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21.
Expected wavelengths and intensities are computed for 1s2l–1snl transitions in helium-like ions of the abundant elements from oxygen to iron, under coronal conditions. Probable observations of some of these lines, in the spectra of solar flares, are discussed and attention is called to a possible reversal of singlet and triplet intensities as compared to laboratory observations. 相似文献
22.
Sidney O. Kastner 《Astrophysics and Space Science》1984,103(2):331-344
Resonant photoexcitation of one atomic species by another often involves a ground configuration level from which an important fine structure line, used for density or temperature determination, may be emitted. It is pointed out that if the photoexcitation rate is sufficiently high the intensity of the latter may be affected. Estimates of the effect are made for several cases of astrophysical interest. 相似文献
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Matthew J. Wooller John W. Pohlman Benjamin V. Gaglioti Peter Langdon Miriam Jones Katey M. Walter Anthony Kevin W. Becker Kai-Uwe Hinrichs Marcus Elvert 《Journal of Paleolimnology》2012,48(1):27-42
Atmospheric contributions of methane from Arctic wetlands during the Holocene are dynamic and linked to climate oscillations. However, long-term records linking climate variability to methane availability in Arctic wetlands are lacking. We present a multi-proxy ~12,000?year paleoecological reconstruction of intermittent methane availability from a radiocarbon-dated sediment core (LQ-West) taken from a shallow tundra lake (Qalluuraq Lake) in Arctic Alaska. Specifically, stable carbon isotopic values of photosynthetic biomarkers and methane are utilized to estimate the proportional contribution of methane-derived carbon to lake-sediment-preserved benthic (chironomids) and pelagic (cladocerans) components over the last ~12,000?years. These results were compared to temperature, hydrologic, and habitat reconstructions from the same site using chironomid assemblage data, oxygen isotopes of chironomid head capsules, and radiocarbon ages of plant macrofossils. Cladoceran ephippia from ~4,000?cal?year BP sediments have ??13C values that range from ~?39 to ?31??, suggesting peak methane carbon assimilation at that time. These low ??13C values coincide with an apparent decrease in effective moisture and development of a wetland that included Sphagnum subsecundum. Incorporation of methane-derived carbon by chironomids and cladocerans decreased from ~2,500 to 1,500?cal?year BP, coinciding with a temperature decrease. Live-collected chironomids with a radiocarbon age of 1,640?cal?year BP, and fossil chironomids from 1,500?cal?year BP in the core illustrate that ??old?? carbon has also contributed to the development of the aquatic ecosystem since ~1,500?cal?year BP. The relatively low ??13C values of aquatic invertebrates (as low as ?40.5??) provide evidence of methane incorporation by lake invertebrates, and suggest intermittent climate-linked methane release from the lake throughout the Holocene. 相似文献
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Thomas G. Müller Emmanuel Lellouch Hermann Böhnhardt John Stansberry Antonella Barucci Jacques Crovisier Audrey Delsanti Alain Doressoundiram Elisabetta Dotto René Duffard Sonia Fornasier Olivier Groussin Pedro J. Gutiérrez Olivier Hainaut Alan W. Harris Paul Hartogh Daniel Hestroffer Jonathan Horner Dave Jewitt Mark Kidger Csaba Kiss Pedro Lacerda Luisa Lara Tanya Lim Michael Mueller Raphael Moreno Jose-Luis Ortiz Miriam Rengel Pablo Santos-Sanz Bruce Swinyard Nicolas Thomas Audrey Thirouin David Trilling 《Earth, Moon, and Planets》2009,105(2-4):209-219
Over one thousand objects have so far been discovered orbiting beyond Neptune. These trans-Neptunian objects (TNOs) represent the primitive remnants of the planetesimal disk from which the planets formed and are perhaps analogous to the unseen dust parent-bodies in debris disks observed around other main-sequence stars. The dynamical and physical properties of these bodies provide unique and important constraints on formation and evolution models of the Solar System. While the dynamical architecture in this region (also known as the Kuiper Belt) is becoming relatively clear, the physical properties of the objects are still largely unexplored. In particular, fundamental parameters such as size, albedo, density and thermal properties are difficult to measure. Measurements of thermal emission, which peaks at far-IR wavelengths, offer the best means available to determine the physical properties. While Spitzer has provided some results, notably revealing a large albedo diversity in this population, the increased sensitivity of Herschel and its superior wavelength coverage should permit profound advances in the field. Within our accepted project we propose to perform radiometric measurements of 139 objects, including 25 known multiple systems. When combined with measurements of the dust population beyond Neptune (e.g. from the New Horizons mission to Pluto), our results will provide a benchmark for understanding the Solar debris disk, and extra-solar ones as well. 相似文献
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Katherine?M.?PrimmEmail author Gregory?P.?Schill Daniel?P.?Veghte Miriam?Arak?Freedman Margaret?A.?Tolbert 《Journal of Atmospheric Chemistry》2017,74(1):55-69
Mineral dust particles are known to be efficient ice nuclei in the atmosphere. Previous work has probed heterogeneous ice nucleation on various laboratory dust samples including Arizona Test Dust, kaolinite, montmorillonite, and illite as atmospheric dust surrogates. However, it has recently been suggested that NX illite may be a better representation of atmospheric dust. Hiranuma et al. (2015) performed a laboratory comparison for immersion ice nucleation on NX illite, but here we focus on depositional ice nucleation because of its importance in low temperature cirrus cloud formation. A Raman microscope setup was used to examine the ice-nucleating efficiency of NX illite. Organic coatings on the NX illite particles were also investigated using a mixture of 5 dicarboxylic acids (M5). The ratio of NX illite to M5 was varied from 1:10 to 100:1. It was found that NX illite efficiently nucleates ice with Sice = 1.07 ± 0.01 at ?47 °C, with Sice slightly increasing at lower temperatures. In contrast, pure M5 is a poorer ice nucleus with Sice = 1.30 ± 0.02 at ?40 °C, relatively independent of temperature. Further, it was found that M5 coatings on the order of several monolayers thick hindered the ice nucleating ability of NX illite. Optical images suggest that at colder temperatures (< ?50 °C) 1:1 NX illite:M5 particles and pure M5 particles nucleate ice depositionally, while at warmer temperatures (> ?50 °C) subsaturated immersion ice nucleation dominates. These experiments suggest that mineral dust particles may become less active towards ice nucleation as they age in the atmosphere. 相似文献
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Miriam Marzen Mario Kirchhoff Irene Marzolff Ali Aït Hssaine Johannes B. Ries 《地球表面变化过程与地形》2020,45(15):3808-3823
The endemic argan woodlands cover large parts of South Morocco and create a characteristic landscape with areas of sparsely vegetated and bare soil surfaces between single trees. This unique ecosystem has been under extensive agrosilvopastoral management for centuries and is now at risk of degradation caused by overgrazing and increasing scarcity and variability of rainfall. To investigate susceptibility to wind erosion, we conducted an experimental–empirical study including wind tunnel tests and a drone-generated digital elevation model and quantified wind-erodible material on five different associated surface types by means of sediment catchers. The highest emission flux was measured on freshly ploughed surfaces (1875 g m–2 h–1), while older ploughed areas with a re-established crust produced a much lower emission flux (795 g m–2 h–1). Extensive tillage may have been a sustainable practice for generations, but increasing drought and uncertainty of rainfall now lead to an acute risk of severe soil erosion and dust production. The typical crusted surfaces characterized by residual rock fragment accumulation and wash processes produced the second highest emission flux (1,354 g m–2 h–1). Material collected from tree-shaded areas (933 g m–2 h–1) was revealed to be a considerable source of organic material, possibly affecting substrate conditions positively on a larger regional scale. The lowest flux was measured on rock fragment-covered surfaces (301 g m–2 h–1). The data show that open argan woodland may be a considerable source for wind erosion and dust production, depending on surface characteristics strongly related to management. An adapted management must include the conservation of argan trees to offer a promising approach to prevent severe wind erosion and dust production and mitigate possible impacts of land-use change and climate change related shifts in wind and rainfall patterns. © 2020 The Authors. Earth Surface Processes and Landforms published by John Wiley & Sons Ltd 相似文献
30.
Anomalous patterns of the sedimentary architecture have been recognized in passive margins, and only recently they have been associated with plate reorganization or compressional deformations propagating from distant margins. With the aim of discussing the sedimentary architecture and the potential tectonic perturbations to the passive margin pattern, we present the revision of the stratigraphy of a fossil passive margin, involved in the retrobelt of the Alpine orogeny. The main events at the transition from rifted to passive margin have been controlled by palaeoceanography, i.e. the trophic state of surface waters that hampered the carbonate photozoan productivity for a long period between Toarcian and Callovian. Toward the latest Bajocian–earliest Bathonian, the platform productivity increased, dominated by ooids. A regressional trend up to the Middle Bathonian allowed the rapid infilling of the previous rift basin. The successive aggradation in the platform was still dominated by non-skeletal grains until the Early Oxfordian. The Middle Oxfordian to Early Kimmeridgian was a time of recovery of the palaeoceanographic conditions allowing the establishment of a hydrozoan/coral rich platform. The sedimentation rates in the platform increased at the margin of the productive Friuli–Adriatic Platform. From Late Kimmeridgian on, the sedimentation rates at the platform margin returned to the pre-Oxfordian values. At the scale of the whole Adriatic Platform, the Middle Oxfordian to Early Kimmeridgian interval is variable in thickness from 0 to 800 m, and it depicts a couple of folds of around 80–100 km of wavelength. The subsidence analysis of wells and composite logs from literature suggests this interval as a perturbation to the passive margin trend of around 3 Myr of duration. We interpret this folding event, superimposed to the passive margin subsidence, as the far field expression of the transition from intraoceanic to continental obduction, occurred at the eastern Adria active margin. 相似文献