Nature of sedimentary deposits in the western Makgadikgadi basin, Botswana     

Susan Ringrose  Bill Downey  Dane Genecke  Francis Sefe  Bernard Vink 《Journal of Arid Environments》1999,43(4):375

Quaternary sedimentation in the western Makgadikgadi basin of north central Botswana is evaluated on the basis of new evidence from satellite imagery and sedimentological analyses. Thematic Mapper imagery interpretation, combined with field evidence, has led to the identification of geomorphological features which are mainly composed of light grey calcareous sandstones (formerly calcretes) overlain by dark grey sands. The literature suggests that palaeolake Makgadikgadi I formed and developed intermittently after initial downwarping in the early–mid Pleistocene. The calcareous sandstones were formed when calcium carbonate precipitation took place in pre-existing Kalahari sands along the western shoreline of Makgadikgadi I. Field evidence, supported by X-ray diffraction and SEM analyses, indicates that CaCO₃precipitated mainly in marshy conditions around plant roots and stems and in association with bacteria in embayments along the lakeshore. The sandstones thickened and became partially indurated as a result of increasing palaeolake levels. Deposition was terminated by renewed tectonism which uplifted the shoreline zone relative to the lake basin, leading to falling palaeolake levels. Post-uplift reworking led to case hardening and pedogenic calcrete formation in the upper sections of the calcareous sandstones. Sedimentary conditions altered during the late Pleistocene. Extensive distributaries from the proto-Okavango system incised the shoreline ridge contributing to the filling of Makgadikgadi II. Satellite data suggest that the proto-Okavango rivers formed a series of fan deltas at this time along the western Makgadikgadi basin. Widespread dispersal of fluvial grey sands took place as a result of basin tilting which led to anastomosing channels flowing southward possibly around 18,000 B.P. These results, although preliminary in nature, augment previous geomorphological analyses by adding some detail in terms of depositional environments and by providing a tentative age and origin for the ubiquitous grey sands.  相似文献   

A New Magnetically Active Binary System Discovered in Yunnan-Hong Kong Wide Field Survey     

WANG Jian-hua  GU Sheng-hong  WANG Xiao-bin  Bill YEUNG  Eric NG 《Chinese Astronomy and Astrophysics》2021,45(1):67-81

We present a newly discovered magnetically active binary system detected by Yunnan-Hong Kong wide field survey, with an orbital period of 0.60286 days. Two color photometry for the system was performed using the 1 m Cassegrain telescope of Yunnan Observatories with its CCD (Charge-Coupled Device) camera. In the observed light curves, there are clearly different light maxima existed in the out-of-eclipse regions. We made spectroscopic observations for the binary system using the 2.4 m telescope and YFOSC (Yunnan Faint Object Spectrograph and Camera) of Lijiang station of Yunnan Observatories, China. The radial velocity curve was derived for primary star of the binary system. The primary star exhibited strong chromospheric activity, which confirms that the distortion of the light curves results from the starspot activity on the primary star. Through analyzing the light curves and RV (Radial Velocity) curve mentioned above by means of the W-D (Wilson-Devinney) code, orbital parameters and starspot configuration of the binary system are obtained. Finally, we have discussed the properties of the binary system, and given the prospects on the future work.  相似文献   

Submillisecond fireball timing using de Bruijn timecodes          下载免费PDF全文

Robert M. Howie  Jonathan Paxman  Philip A. Bland  Martin C. Towner  Eleanor K. Sansom  Hadrien A. R. Devillepoix 《Meteoritics & planetary science》2017,52(8):1669-1682

Long‐exposure fireball photographs have been used to systematically record meteoroid trajectories, calculate heliocentric orbits, and determine meteorite fall positions since the mid‐20th century. Periodic shuttering is used to determine meteoroid velocity, but up until this point, a separate method of precisely determining the arrival time of a meteoroid was required. We show it is possible to encode precise arrival times directly into the meteor image by driving the periodic shutter according to a particular pattern—a de Bruijn sequence—and eliminate the need for a separate subsystem to record absolute fireball timing. The Desert Fireball Network has implemented this approach using a microcontroller driven electro‐optic shutter synchronized with GNSS UTC time to create small, simple, and cost‐effective high‐precision fireball observatories with submillisecond timing accuracy.  相似文献   

The British seas: Hardisty, J. London, Routledge, 1990. 272 pp. £35.00 hardback     

Bill Carter 《Applied geography (Sevenoaks, England)》1991,11(4)

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CAL PACKAGES FROM UKESCC     

Dave Bryon  Bill Sowerbutts 《地学学报》1996,8(3):293-295

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Airglow from Jupiter's nightside and crescent: Ultraviolet spectrometer observations from Voyager 2     

John C. McConnell  Bill R. Sandel  A.Lyle Broadfoot 《Icarus》1980,43(2):128-142

The Voyager Ultraviolet Spectrometer has made extensive observations of airglow from Jupiter's dark-side equatorial latitudes. The brightness of H Lyman α (Lyα), the only emission detected, varies between 700 and 1000 rayleighs (R) as a function of lungitude. The dark side of Jupiter is illuminated by sky background Lyα arising from resonance scattering of the solar Lyα line by the neutral hydrogen of the interstellar medium. Calculations show that resonance scattering of this sky background by hydrogen in Jupiter's thermosphere will produce about 300 R of Lyα emission. The additional Lyα observed is probably excited by electrons and protons precipitating at equatorial latitudes. Based on the 500-R upper limit set here on the dark-side H₂ Lyman and Werner bands, and the Lyα measurements, the exciting particles are thought to have a soft energy spectrum and deposit about 0.04 erg cm^?2 sec^?1 in the atmosphere. There is evidence for an asymmetrical precipitation pattern associated with the longitudinal variation in Ly α emission, and a suggestion of a strong day-night difference in precipitation as well.  相似文献   

The Hard X-ray Imaging Spectrometer (HXIS)     

H. Frank Van Beek  Peter Hoyng  Bill Lafleur  George M. Simnett 《Solar physics》1980,65(1):39-52

The HXIS, a joint instrument of the Space Research Laboratory at Utrecht, The Netherlands, and the Department of Space Research of the University of Birmingham, U.K., images the Sun in hard X-rays: Six energy bands in energy range 3.5–30 keV, spatial resolution 8 over Ø 240 and 32 over Ø 624 field of view, and time resolution of 0.5–7 s depending on the mode of operation. By means of a flare flag it alerts all the other SMM instruments when a flare sets in and informs them about the location of the X-ray emission. The experiment should yield information about the position, extension and spectrum of the hard X-ray bursts in flares, their relation to the magnetic field structure and to the quasi-thermal soft X-rays, and about the characteristics and development of type IV electron clouds above flare regions.  相似文献   

Mind mapping as an interactive tool for engaging complex geographical issues     

David Lloyd  Bill Boyd  Kristin Den Exter 《New Zealand geographer》2010,66(3):181-188

Students and communities engaging with complex geographical issues require tools to assist them structure and contextualise data. This paper examines the use of mind mapping as a tool in geographical education. Case studies are presented from a sample of undergraduate and cross-cultural educational settings demonstrating the use of mind mapping in communication, analysis and problem solving. In critiquing this educational tool, we conclude that mind mapping is particularly effective with communities whose cultures are strong on visual communication but weaker on written communication and in cross-cultural situations where use of written English would disadvantage learners.  相似文献   

Precise orbit determination for the FORMOSAT-3/COSMIC satellite mission using GPS   总被引：2，自引：1，他引：1  

Cheinway Hwang  Tzu-Pang Tseng  Tingjung Lin  Dražen Švehla  Bill Schreiner 《Journal of Geodesy》2009,83(5):477-489

The joint Taiwan–US mission FORMOSAT-3/ COSMIC (COSMIC) was launched on April 17, 2006. Each of the six satellites is equipped with two POD antennas. The orbits of the six satellites are determined from GPS data using zero-difference carrier-phase measurements by the reduced dynamic and kinematic methods. The effects of satellite center of mass (COM) variation, satellite attitude, GPS antenna phase center variation (PCV), and cable delay difference on the COSMIC orbit determination are studied. Nominal attitudes estimated from satellite state vectors deliver a better orbit accuracy when compared to observed attitude. Numerical tests show that the COSMIC COM must be precisely calibrated in order not to corrupt orbit determination. Based on the analyses of the 5 and 6-h orbit overlaps of two 30-h arcs, orbit accuracies from the reduced dynamic and kinematic solutions are nearly identical and are at the 2–3 cm level. The mean RMS difference between the orbits from this paper and those from UCAR (near real-time) and WHU (post-processed) is about 10 cm, which is largely due to different uses of GPS ephemerides, high-rate GPS clocks and force models. The kinematic orbits of COSMIC are expected to be used for recovery of temporal variations in the gravity field.  相似文献   

Erratum: Simulating flow in karst aquifers at laboratory and sub-regional scales using MODFLOW-CFP   总被引：1，自引：0，他引：1  

Josue Jacob Gallegos  Bill X. Hu  Hal Davis 《Hydrogeology Journal》2013,21(8):1911-1912

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