1:1 Ground-track resonance in a uniformly rotating 4th degree and order gravitational field     

Jinglang Feng  Ron Noomen  Xiyun Hou  Pieter Visser  Jianping Yuan 《Celestial Mechanics and Dynamical Astronomy》2017,127(1):67-93

Using a gravitational field truncated at the 4th degree and order, the 1:1 ground-track resonance is studied. To address the main properties of this resonance, a 1-degree of freedom (1-DOF) system is firstly studied. Equilibrium points (EPs), stability and resonance width are obtained. Different from previous studies, the inclusion of non-spherical terms higher than degree and order 2 introduces new phenomena. For a further study about this resonance, a 2-DOF model which includes a main resonance term (the 1-DOF system) and a perturbing resonance term is studied. With the aid of Poincaré sections, the generation of chaos in the phase space is studied in detail by addressing the overlap process of these two resonances with arbitrary combinations of eccentricity (e) and inclination (i). Retrograde orbits, near circular orbits and near polar orbits are found to have better stability against the perturbation of the second resonance. The situations of complete chaos are estimated in the \(e-i\) plane. By applying the maximum Lyapunov Characteristic Exponent (LCE), chaos is characterized quantitatively and similar conclusions can be achieved. This study is applied to three asteroids 1996 HW1, Vesta and Betulia, but the conclusions are not restricted to them.  相似文献   

Assessing “Urban Karst” Effects from Groundwater–Storm Sewer System Interaction in a Till Aquitard     

Martin G. Shepley  Nick Schmidt  Matt J. Senior  Stephen R. H. Worthington  Ron B. Scheckenberger 《Ground water》2020,58(2):269-277

Storm sewer systems and their associated utility trenches may strongly influence the effects of urbanization on a groundwater system. This study was undertaken to identify the causes of district-wide basement infiltration in an aquitard system. It comprised widespread continuous monitoring of utility trench wells and dye tracing from storm sewer system exfiltration tests. The results indicate that a major effect of urbanization on shallow groundwater is related to storm sewer system exfiltration, which is marked by a characteristic pattern of head variations in the aquitard unrelated to distributed surface infiltration. The aquitard constrains flow from storm sewer system exfiltration to the utility trench, creating an urban flow path for groundwater discharge. Temporary buildup of water levels in the utility trench drives relatively high-velocity flow through the permeable sewer bedding material of the utility trench to a separate foundation drainage collector system, ultimately causing a severe “urban karst” effect that produces system surcharging and widespread basement water infiltration. The main conditions causing the “urban karst” are the large hydraulic conductivity ratio between the utility trench material and the aquitard, and the shallow depth and low gradient of the storm sewer system imposed by a very flat drainage basin.  相似文献   

Overlooked small and juvenile piscivores dominate shallow-water estuarine “refuges” in tropical Australia     

Ron Baker  Marcus Sheaves 《Estuarine, Coastal and Shelf Science》2009

A model was developed to estimate the relative impacts of different functional groups of piscivores on the shallow-water estuarine nursery assemblage of tropical north-eastern Australia. Data on variability in the occurrence, number and type of fish in the diet of different piscivores was combined with estimates of the abundance of each group. The model predicts that previously overlooked small and occasional or ‘minor’ piscivores, such as sillaginids, ambassids, sparids and small juvenile carangids, inflict mortality on new recruits to shallow-water nursery habitats that is orders of magnitude greater than that imparted by more conspicuous larger piscivores. Because of their high abundance, a shift in the diet of minor piscivores to prey on new recruits results in a massive increase in the consumption of fish prey by the piscivore assemblage as a whole. Even if the evidence that minor piscivores switch to target new recruits was rejected, the model shows that the highly abundant minor piscivores must still exert a significant proportion of predation mortality experienced by recruiting fishes. As a broad functional group, minor piscivores occur in most aquatic systems around the globe and are likely to play an important but largely overlooked role as predators that shape communities through predation on critical early life stages of other fishes.  相似文献   

The Geography of American Poverty: Is There a Need for Place‐Based Policies? by Mark D. Partridge and Dan S. Rickman     

Ron Mitchelson 《The Professional geographer》2007,59(4):551-553

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Geography and Revolution – Edited by David N. Livingstone and Charles W. J. Withers     

Ron Johnston 《Singapore journal of tropical geography》2007,28(2):246-248

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Possibilities of soil spectroscopy for the classification of contaminated areas in river floodplains     

Lammert Kooistra  Ron Wehrens  Lutgarde MC Buydens  Rob S. E. W. Leuven  Piet H Nienhuis 《International Journal of Applied Earth Observation and Geoinformation》2001,3(4):337

During the past decades, large amounts of diffuse contaminated soil material have been deposited in the floodplains of the river Rhine in the Netherlands. The dynamic character of this river causes a large spatial variability in the contamination level of its floodplain soils. Characterisation of the spatial variability exclusively based on soil sampling and analysis is often insufficient and expensive. Hyperspectral images can provide additional spatial information for a proper characterisation of the contamination situation of river floodplains. This paper describes the possible application of soil spectroscopy to estimate metal concentration levels in river floodplains. Soil reflectance spectra in the visible-near infrared region (VNIR) were measured in the laboratory for soil samples taken from two river floodplains along the river Waal, the main tributary of the river Rhine in the Netherlands. A multivariate calibration procedure using partial least squares (PLS) regression was applied to establish a relationship between reflectance spectra in the visible-near infrared (VNIR) region and spectrally active soil characteristics (organic matter and clay content) that are intercorrelated with concentration levels of Cd and Zn. Results of the analysis of two river floodplains are summarised and the influence of scale-level and sub soil material on the prediction capability is discussed.  相似文献   

Philip W Hill 1937–1999     

Ron Hilditch 《Astronomy& Geophysics》2000,41(1):1.35-1.36

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Polarimetry and spectroscopy of RX J1141.3−6410: a single‐pole AM Her system     

David A. H. Buckley  Mark Cropper  Kurt van der Heyden  S. B. Potter  Dayal T. Wickramasinghe 《Monthly notices of the Royal Astronomical Society》2000,318(1):187-194

We present polarimetric and spectroscopic observations of the ROSAT source RX J1141.3−6410, recently identified as a polar. The detection of circular polarization variations, with an amplitude of 10 per cent, over a 3.16-h period confirms that the system is a polar (AM Herculis star). Supporting evidence comes from the nature of the emission lines and their radial velocity variability. In addition, we observe continuum slope changes in the far-red spectral region (∼6000–8200 Å), indicative of phase dependent cyclotron emission. Polarimetric modelling at two wavelengths establishes RX J1141.3−6410 as a single-pole system, with i ∼ β ∼70°. The accretion region is extended in magnetic longitude, and is totally self-occulted for ∼25 per cent of the orbit. The radial velocity curves derived from the emission lines show a phasing with maximum blueshift occurring with Δ φ ∼0.05 of maximum intensity and circular polarisation. In addition, the broader component of the lines exhibit a substantial radial velocity phase shift with respect to the narrower component, in the sense that the broad component preceeds the narrow. This can be readily understood if the narrower component is principally a result of orbital motion of the stream material and the broad component mainly a result of streaming motion near the coupling region. The phasing of the Ca  ii near-infrared line radial velocities also supports this general picture.  相似文献   

Reply to discussion of Jamaican cenozoic ichnology: review and prospectus: (v. 50, p. 364–382)     

Stephen K. Donovan  Ron K. Pickerill 《Geological Journal》2015,50(4):542-544

Mitchell and Ramsook comment on the lithostratigraphic assignment of Jamaican Cenozoic ichnofossils discussed in Donovan et al. They argue that the Paleogene Richmond Formation should be subdivided to produce a ‘Moore Town formation’ in eastern Jamaica, but the latter remains undefined as a lithostratigraphic unit and no new lithostratigraphic evidence is produced to support their supposition. Further, their use of a flawed table of trace fossil distributions does not support their thesis. The distribution of trace fossils in the White Limestone Group presented by Donovan et al. follows the lithostratigraphic scheme that was current at the time that the research was originally undertaken in the early 2000s. Yet, whatever lithostratigraphic scheme is utilised for the island, it is apparent that the more accurate data is provided by the biostratigraphy of the larger benthic foraminifers. Copyright © 2015 John Wiley & Sons, Ltd.  相似文献   

Geographic Thought: A Critical Introduction by Tim Cresswell. Wiley‐Blackwell,Chichester, 2013, pp. viii + 290 (ISBN 978‐1‐4051‐6940‐0) (hbk) (ISBN 978‐1‐4051‐6939‐4) (pbk).     

Ron Johnston 《Singapore journal of tropical geography》2014,35(2):282-285

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