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1.
Using statistical orbital ranging, we systematically study the orbit computation problem for transneptunian objects (TNOs). We have automated orbit computation for large numbers of objects, and, more importantly, we are able to obtain orbits even for the most sparsely observed objects (observational arcs of a few days). For such objects, the resulting orbit distributions include a large number of high-eccentricity orbits, in which TNOs can be perturbed by close encounters with Neptune. The stability of bodies on the computed orbits has therefore been ascertained by performing a study of close encounters with the major planets. We classify TNO orbit distributions statistically, and we study the evolution of their ephemeris uncertainties. We find that the orbital element distributions for the most numerous single-apparition TNOs do not support the existence of a postulated sharp edge to the belt beyond 50 AU. The technique of statistical ranging provides ephemeris predictions more generally than previously possible also for poorly observed TNOs. 相似文献
2.
Rinse L. de Swart Francesca Ribas Gonzalo Simarro Jorge Guillén Daniel Calvete 《地球表面变化过程与地形》2021,46(15):3252-3270
Nearshore sandbars are important features in the surf zone of many beaches because they strongly influence the mean circulation and evolving morphology. Due to variations in wave conditions, sandbars can experience cross-shore migration and vary in shape from alongshore uniform (shore-parallel) to alongshore rhythmic (crescentic). Sandbar dynamics have been studied extensively, but existing observational studies usually do not quantify the processes leading to crescentic bar formation and straightening. This study analyses the dynamics of crescentic bar events at the fetch-limited beach of Castelldefels (northwestern Mediterranean Sea, Spain) using 7.5 years of hourly time-exposure video images and detailed wave conditions. The results show that, despite the generally calm wave conditions, the sandbars were very dynamic in the cross-shore and longshore directions. They often migrated rapidly offshore during storms (up to 70 m in one day) and more slowly onshore during post-storm conditions. Crescentic bars were often present at the study site (48% of the time), but only when the sandbar was at least 10 m from the shoreline. They displayed a large variability in wavelengths (100–700 m), alongshore migration speeds (0–50 m/day) and cross-shore amplitudes (5–20 m). Wavelengths increased for larger bar–shoreline distances and the alongshore migration speeds were strongly correlated with the alongshore component of the radiation stresses. Crescentic patterns typically developed during low–medium energetic waves with limited obliquity ( ° at 10 m depth), while bar straightening occurred during medium–high energetic waves with strong oblique angles of incidence ( °). Overall, this study provides further proof for the important role of wave direction in crescentic bar dynamics and highlights the strong dependence of crescentic bar development on the initial bathymetric configuration. 相似文献
3.
Ionospheric delays compensation is a mandatory step for precise absolute and relative positioning of Low Earth Orbit Satellites (LEO) by GPS measurements. The most frequently used ionosphere model for real-time GPS-based navigation in LEO is an isotropic model proposed by Lear, which uses the Vertical Total Electron Content (VTEC) above the receiver and a mapping function for TEC evaluation along a given ray path. Based on significant assessed results available for ground-based GPS receivers, we propose the use of a different model relying on the thin shell assumption and a bilinear horizontal variation of the VTEC as a function of latitude and longitude in the shell. It is expected that this model is capable of better describing horizontal gradients in the ionosphere, thus improving ionospheric delay estimation, especially in intense ionospheric conditions. This model is referred to as Linear Thin Shell (LTS). LTS performance in estimating undifferenced and double-differenced ionospheric delays is checked by comparing measured and predicted delays computed using flight data from the GRACE mission. Results show that the LTS always outperforms the isotropic model, especially in case of high solar activity. Moreover, the LTS model provides a higher performance uniformity over a wide range of ionospheric delays, thus ensuring good performance in different conditions. The results obtained demonstrate that the LTS model improves the ionosphere delays estimation accuracy by 20 and 40% for undifferenced and double-differenced delays, respectively. This suggests the LTS model can effectively contribute to improving precision in LEO positioning applications. 相似文献
4.
Quintana Gonzalo Iñaki Tandeo Pierre Drumetz Lucas Leballeur Laurent Pavec Marc 《Natural Hazards》2021,108(3):2905-2917
Natural Hazards - This paper studies different machine learning methods for solving the regression problem of estimating the marine surge value given meteorological data. The marine surge is... 相似文献
5.
Dr. J. Mortatti Dr. J. R. Ferreira Dr. L. A. Martinelli Dr. R. L. Victoria A. C. F. Tancredi 《GeoJournal》1989,19(4):391-397
A biogeochemical characterization of the Madeira river basin has been made to evaluate the local and global effects of possible alterations in the ecosystem caused by recent intensive occupation in Rondonia state. During the period April 1983—January 1986, sampling was made both by land and river along the tributaries and main channel of the Madeira river. The parameters analysed lead to a detailed study of the physicochemical quality of the waters of the basin and their relationship with the local geology, associated with the transport of solid material and the hydrological behavior of the ecosystem.Penman's method adapted to tropical rainforest conditions was used to evaluate the potential evapotranspiration for the basin. Estimated potential evapotranspiration was 1420 mm/y, 77% due to the energy balance. Real evapotranspiration was 94% of the estimated potential and the main residence time of the rain water in the basin was 2 months. The isotopic behavior of Hydrogen and Oxygen in the river waters of the region was typical of great rivers, the values being more positive during the dry season and more negative during the rainy season. An isotopic gradient of 18O 0.038 ()/100 km, was established from Porto Velho station to the estuary, which was considered low when compared with the value of 0.063 ()/100 km, obtained for the Amazon river.In general, the waters of the tributaries were poor in dissolved ion species when compared with the main channel of the Madeira river. Seasonal variation in the transport of suspended sediment kept the same pattern, greater transport being observed on rising water than during high water. A transport of 2.85 million tons per day was observed in the Madeira river near the mouth. 相似文献
6.
Gonzalo Alcaino 《Astrophysics and Space Science》1980,70(2):363-375
A critical review is presented of fundamental conclusions on the evolution of galaxies as derived from the information provided by studies of globular clusters. It is stressed that most of the essential results concerning our Galaxy — as well as extragalactic systems — depend strongly on effects involving the accuracies of the data and sample incompleteness. High-quality observations of a much larger group of objects than presently available are required before any definite conclusions can be drawn. 相似文献
7.
G. Tancredi 《Celestial Mechanics and Dynamical Astronomy》1998,70(3):181-200
The dynamics of two families of minor inner solar system bodies that suffer frequent close encounters with the planets is
analyzed. These families are: Jupiter family comets (JF comets) and Near Earth Asteroids (NEAs).
The motion of these objects has been considered to be chaotic in a short time scale,and the close encounters are supposed
to be the cause of the fast chaos. For a better understanding of the chaotic behavior we have computed Lyapunov Characteristic
Exponents (LCEs) for all the observed members of both populations. LCEs are a quantitative measure of the exponential divergence
of initially close orbits. We have observed that most members of the two families show a concentration of Lyapunov times (inverse
of LCE) around 50–100yr. The concentration is more pronounced for JF comets than for NEAs, among which a lesser spread is
observed for those that actually cross the Earth's orbit (mean perihelion distance q < 1.05 AU). It is also observed that
a general correspondence exists between Lyapunov times and the time between consecutive encounters.
A simple model is introduced to describe the basic characteristics of the dynamical evolution. This model considers an impulsive
approach, where the particles evolve unperturbedly between encounters and suffer ‘kicks’ in semimajor axis at the encounters.
It also reproduces successfully the short Lyapunov times observed in the numerical integrations and is able to estimate the
dynamical lifetimes of comets during a stay in the Jupiter family in correspondence with previous estimates.
It has been demonstrated with the model that the encounters with the largest effect on the exponential growth of the distance
between initially nearby orbits are neither the infrequent deep encounters, nor the frequent and far ones; instead, the intermediate
approaches have the most relevant contribution to the error growth. Such encounters are at a distance a few times the radius
of the Hill's sphere of the planet (e.g. 3).
An even simpler model allows us to get analytical estimates of the Lyapunov times in good agreement with the values coming
from the model above and the numerical integrations.
The predictability of the medium‐term evolution and the hazard posed to the Earth by those objects are analysed in the Discussion
section.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
8.
Carolina Boix‐Fayos Joris de Vente María Martínez‐Mena Gonzalo G. Barberá Víctor Castillo 《水文研究》2008,22(25):4922-4935
Extensive land use changes have occurred in many areas of SE Spain as a result of reforestation and the abandonment of agricultural activities. Parallel to this the Spanish Administration spends large funds on hydrological control works to reduce erosion and sediment transport. However, it remains untested how these large land use changes affect the erosion processes at the catchment scale and if the hydrological control works efficiently reduce sediment export. A combination of field work, mapping and modelling was used to test the influence of land use scenarios with and without sediment control structures (check‐dams) on sediment yield at the catchment scale. The study catchment is located in SE Spain and suffered important land use changes, increasing the forest cover 3‐fold and decreasing the agricultural land 2·5‐fold from 1956 to 1997. In addition 58 check‐dams were constructed in the catchment in the 1970s accompanying reforestation works. The erosion model WATEM‐SEDEM was applied using six land use scenarios: land use in 1956, 1981 and 1997, each with and without check‐dams. Calibration of the model provided a model efficiency of 0·84 for absolute sediment yield. Model application showed that in a scenario without check dams, the land use changes between 1956 and 1997 caused a progressive decrease in sediment yield of 54%. In a scenario without land use changes but with check‐dams, about 77% of the sediment yield was retained behind the dams. Check‐dams can be efficient sediment control measures, but with a short‐lived effect. They have important side‐effects, such as inducing channel erosion downstream. While also having side‐effects, land use changes can have important long‐term effects on sediment yield. The application of either land use changes (i.e. reforestation) or check‐dams to control sediment yield depends on the objective of the management and the specific environmental conditions of each area. Copyright © 2008 John Wiley & Sons, Ltd. 相似文献
9.
Juan Pedro Rodríguez‐López Lars B. Clemmensen Nick Lancaster Nigel P. Mountney Gonzalo D. Veiga 《Sedimentology》2014,61(6):1487-1534
The sedimentary record of aeolian sand systems extends from the Archean to the Quaternary, yet current understanding of aeolian sedimentary processes and product remains limited. Most preserved aeolian successions represent inland sand‐sea or dunefield (erg) deposits, whereas coastal systems are primarily known from the Cenozoic. The complexity of aeolian sedimentary processes and facies variability are under‐represented and excessively simplified in current facies models, which are not sufficiently refined to reliably account for the complexity inherent in bedform morphology and migratory behaviour, and therefore cannot be used to consistently account for and predict the nature of the preserved sedimentary record in terms of formative processes. Archean and Neoproterozoic aeolian successions remain poorly constrained. Palaeozoic ergs developed and accumulated in relation to the palaeogeographical location of land masses and desert belts. During the Triassic, widespread desert conditions prevailed across much of Europe. During the Jurassic, extensive ergs developed in North America and gave rise to anomalously thick aeolian successions. Cretaceous aeolian successions are widespread in South America, Africa, Asia, and locally in Europe (Spain) and the USA. Several Eocene to Pliocene successions represent the direct precursors to the present‐day systems. Quaternary systems include major sand seas (ergs) in low‐lattitude and mid‐latitude arid regions, Pleistocene carbonate and Holocene–Modern siliciclastic coastal systems. The sedimentary record of most modern aeolian systems remains largely unknown. The majority of palaeoenvironmental reconstructions of aeolian systems envisage transverse dunes, whereas successions representing linear and star dunes remain under‐recognized. Research questions that remain to be answered include: (i) what factors control the preservation potential of different types of aeolian bedforms and what are the characteristics of the deposits of different bedform types that can be used for effective reconstruction of original bedform morphology; (ii) what specific set of controlling conditions allow for sustained bedform climb versus episodic sequence accumulation and preservation; (iii) can sophisticated four‐dimensional models be developed for complex patterns of spatial and temporal transition between different mechanisms of accumulation and preservation; and (iv) is it reasonable to assume that the deposits of preserved aeolian successions necessarily represent an unbiased record of the conditions that prevailed during episodes of Earth history when large‐scale aeolian systems were active, or has the evidence to support the existence of other major desert basins been lost for many periods throughout Earth history? 相似文献
10.
The Uppsala-ESO Survey of Asteroids and Comets was undertaken to find previously undetected comets in the vicinity of Jupiter. Over 15000 positions of moving objects have been detected on 74 plates obtained from the European Southern Observatory in Chile and from the Anglo-Australian Observatory in Australia in 1992 and 1993. Two or more positions were secured for about 3300 asteroids and orbits have so far been calculated for 1944 asteroids. The main bulk of these asteroids are previously undetected. We present absolute magnitudes and diameters for asteroids which have an accurate orbit. The magnitude and diameter distributions are compared to the results of the Palomar-Leiden Survey of Faint Minor Planets. 相似文献