Photometric observations of 107P/Wilson-Harrington     

Seitaro Urakawa  Shin-ichiro Okumura  Tsuyoshi Sakamoto  Shinsuke Abe  Kohei Kitazato  Sunao Hasegawa  Nobuyuki Kawai  Shogo Nagayama  Michitoshi Yoshida 《Icarus》2011,215(1):17-26

We present lightcurve observations and multiband photometry for 107P/Wilson-Harrington using five small- and medium-sized telescopes. The lightcurve has shown a periodicity of 0.2979 day (7.15 h) and 0.0993 day (2.38 h), which has a commensurability of 3:1. The physical properties of the lightcurve indicate two models: (1) 107P/Wilson-Harrington is a tumbling object with a sidereal rotation period of 0.2979 day and a precession period of 0.0993 day. The shape has a long axis mode (LAM) of L₁:L₂:L₃ = 1.0:1.0:1.6. The direction of the total rotational angular momentum is around λ = 310°, β = −10°, or λ = 132°, β = −17°. The nutation angle is approximately constant at 65°. (2) 107P/Wilson-Harrington is not a tumbler. The sidereal rotation period is 0.2979 day. The shape is nearly spherical but slightly hexagonal with a short axis mode (SAM) of L₁:L₂:L₃ = 1.5:1.5:1.0. The pole orientation is around λ = 330°, β = −27°. In addition, the model includes the possibility of binary hosting. For both models, the sense of rotation is retrograde. Furthermore, multiband photometry indicates that the taxonomy class of 107P/Wilson-Harrington is C-type. No clear rotational color variations are confirmed on the surface.  相似文献   

Simultaneous MITSuME g'R_CI_C monitoring of S5 0716+714     

C. S. Stalin  Koji. S. Kawabata  Makoto Uemura  Michitoshi Yoshida  Nobuyuki Kawai  Kenshi Yanagisawa  Yasuhiro Shimizu  Daisuke Kuroda  Shogo Nagayama  Hiroyuki Toda 《Monthly notices of the Royal Astronomical Society》2009,399(3):1357-1366

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Evaluation of the Diurnal Warming of Sea Surface Temperature Using Satellite-Derived Marine Meteorological Data   总被引：2，自引：0，他引：2  

Yoshimi Kawai  Hiroshi Kawamura 《Journal of Oceanography》2002,58(6):805-814

In order to produce a high-quality sea surface temperature (SST) data set, the daily amplitude of SST (ΔSST) should be accurately known. The purpose of this study was to evaluate the diurnal variation of sea surface temperature in a simple manner. The authors first simulated ΔSST with a one-dimensional numerical model using buoy-observed meteorological data and satellite-derived solar radiation data. When insolation is strong, the model-simulated 1-m-depth ΔSST becomes much smaller than the in situ value as wind speed decreases. By forcibly mixing the sea surface layer, the model ΔSST becomes closer to the in situ value. It can be considered that part of this difference is due to the turbulence induced by the buoy hull. Then, on the assumption that the model results were reliable, the authors derived a regression equation to evaluate ΔSST at the skin and 1-m depth from daily mean wind speed (U) and daily peak solar radiation (PS). ΔSST is approximately proportional to In(U) and (PS)², and the skin ΔSST estimated by the equation is not inconsistent with in situ observation results reported in past studies. The authors prepared maps of PS and U using only satellite data, and demonstrated the ΔSST evaluation over a wide area. The result showed that some wide patchy areas where the skin ΔSST exceeds 3.0 K can appear in the tropics and the mid-latitudes in summer. This revised version was published online in July 2006 with corrections to the Cover Date.  相似文献   

Scale dependence of divergence and vorticity of near-surface flows in the sea     

Hideo Kawai 《Journal of Oceanography》1985,41(3):157-166

To area-average the horizontal divergence and the vertical component of vorticity, three methods are proposed and examined. The polygon method is based on deformations of a polygon made by connecting drifters. They are deployed at widely different scales of 1 cm^– 1,000 km and tracked using various procedures. The loop method is adopted when a drifter completes at least two loops of trajectory in a tidal vortex, a ring or a gyre. Even if data for a drifter completing only one loop is available, the vorticity can be calculated. The crossing method is applied to the GEK data on the circumference of a Kuroshio ring. The data which will be used to calculate them in Part 2 are summarized in tables. Offset dispositions of positive and negative divergences or vorticities on a horizontal plane and in a water column are shown. Probably, the vertical offset of vorticities does not occur in general. The area-turnover of a polygon of drifters are discussed. Sampling time-intervals, appropriate to the scale of the area, for the polygon and loop methods are examined. A first impression of Rhines' (1979) sketch has produced a misunderstanding that the polygon method would be useless because a limited number of drifters cannot follow such a complicated deformation of the material line over a long period. It is shown that adopting a short time-scale appropriate to the length scale furnishes a practical solution to the problem.  相似文献   

Measurement of skin friction distribution along the surface of wind waves     

Kuniaki Okuda  Sanshiro Kawai  Yoshiaki Toba 《Journal of Oceanography》1977,33(4):190-198

Measurements of local values of the skin friction have been made at many points along the surface of representative wind wave crests in a wind wave tunnel, by use of the distortion of hydrogen-bubble lines. The results obtained at 2.85-m fetch under 6.2 m s^–1 mean wind speed show that the intensity of the skin friction varies greatly along the surface of wind waves as a function of the phase angle. It increases rather continuously at the windward surface toward the crest, attains a value of about 12 dyn cm^–2 near the crest, decreases suddenly just past the crest, and the value at the lee surface is substantially zero Values of the skin friction thus determined along the representative wind waves give an average value of 3.6 dyn cm^–2, rather exceeding the overall stress value of 3.0 dyn cm^–2, which has been estimated from the wind profile. The results are interpreted as that the skin friction bears most of the shearing stress of wind, and that it exerts most intensively around the representative wave crests at their windward faces.  相似文献   

Interpretation of Repulsive Bifurcation of the Kuroshio and Gulf Stream with Classification of Their Branches     

Hideo Kawai 《Journal of Oceanography》2003,59(3):303-315

Distinctive features of the repulsive bifurcation of Kuroshio and Gulf Stream jets, induced from various observations, have been examined by equations of motion of a 1 1/2 -layer model in natural coordinates, although causes of the bifurcation are discussed little. From the conservation of volume flux through paths above a main thermocline, we have derived the equation M _n K _In + M _s K _Is = 0 for northern (n) and southern (s), steady-state curved jets, just downstream of the bifurcation from a straight jet, where M is momentum flux through the path and K _I, positive for cyclonic deflection of the streamline, is the curvature of a certain intermediate streamline in the laminar path. Assuming concentric streamlines in the curved path, which is narrower than 1–0.5 times the radius of curvature of the center streamline of the path, and symmetric momentum flux through a path element of unit width with respect to the center streamline, we found that K _I approximates to the curvature averaged across the path, relative errors being <0.25−(0.25)². Bifurcation starts with a ‘Big Bang’ and the leading turbulent front of a split jet may be unsteady, but the succeeding laminar part can be regarded as stationary or as translating at a constant velocity, as if frozen. Thus, such features are described by the equation. A provisional classification is given of various warm extrusions along branches of both Streams, all of which exemplify the repulsive bifurcation, except for the mainstreams with very narrow (< 30–40 km) branches, possibly due to misapplication of the model. This revised version was published online in August 2006 with corrections to the Cover Date.  相似文献   

Field data support of three-seconds power law andgu *σ−4-spectral form for growing wind waves     

Sanshiro Kawai  Kozo Okada  Yoshiaki Toba 《Journal of Oceanography》1977,33(3):137-150

Observational data on air-sea boundary processes at the Shirahama Oceanographic Tower Station, Kyoto University, obtained in November, 1969, was analyzed and presented as an example representing the structure of growing wind-wave field. The condition was an ideal onshore wind, and the data contained continuous records of the wind speed at four heights, the wind direction, the air and water temperatures, the tides, and the growing wind waves, for more than six hours. The main results are as follows. Firstly, in both of the wind speed and the sea surface wind stress, rather conspicuous variations of about six-minute period were appreciable. Secondly, the three-seconds power law and its lemma expressed byH ^*=BT ^*3/2 and=2BT ^*–1/2, respectively, are very well supported by the data, whereH ^*(gH/u _* ²) andT ^*(gT/u _*) are the dimensionless significant wave height and period, respectively, the wave steepness,u _* the friction velocity of air,g the acceleration of gravity, andB=0.062 is a universal constant. Thirdly, the spectral form for the high-frequency side of the spectral maximum is well expressed by the form of()= _sgu_*^–4, where is the angular frequency and() the spectral density. The value of _s is determined as 0.062±0.010 from the observational data. There is a conspicuous discrepancy between the spectral shape of wind waves obtained in wind-wave tunnels and those in the sea, the former containing well-defined higher harmonics of the spectral peak, and consequently there is an apparent difference in the values of _s also. However, it is shown that the discrepancy of _s may be eliminated by evaluating properly the energy level of the spectral form containing higher harmonics.  相似文献   

Wide field X-ray monitor on board the High Energy Transient Experiment (HETE)     

A. Yoshida  M. Matsuoka  N. Kawai  F. Tokanai  E. E. Fenimore  J. J. M. In 'T Zand  M. Galassi  M. Yamauchi  K. Takagishi  I. Hatsukade 《Astrophysics and Space Science》1995,231(1-2):463-466

The High-Energy Transient Experiment (HETE) is designed for the multiwavelengths study of Gamma-Ray Bursts (GRBs) in UV, X-ray and gamma-ray range with three scientific instruments. The X-ray instrument, Wide-field X-ray Monitor (WXM), consists of four units of one-dimensional position sensitive gas proportional counters and two perpendicularly oriented one-dimensional coded apertures. The WXM has a wide FOV of 1.5 steradian together with the capability to locate GRBs with 10 arcmin accuracy, and covers photon energies of 2 to 25 keV with an energy resolution of typically 18 % at 6 keV, measuring wide band spectra together with the gamma-ray spectrometer (FREGATE). The coded X-ray image will be deconvolved on board and the GRB location will be provided to the UV camera within 1 sec . GRB locations will also be broadcast in real time to ground-based observers for follow-up observations.  相似文献   

Orbital signals found in physical and chemical properties of bottom sediments from Lake Baikal     

Kenji Kashiwaya  Toshio Nakamura  Nobuki Takamatsu  Hideo Sakai  Manabu Nakamura  Takayoshi Kawai 《Journal of Paleolimnology》1997,18(3):293-297

Physical and chemical properties of two 100 m sediment cores (BDP-93-1, 93-2) obtained from the Buguldeika saddle of Lake Baikal in the eastern Siberia and a ¹⁴C-based age scale for the core show that the core bottom is about 400000 years ago and that the changes in the sedimentological environment of the area during the interval were that comparatively coarse and high C/N ratio sediments accumulated in the lake during interglacial periods, and fine material and low C/N ratio during glacial periods. The tentative age scale suggests that the first excursion in the earth's magnetic field at about 26 m (BDP-93-1 and 93-2) from the sediment surface corresponds to the Blake event. Statistical analyses of the data-sets for the some properties show that the fluctuations have distinct periods; 20000 years, 40000 years and 100000 years, that are related to the Milankovitch parameters and support that the tentative age scale is approximately acceptable.  相似文献   

Meridional flow in thick accretion disks     

Yasushi Kawai 《Astrophysics and Space Science》1989,151(1):81-101

We have studied the effect of the flow in the accretion disk. The specific angular momentum of the disk is assumed to be constant and the polytropic relation is used. We have solved the structure of the disk and the flow patterns of the irrotational perfect fluid.As far as the obtained results are concerned, the flow does not affect the shape of the configuration in the bulk of the disk, although the flow velocity reaches even a half of the sound velocity at the inner edge of the disk. Therefore, in order to study accretion disk models with the moderate mass accretion rate—i.e.,

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