Characterization of zonal winds in the stratosphere of Titan with UVES     

D. Luz  T. Civeit  J.-P. Lebreton  P. Rannou  O. Witasse  F. Ferri 《Icarus》2005,179(2):497-510

Titan has been observed with UVES, the UV-Visual Echelle Spectrograph at the Very Large Telescope, with the aim of characterizing the zonal wind flow. We use a retrieval scheme originally developed for absolute stellar accelerometry [Connes, P., 1985. Astrophys. Space Sci., 110, 211-255] to extract the velocity signal by simultaneously taking into account all the lines present in the spectrum. The method allows to measure the Doppler shift induced at a given point by the zonal wind flow, with high precision. The short-wavelength channel (4200-5200 Å) probes one scale height higher than the long-wavelength one (5200-6200 Å), and we observe statistically significant evidence for stronger winds at higher altitudes. The results show a high dispersion. Globally, we detect prograde zonal winds, with lower limits of 62 and 50 m s⁻¹ at the regions centered at 200 and 170 km altitude, but approximately a quarter of the measurements indicates null or retrograde winds.  相似文献   

Diurnal variation of the dayside, ionospheric, mid-latitude trough in the southern hemisphere at 800 km: Model and measurement comparison   总被引：1，自引：0，他引：1  

J.J. Sojka  W.J. Raitt  R.W. Schunk  J.L. Parish  F.J. Rich 《Planetary and Space Science》1985,33(12):1375-1382

Our high latitude ionospheric model predicts the existence of a pronounced “dayside” trough in plasma concentration equatorward of the auroral oval in both the Northern and Southern Hemispheres for solar maximum, winter, and low geomagnetic activity conditions. The trough in the Southern Hemisphere is much deeper than that in the Northern Hemisphere, with the minimum trough density at 800 km being 2 × 10³ cm⁻³ in the Southern Hemisphere and 10⁴ cm⁻³ in the Northern Hemisphere. The dayside trough has a strong longitudinal (diurnal) dependence and appears between 11:00 and 19:00 U.T. in the Southern Hemisphere and between 02:00 and 08:00 U.T. in the Northern Hemisphere. This dayside trough is a result of the auroral oval moving to larger solar zenith angles at those universal times when the magnetic pole is on the antisunward side of the geographic pole. As the auroral ionization source moves to higher geographic latitudes, it leaves a region of declining photoionization on the dayside. For low convection speeds, the ionosphere decays and a dayside trough forms. The trough is deeper in the Southern Hemisphere than in the Northern Hemisphere because of the greater offset between the geomagnetic and geographic poles. Satellite data taken in both the Northern and Southern Hemispheres confirm the gross features of the dayside trough, including its strong longitudinal dependence, its depth, and the asymmetry between the Northern and Southern Hemisphere troughs.  相似文献   

Seismic reflection measurements behind the Hikurangi convergent margin, southern North Island, New Zealand.     

F.J. Davey 《Geophysical Journal International》1987,89(1):443-448

Summary. The active Australian-Pacific plate boundary passes through New Zealand. In the north, the Pacific plate subducts beneath the Australian plate with an accretionary wedge forming the eastern continental (Hikurangi) margin of the North Island. The structure of the region behind the Hikurangi margin changes from the extensional back-arc basin under central North Island to a postulated crustal downwarp under the southern North Island. A 100 km long multichannel seismic reflection profile was recorded across the region of crustal downwarp. The data show discontinuous coherent reflectors dipping westwards at the east end of the profile, and east dipping reflectors at the west end, from depths of 9 to 15 s two way time. Simple hand migration of these events indicate that the east dipping reflectors, interpreted as the base of the Australian plate crust, abut against the west dipping reflectors which are interpreted as marking the top of the subducted Pacific plate. Detailed earthquake hypocentre locations in the area show a dipping zone of high seismicity, the top of which coincides closely with the west dipping events, thus supporting this interpretation.  相似文献   

Chronology and evolution of the northern Iceland Plateau     

J. CAMPSIE  M. H. RASMUSSEN  L. C. KOVACS  F. DITTMER  J. C. BAILEY  N. O. HANSEN  J. LAURSEN  L. JOHNSON 《Polar research》1990,8(2):237-243

New aeromagnetic data, K-Ar age determinations of dredged marine igneous rocks, as well as other geophysical evidence have shed light on the chronology, nature and evolution of the northern Iceland Plateau. Correspondence between seismic refraction profiles taken on the Jan Mayen Ridge and westward through Jan Mayen Island, suppressed aeromagnetic anomalies, earthquake surface wave studies, and ages of dredged igneous rocks suggest these strata may form an extended region of thickened crust, possibly of Caledonian age, extending westward toward the Kolbeinsey Ridge and northwest to the south wall of the Jan Mayen Fracture Zone.  相似文献   

Modeling X-Ray Photoionized Plasmas Produced at the Sandia Z-Facility     

P. A. M. Van Hoof  M. E. Foord  R. F. Heeter  J. E. Bailey  H.-K. Chung  M. E. Cuneo  W. H. Goldstein  V. Jonauskas  F. P. Keenan  R. Kisielius  D. A. Liedahl  C. Ramsbottom  S. J. Rose  P. T. Springer  R. S. Thoe 《Astrophysics and Space Science》2005,298(1-2):147-153

In experiments at the high-power Z-facility at Sandia National Laboratory in Albuquerque, New Mexico, we have been able to produce a low density photoionized laboratory plasma of Fe mixed with NaF. The conditions in the experiment allow a meaningful comparison with X-ray emission from astrophysical sources. The charge state distributions of Fe, Na and F are determined in this plasma using high resolution X-ray spectroscopy. Independent measurements of the density and radiation flux indicate unprecedented values for the ionization parameter ξ = 20–25 erg cm s⁻¹ under nearly steady-state conditions. First comparisons of the measured charge state distributions with X-ray photoionization models show reasonable agreement, although many questions remain.  相似文献   

Weathering process at the natural fission reactor of Bangombé     

G. Bracke  S. Salah  F. Gauthier-Lafaye 《Environmental Geology》2001,40(4-5):403-408

 The uranium deposits in the basin of Franceville (Gabon) host the only natural fission reactors known in the world. Unique geological conditions favoured a natural fission reaction 2 Ga ago. This was detected by anomalous isotopic compositions of uranium and rare earth elements (REE), which are produced by the fission reaction. In total, 16 reactor zones were found. Most of them are mined out. The reactor zone of Bangombé, is only 10–11 m below the surface. This site has been influenced by surface weathering processes. Six drill cores have been sampled at the site of the reactor zone of Bangombé during the course of the study and only one drill core (BAX 08) hit the core of the reactor. From these data and previous drilling campaigns, the reactor size is estimated to be 10 cm thick, 2–3 m wide and 4–6 m long. The migration of fission products can be traced by the anomalous isotope ratios of REE because of the fission process. The ¹⁴⁹Sm/¹⁴⁷Sm ratio close to the reactor zone is only 0.28 (normal: 0.92) because of the intense neutron capture of ¹⁴⁹Sm and subsequent transmutation, whereas ¹⁴⁷Sm is enriched by the fission reaction. Similar changes in isotopic patterns are detectable on other REE. The isotope ratios of Sm and Nd of whole rock and fracture samples surrounding the reactor indicate that fission-genic REE migrated only a few decimetres above and mainly below the reactor zone. Organic matter (bitumen) seems to act as a trap for fission-genic REE. Additional REE-patterns show less intense weathering with increasing depth in the log profile and support a simple weathering model. Received: 26 November 1999 · Accepted: 2 May 2000  相似文献   

Optical and X-Ray Observations of the Symbiotic System Ag Draconis During Quiescence and Outburst     

Roberto F. Viotti  Rosario González-Riestra  Takashi Iijima  Stefano Bernabei  Riccardo Claudi  Jochen Greiner  Michael Friedjung  Vito Francesco Polcaro  Corinne Rossi 《Astrophysics and Space Science》2005,296(1-4):435-439

We review the main characteristics of the symbiotic system AG Draconis, with special emphasis on its optical and X-ray variations. We also discuss the X-ray to visual energy distribution during quiescence and outburst and describe our spectroscopic and X-ray observations during the 2003 outburst. Based on X-ray observations collected with the XMM–Newton Observatory, on INES data from the IUE satellite, and on optical spectra collected with the Asiago–Cima Ekar, Bologna–Loiano and La Palma–Galileo Italian telescopes.  相似文献   

Cosmic metal production and the mean metallicity of the Universe     

F. Calura  F. Matteucci 《Monthly notices of the Royal Astronomical Society》2004,350(1):351-364

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Simultaneous operation of the Sea Beam multibeam echo-sounder andthe SeaMARC II bathymetric sidescan sonar system     

de Moustier  C. Lonsdale  P.F. Shor  A.N. 《Oceanic Engineering, IEEE Journal of》1990,15(2):84-94

An experiment aboard the Scripps Institution of Oceanography's RV Thomas Washington has demonstrated the seafloor mapping advantages to be derived from combining the high-resolution bathymetry of a multibeam echo-sounder with the sidescan acoustic imaging plus wide-swath bathymetry of a shallow-towed bathymetric sidescan sonar. To a void acoustic interference between the ship's 12-kHz Sea Beam multibeam echo-sounder and the 11-12-kHz SeaMARC II bathymetric sidescan sonar system during simultaneous operations, Sea Beam transmit cycles were scheduled around SeaMARC II timing events with a sound source synchronization unit originally developed for concurrent single-channel seismic, Sea Beam, and 3.5-kHz profile operations. The scheduling algorithm implemented for Sea Beam plus SeaMARC II operations is discussed, and the initial results showing their combined seafloor mapping capabilities are presented  相似文献   

X-shaped radio galaxies as observational evidence for the interaction of supermassive binary black holes and accretion disc at parsec scale     

F. K. Liu 《Monthly notices of the Royal Astronomical Society》2004,347(4):1357-1369

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