larvae in sandy beaches are needed.  相似文献   

Mineralogy of new Antarctic achondrites with affinity to Lodran and a model of their evolution in an asteroid     

Hiroshi Takeda  Hiroshi Mori  Takahiro Hiroi  Jun Saito 《Meteoritics & planetary science》1994,29(6):830-842

Abstract— We studied five new Antarctic achondrites, MacAlpine Hills (MAC) 88177, Yamato (Y)74357, Y75274, Y791491 and Elephant Moraine (EET)84302 by mineralogical techniques to gain a better understanding of the mineral assemblages of a group of meteorites with an affinity to Lodran (stony-iron meteorite) and their formation processes. This group is being called lodranites. These meteorites contain major coarse-grained orthopyroxene (Opx) and olivine as in Lodran and variable amounts of FeNi metal and troilite etc. MAC88177 has more augite and less FeNi than Lodran; Y74357 has more olivine and contains minor augite; Y791491 contains in addition plagioclase. EET84302 has an Acapulco-like chondritic mineral assembladge and is enriched in FeNi metal and plagioclase, but one part is enriched in Opx and chromite. The EET84302 and MAC88177 Opx crystals have dusty cores as in Acapulco. EET84302 and Y75274 are more Mg-rich than other members of the lodranite group, and Y74357 is intermediate. Since these meteorites all have coarse-grained textures, similar major mineral assemblages, variable amounts of augite, plagioclase, FeNi metal, chromite and olivine, we suggest that they are related and are linked to a parent body with modified chondritic compositions. The variability of the abundances of these minerals are in line with a proposed model of the surface mineral assemblages of the S asteroids. The mineral assemblages can best be explained by differing degrees of loss or movements of lower temperature partial melts and recrystallization, and reduction. A portion of EET84302 rich in metal and plagioclase may represent a type of component removed from the lodranite group meteorites. Y791058 and Caddo County, which were studied for comparison, are plagioclase-rich silicate inclusions in IAB iron meteorites and may have been derived by a similar process but in a different body.  相似文献   

Demonstration of crystalline forsterite grain formation due to coalescence growth of Mg and SiO smoke particles     

C. Kaito  Y. Ojima  K. Kamitsuji  O. Kido  Y. Kimura  H. Suzuki  T. Sato  T. Nakada  Y. Saito  C. Koike 《Meteoritics & planetary science》2003,38(1):49-57

Abstract— Experimental studies of coalescence between Mg grains and SiO grains in smoke reveal the direct production of crystalline forsterite grains. The present results also show that different materials can be produced by grain‐grain collisions, which have been considered one of the models of grain formation in the interstellar medium. The fundamentals of coalescence growth in smoke, which have been developed in our series of experiments, are presented in this paper. Mg₂Si polyhedral grains were obtained in a Mg grain‐rich atmosphere. Mg₂SiO₄ polyhedral grains were obtained in a SiO grain‐rich atmosphere. The IR spectra of the resultant grains showed the characteristics of crystalline forsterite.  相似文献   

Influence of plasma on formation of crystalline Fe₂SiO₄ grains     

T. Sato  M. Kurumada  O. Kido  M. Shintaku  Y. Saito 《Planetary and Space Science》2006,54(6):612-616

The production of Fe₂SiO₄ (fayalite) crystalline grains was performed by two processes, namely, grain formation in a plasma field by evaporating a mixture powder of Fe and SiO and heat treatment of the product collected on the radio-frequency (RF) electrode side. Fe grains <20 nm in size covered with an amorphous SiO layer selectively formed Fe₂SiO₄ grains by heating at 800 °C. By heating at 600 °C, in addition to the formation of Fe₂SiO₄ crystal grains, the FeO phase appeared. The doping effect of excited oxygen in a plasma field into the Fe small grains may be the trigger on the formation of fayalite through the FeO phase formation. The present experimental result suggests that the probability of Fe₂SiO₄ grain formation in space is low.  相似文献   

Hydromagnetic waves driven by velocity shear instability in the magnetospheric boundary layer     

Kiyohumi Yumoto  Takao Saito 《Planetary and Space Science》1980,28(8):789-798

The bulk flow of the solar wind plasma in the flank-side of the magnetospheric boundary layer, where the magnetic field lines are closed, has a component transverse to the ambient field. There is quite a strong velocity shear. The theoretical model ignores inhomogeneities in the ambient field and the mass density which occur at the magnetopause on about the same length scale as that of the velocity shear.Consideration is restricted to hydromagnetic waves which have a k-vector nearly normal to the B_o-V_o plane, i.e., approximately the magnetopause surface (k_x >k_z ～ k_y′k_xL_B > 1 and L_B = 0.1 ～ 1.0 R_E where L_B is a characteristic length of the boundary layer). It is found that a long-period (T ? 40 sec) hydromagnetic wave [the Alfvén-like wave ($\text{Ω}{}_{\mathrm{<mtext>A</mtext>}}$)] driven by velocity shear instability can be excited in the shear plasma. It is also found that the group velocity of the HM-wave is directed almost along the magnetic field line and that the magnetic variance in the shear plasma tends to be parallel to the B_o-V_o plane. The velocity shear instability in the magnetospheric boundary layer is judged to be a likely source of long-period magnetic pulsations.  相似文献   

Experimental Study of the Applicability of the Remotely Positioned Laser Doppler Vibrometer to Rock-Block Stability Assessment     

Gui-Chen Ma  Kazuhide Sawada  Atsushi Yashima  Hideki Saito 《Rock Mechanics and Rock Engineering》2015,48(2):787-802

This paper examines a new method for evaluating the stability of rock blocks on slopes using a remotely positioned Laser Doppler Vibrometer (LDV). A series of experiments using physical models were conducted to evaluate the validity of this new method. Based on the experimental studies, the applicability of LDV was examined by comparing results with a conventional seismometer measurement. To examine the quantitative correlations between vibration properties and the stability of a rock block, the effects on the vibration properties of the size of the rock block, the initial block position, the slope incline, and the type of ground surface were studied. The experimental results showed that LDV measurements agreed with conventional seismometer measurements. There was also a good correlation between vibration properties and rock-block stability. On the other hand, it was found that for a boulder on tightly compacted ground, the application of block stability assessment by tonometry was difficult when measuring microtremors or sloppy vibration due to nearby vehicle traffic. Furthermore, numerical analysis of the slope model was carried out to examine the validity of the model experiment and application of the suggested technique. The results of the analysis demonstrated that the suggested technique was effective for application to stability monitoring of a block and evaluation of the effect of stability measures.  相似文献