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471.
The KASKADE and CORSIKA air shower generators are compared to the data collected by MASS2 balloon experiment in 1991. The test of longitudinal profile for proton, helium and muon flux production provide good constraints on these air shower generators. KASKADE and CORSIKA especially with the new simulator UrQMD for low energies are found to fit these data well. This study is limited to a comparison of longitudinal profiles and therefore does not provide constraints on the overall shower development. 相似文献
472.
Christian Koeberl Wolf Uwe Reimold Gordon Cooper Duncan Cowan Pierre M. Vincent 《Meteoritics & planetary science》2005,40(9-10):1455-1471
Abstract— Shuttle Radar Topography Mission data was used to investigate the Aorounga and Gweni Fada impact structures in Chad as part of a new remote sensing study. We believe the results of various data treatments provide extensive new perspective on the macro‐structural and topographic divisions for these two impact structures. Our remote sensing studies indicate revised diameters of Aorounga and Gweni Fada of 16 and 22 km, respectively. We selected samples from these two structures for their petrographic, geochemical, and Rb‐Sr and Sm‐Nd isotopic characteristics. In samples from both structures, evidence for shock metamorphism was found in the form of single or multiple sets of planar deformation features in quartz, which confirms the impact origin for both the Aorounga and Gweni Fada structures. The crystallographic orientations of PDFs indicate maximum shock levels of 20–30 GPa for samples from the central parts of both structures. The PDF orientations are characteristic for the orientations observed elsewhere in shocked sandstones, with the higher angles in the orientation histograms being fairly abundant. Geochemically, the rocks are typical upper‐crustal sandstones. 相似文献
473.
Sylvie Jancart Anne Lemaitre Vincent Letocart 《Celestial Mechanics and Dynamical Astronomy》2003,86(4):363-383
Interested in the role of the inclination in our results (Jancart and Lemaitre, 2001), we analyze the process of resonance trapping due to general dissipation forces in the frame of the spatial restricted three body problem and in the case of external mean motion resonances. We compute our simulations by using the three-dimensional Extended Schubart Averaging integrator developed by Moons (1994) for all mean motion resonances. We complete it by adding the averaged contributions of general dissipative forces like Murray has proposed in the article (1994). The behavior of the inclination is especially pointed out. 相似文献
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478.
Guneshwar Thangjam Andreas Nathues Kurt Mengel Martin Hoffmann Michael Schäfer Vishnu Reddy Edward A. Cloutis Ulrich Christensen Holger Sierks Lucille Le Corre Jean‐Baptiste Vincent Christopher T. Russell 《Meteoritics & planetary science》2014,49(10):1831-1850
We present an analysis of olivine‐rich exposures at Bellicia and Arruntia craters using Dawn Framing Camera (FC) color data. Our results confirm the existence of olivine‐rich materials at these localities as described by Ammannito et al. ( 2013a ) using Visual Infrared Spectrometer (VIR) data. Analyzing laboratory spectra of various howardite–eucrite–diogenite meteorites, high‐Ca pyroxenes, olivines, and olivine‐orthopyroxene mixtures, we derive three FC spectral band parameters that are indicators of olivine‐rich materials. Combining the three band parameters allows us, for the first time, to reliably identify sites showing modal olivine contents >40%. The olivine‐rich exposures at Bellicia and Arruntia are mapped using higher spatial resolution FC data. The exposures are located on the slopes of outer/inner crater walls, on the floor of Arruntia, in the ejecta, as well as in nearby fresh small impact craters. The spatial extent of the exposures ranges from a few hundred meters to few kilometers. The olivine‐rich exposures are in accordance with both the magma ocean and the serial magmatism model (e.g., Righter and Drake 1997 ; Yamaguchi et al. 1997 ). However, it remains unsolved why the olivine‐rich materials are mainly concentrated in the northern hemisphere (approximately 36–42°N, 46–74°E) and are almost absent in the Rheasilvia basin. 相似文献
479.
Data assimilation techniques, developed in the past two decades mainly for weather prediction, produce better forecasts by
taking advantage of both theoretical/numerical models and real-time observations. In this paper, we explore the possibility
of applying the four-dimensional variational data assimilation (4D-VAR) technique to the prediction of solar flares. We do
so in the context of a continuous version of the classical cellular-automaton-based self-organized critical avalanche models
of solar flares introduced by Lu and Hamilton (Astrophys. J.
380, L89, 1991). Such models, although a priori far removed from the physics of magnetic reconnection and magnetohydrodynamical evolution of coronal structures, nonetheless
reproduce quite well the observed statistical distribution of flare characteristics. We report here on a large set of data
assimilation runs on synthetic energy release time series. Our results indicate that, despite the unpredictable (and unobservable)
stochastic nature of the driving/triggering mechanism within the avalanche model, 4D-VAR succeeds in producing optimal initial
conditions that reproduce adequately the time series of energy released by avalanches and flares. This is an essential first
step toward forecasting real flares. 相似文献
480.
Kenneth F. Rijsdijk Julian P. Hume Frans Bunnik F.B. Vincent Florens Claudia Baider Beth Shapiro Johannes van der Plicht Anwar Janoo Owen Griffiths Lars W. van den Hoek Ostende Holger Cremer Tamara Vernimmen Perry G.B. De Louw Assenjee Bholah Salem Saumtally Nicolas Porch James Haile Mike Buckley Matthew Collins Edmund Gittenberger 《Quaternary Science Reviews》2009,28(1-2):14-24
Although the recent history of human colonisation and impact on Mauritius is well documented, virtually no records of the pre-human native ecosystem exist, making it difficult to assess the magnitude of the changes brought about by human settlement. Here, we describe a 4000-year-old fossil bed at Mare aux Songes (MAS) in south-eastern Mauritius that contains both macrofossils (vertebrate fauna, gastropods, insects and flora) and microfossils (diatoms, pollen, spores and phytoliths). With >250 bone fragments/m2 and comprising 50% of all known extinct and extant vertebrate species (ns = 44) of Mauritius, MAS may constitute the first Holocene vertebrate bone Concentration-Lagerstätte identified on an oceanic volcanic island. Fossil remains are dominated by extinct giant tortoises Cylindraspis spp. (63%), passerines (~10%), small bats (7.8%) and dodo Raphus cucullatus (7.1%). Twelve radiocarbon ages [four of them duplicates] from bones and other material suggest that accumulation of fossils took place within several centuries. An exceptional combination of abiotic conditions led to preservation of bones, bone collagen, plant tissue and microfossils. Although bone collagen is well preserved, DNA from dodo and other Mauritian vertebrates has proved difficult. Our analysis suggests that from ca 4000 years ago (4 ka), rising sea levels created a freshwater lake at MAS, generating an oasis in an otherwise dry environment which attracted a diverse vertebrate fauna. Subsequent aridification in the south-west Indian Ocean region may have increased carcass accumulation during droughts, contributing to the exceptionally high fossil concentration. The abundance of floral and faunal remains in this Lagerstätte offers a unique opportunity to reconstruct a pre-human ecosystem on an oceanic island, providing a key foundation for assessing the vulnerability of island ecosystems to human impact. 相似文献