Automated cloud tracking system for the Akatsuki Venus Climate Orbiter data     

Kazunori Ogohara  Toru Kouyama  Hiroki Yamamoto  Naoki Sato  Masahiro Takagi  Takeshi Imamura 《Icarus》2012,217(2):661-668

Japanese Venus Climate Orbiter, Akatsuki, is cruising to approach to Venus again although its first Venus orbital insertion (VOI) has been failed. At present, we focus on the next opportunity of VOI and the following scientific observations.We have constructed an automated cloud tracking system for processing data obtained by Akatsuki in the present study. In this system, correction of the pointing of the satellite is essentially important for improving accuracy of the cloud motion vectors derived using the cloud tracking. Attitude errors of the satellite are reduced by fitting an ellipse to limb of an imaged Venus disk. Next, longitude–latitude distributions of brightness (cloud patterns) are calculated to make it easy to derive the cloud motion vectors. The grid points are distributed at regular intervals in the longitude–latitude coordinate. After applying the solar zenith correction and a highpass filter to the derived longitude–latitude distributions of brightness, the cloud features are tracked using pairs of images. As a result, we obtain cloud motion vectors on longitude–latitude grid points equally spaced. These entire processes are pipelined and automated, and are applied to all data obtained by combinations of cameras and filters onboard Akatsuki. It is shown by several tests that the cloud motion vectors are determined with a sufficient accuracy. We expect that longitude–latitude data sets created by the automated cloud tracking system will contribute to the Venus meteorology.  相似文献   

Resolving power of surface wave polarization data for higher-order heterogeneities     

Kazunori Yoshizawa  Kiyoshi Yomogida  & Seiji Tsuboi 《Geophysical Journal International》1999,138(1):205-220

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Electron beam emission from the EXOS-B (JIKIKEN) satellite as a powerful diagnostic tool in the magnetosphere     

Nobuki Kawashima  Kazunori Akai  Yukio Murasato  Susumu Sasaki 《Astrophysics and Space Science》1984,106(1):117-123

In an electron beam emission experiment on board the EXOS-B (JIKIKEN) satellite (200 V, 1 mA-maximum), several types of waves are strongly excited by the beam such as plasma frequency, upper hybrid frequency, electron cyclotron frequency, their harmonics and nonlinear coupling of these waves. Measurements of these waves give information on local plasma density and magnetic field strength and it is revealed that the electron beam emission from the spacecraft is a powerful diagnostic tool in the magnetosphere. A long term observation in this electron beam experiment has provided us with the average plasma density profile in the magnetosphere. It is also useful for the detection of the plasmapause. Plasma density measurements down to the order of 10 cm^–3 are possible. The instrument itself is very simple and compact, so that it will be a powerful plasma diagnostic tool in future magnetospheric and planetary explorations.  相似文献   

Path Transition of the Kuroshio Due to Mesoscale Eddies: A Two-Layer, Wind-Driven Experiment   总被引：1，自引：0，他引：1  

Kazunori Akitomo  Masao Kurogi 《Journal of Oceanography》2001,57(6):735-741

We have executed numerical experiments using a two-layer, wind-driven ocean model with simplified coastal geometry and bottom topography to investigate the possibility of the Kuroshio path transition due to mesoscale eddies. A straight path easily changes into a meandering path due to the eddy action. For this transition, an anticyclonic eddy is preferable to a cyclonic one when imposed in the beginning region of the Kuroshio (east of Taiwan). When imposed southeast of Kyushu, on the other hand, a cyclonic eddy is more effective than an anticyclonic one. The reverse transition, from a meandering to a straight path, did not occur at all in this experiment. This revised version was published online in August 2006 with corrections to the Cover Date.  相似文献   

Deep convection in a lake triggered by wind: Two-dimensional numerical experiments with a nonhydrostatic model     

Kazunori Akitomo  Kiyoshi Tanaka  Toshiyuki Awaji  Norihisa Imasato 《Journal of Oceanography》1995,51(2):171-185

We have investigated the fundamental processes of deep convection in a lake at high latitudes triggered by wind during spring or autumn and the associated deep water formation, executing vertically two-dimensional numerical experiments with a nonhydrostatic model. The water column in which a relatively cold mixed layer overlies a relatively warm layer becomes unstable, when the Ekman convergence on the shore due to along-shore wind deepens the mixed layer below the compensation depth, where water densities in both layers becomes equal to each other because of the thermobaric effect. At the onset of deep convection, the critical Rayleigh number agrees with that predicted by the linear theory. The onset time of deep convection is inversely proportional to the magnitude of wind stress. On the other hand, the onset time is minimal when water temperature in the mixed layer _m is 3.1°C because a change of _m has two effects oppositely acting on the stability of the water column. After the first onset, deep convection occurs intermittently for a few days. The sinking of the mixed layer water occurs in a thermal-like shape, and its amount is 4184% of the time-integrated Ekman transport when _m 3°C while it decreases to less than 10% for _m lower than 1.5°C. The present process can explain 30% of the amount of deep water renewal which is expected from the observation in Lake Baikal.  相似文献   

The galactic warp and rotations of the fundamental system     

M. Miyamoto  M. Yoshizawa  S. Suzuki 《Astrophysics and Space Science》1991,177(1-2):399-408

The proper motion in galactic latitude of O-B stars enables us to detect the kinematic behaviour of an optical counterpart of the large-scale warp of the HI gas layer in our Galaxy. A selected set of the proper motions of about 350 O-B stars within 3kpc from the sun (R₀=8.5kpc) is analyzed on the proper motion systems of N30, FK4, and FK5. A remarkable differece in the kinematic behaviour of the warp appears between the old systems (N30 and FK4) and FK5-system. On the old systems, the O-B stars in the belt 8.5kpcR<9.5kpc exhibit a systematic z-motion upward from the galactic plane forl180° and downward forl>180° with the mean proper motions of about ±0".4/century, respectively. On the other hand, the results on the FK5-system show no meaningful systematic z-motion, even though the O-B star layer exterior to the solar circle is inclined (3°) with respect to the galactic plane. These findings can neither be inferred from the model of the oblique material flow nor from the concepts of the precessional stellar rings and of the bending oscillation of a stellar disk. The remarkable difference in the kinematic behaviour of the warp, appearing between the old and new systems, is caused mainly by the conversion of the proper motions on the old systems into those on the J2000.0 frame. The conversion near the galactic plane is given by µ_b(FK4(J2000.0))–µ_b(FK4)–0.50 sinl/century. The implication of this relation is discussed in connection with the warping motion of stars detected here.  相似文献   

A trial for monitoring temporal variation of seismic velocity using an ACROSS system   总被引：4，自引：0，他引：4  

Koshun Yamaoka  Takahiro Kunitomo  Koji Miyakawa  Kazunori Kobayashi  Mineo Kumazawa 《Island Arc》2001,10(3-4):336-347

Abstract The temporal variation of seismic velocity near the Nojima Fault, which ruptured during the 1995 Kobe earthquake (Hyogo-ken Nanbu earthquake), was detected using an accurately controlled routine-operated seismic source (ACROSS). The source generates elastic waves by a centrifugal force of an eccentric mass rotating around an axis. The mass is driven with an AC servomotor whose angular position is accurately controlled with reference to a very accurate global positioning system (GPS) clock. The error of the mass' position is less than 0.002 radian and does not accumulate. As a result, the source generates sinusoidal waves of very narrow spectral peaks enabling their detection with an excellent signal-to-noise ratio. Although the stability of the rotation is quite excellent, a large daily variation was found, which seems to be caused by changes in atmospheric temperature. The daily variation was 10% in amplitude and 0.1 radian in phase of the signal observed at the 800 m borehole seismometer. A significant variation was found to be due to that of coupling between the rotational source and the foundation made of reinforced concrete in which the source was situated. In order to make a correction on the signal of the 800 m borehole seismometer, the vibration of the foundation was measured and modeled assuming a rigid body movement. The correction successfully reduced the daily variation by approximately 90%, resulting in a variation of 1% in amplitude and 0.01 radian in phase. The phase variation of 0.01 radian corresponds to 100 μs and less than 0.1% in velocity over 1000 m between the source and the receiver.  相似文献   

Early exhumation of the collisional orogen and concurrent infill of foredeep basins in the Miocene Eurasian–Okhotsk Plate boundary, central Hokkaido, Japan: Inferences from K–Ar dating of granitoid clasts     

Gentaro Kawakami  Kazunori Arita  Toshinori Okada  Tetsumaru Itaya 《Island Arc》2004,13(2):359-369

Abstract   Thick Middle (–Upper) Miocene turbiditic deposits filled very deep and narrow foredeep basins formed in the western margin of the Hidaka collision zone in central Hokkaido. Cobble- to boulder-sized clasts of eight monzogranites and a single granodiorite in the Kawabata Formation in the Yubari Mountains area yielded biotite K–Ar ages of 44.4 ± 1.0 to 45.4 ± 1.0 Ma and 42.8 ± 1.1 Ma, respectively. Major elemental compositions of the clasts all fall in the field of S-type granite on an NK/A (Na₂O + K₂O/Al₂O₃ in molecule) versus A/CNK (Al₂O₃/CaO + Na₂O + K₂O in molecule) diagram, verifying their peraluminous granite character (aluminium saturation index (ASI): 1.12–1.19). These geochronological and petrographical features indicate that the granitoid clasts in the Kawabata Formation correlate with Eocene granitic plutons in the northeastern Hidaka Belt, specifically the Uttsudake (43 Ma) and Monbetsu (42 Ma) plutons. Foredeep basins are flexural depressions developed at the frontal side of thickened thrust wedges. The results presented here suggest that deposition of the Middle Miocene turbidites was coeval with rapid westward up-thrusting and exhumation of the Hidaka Belt. This early mountain building may have occurred in response to thrusting in the Tertiary fold-and-thrust system of central Hokkaido.  相似文献   

Dynamical behaviour of planetesimals temporarily captured by a planet from heliocentric orbits: basic formulation and the case of low random velocity     

Kazunori Iwasaki  Keiji Ohtsuki 《Monthly notices of the Royal Astronomical Society》2007,377(4):1763-1771

Planetesimals encountering with a planet cannot be captured permanently unless energy dissipation is taken into account, but some of them can be temporarily captured in the vicinity of the planet for an extended period of time. Such a process would be important for the origin and dynamical evolution of irregular satellites, short-period comets, and Kuiper-belt binaries. In this paper, we describe the basic formulation for the study of temporary capture of planetesimals from heliocentric orbits using three-body orbital integration, such as the definition of the duration and rate of temporary capture, and present results in the case of low random velocity of planetesimals. In the case of planetesimals initially on circular orbits, we find that planetesimals undergo a close encounter with the planet before they become temporarily captured. When planetesimals are scattered by the planet into the vicinity of one of periodic orbits around the planet, the duration of temporary capture tends to be extended. Typically, these capture orbits are in the retrograde direction around the planet. We evaluate the rate of temporary capture of planetesimals, and find that the ratio of this rate to their collision rate on to the planet increases with increasing semimajor axis of the planet. Similar results are obtained for planetesimals with non-zero but small random velocities, as long as Kepler shear dominates the relative velocity between the planet and planetesimals. For larger initial random velocities of planetesimals, temporary capture in both prograde and retrograde directions with much longer duration becomes possible.  相似文献   

Non-linear waveform inversion for surface waves with a neighbourhood algorithm—application to multimode dispersion measurements     

K. Yoshizawa  B. L. N. Kennett 《Geophysical Journal International》2002,149(1):118-133

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