全文获取类型
收费全文 | 448篇 |
免费 | 10篇 |
国内免费 | 2篇 |
专业分类
测绘学 | 8篇 |
大气科学 | 23篇 |
地球物理 | 87篇 |
地质学 | 179篇 |
海洋学 | 32篇 |
天文学 | 78篇 |
综合类 | 2篇 |
自然地理 | 51篇 |
出版年
2021年 | 5篇 |
2019年 | 8篇 |
2018年 | 4篇 |
2017年 | 7篇 |
2016年 | 8篇 |
2015年 | 7篇 |
2014年 | 11篇 |
2013年 | 25篇 |
2012年 | 10篇 |
2011年 | 15篇 |
2010年 | 18篇 |
2009年 | 21篇 |
2008年 | 20篇 |
2007年 | 16篇 |
2006年 | 15篇 |
2005年 | 18篇 |
2004年 | 23篇 |
2003年 | 9篇 |
2002年 | 15篇 |
2001年 | 5篇 |
2000年 | 14篇 |
1999年 | 5篇 |
1998年 | 13篇 |
1997年 | 4篇 |
1996年 | 3篇 |
1995年 | 7篇 |
1994年 | 3篇 |
1993年 | 5篇 |
1992年 | 6篇 |
1991年 | 7篇 |
1990年 | 5篇 |
1989年 | 8篇 |
1988年 | 9篇 |
1987年 | 12篇 |
1986年 | 8篇 |
1985年 | 6篇 |
1984年 | 12篇 |
1983年 | 9篇 |
1982年 | 3篇 |
1981年 | 4篇 |
1980年 | 11篇 |
1979年 | 3篇 |
1978年 | 14篇 |
1976年 | 2篇 |
1974年 | 4篇 |
1973年 | 4篇 |
1972年 | 2篇 |
1971年 | 2篇 |
1970年 | 4篇 |
1912年 | 2篇 |
排序方式: 共有460条查询结果,搜索用时 15 毫秒
71.
Steven J. Ostro Alan W. Harris Donald B. Campbell Irwin I. Shapiro James W. Young 《Icarus》1984,60(2):391-403
Results of 13-cm-wavelength radar observations and V-filter photoelectric observations of Ra- Shalom during its 1981 Aug–Sep apparition are reported. The radar data yid detections of echoes in the same sense of circular polarization as transmitted (i.e., the SC sense) as well as in the opposite (OC) sense. The estimate of the ratio of SC to OC echo power, μc = 0.14 ± 0.02, indicates that most, but certainly not all, of the backscattering is due to single reflections from surface elements that are fairly smooth at decimeter scales. The value obtained for the OC radar cross section on Aug 26 (1.2 ± 0.3 km2) is about three times larger than those obtained on Aug 23, 24, and 25. The echo bandwidth appears to be within about 1.5 Hz of 5.0 Hz on each date. The photoelectric data suggest a value, Psyn = 19.79 hr, for the synodic rotation period, and yield a composite lightcurve with two pairs of extrema. Combining this value for Psyn with a firm lower bound (4 Hz) on the maximum echo bandwidth yields a lower bound of 1.4 km on the maximum distance between Ra-Shalom's spin axis and any point on its surface. 相似文献
72.
J. W. Campbell 《Astrophysics and Space Science》1970,9(1):128-145
Twin channel stellar photometers were flown in two sounding rockets in May 1967 and produced a number of absolute photoelectric measurements in the region 2000–3000 Å.The observations were made at wavelengths centred on 2150 Å and 2550 Å with passbands of 300 Å on 28 early-type stars in the range of spectral types B0-B9.5 down to a limiting visual magnitude of 4.8.A comparison of the experimental data with other observations and with the theoretically derived fluxes of Mihalas for unblanketed models, indicates a satisfactory agreement within the limits of the known photometric accuracies, although there are significant departures for those stars exhibiting emission characteristics and for the magnetic variable 2 Canum Venaticorum. 相似文献
73.
Solubility of Pt in sulphide mattes: Implications for the genesis of PGE-rich horizons in layered intrusions 总被引:1,自引:0,他引:1
Raúl O.C. Fonseca Ian H. Campbell Hugh St. C. ONeill Charlotte M. Allen 《Geochimica et cosmochimica acta》2009,73(19):5764-5777
The partitioning of Pt in sulphide melt (matte) has been studied as a function of fS2 and fO2 at 1200 and 1300 °C. The results show that the solubility of Pt in mattes increases strongly with increasing fS2 and decreases weakly with increasing fO2. The increase in Pt solubility with increasing fS2 is attributed to Pt dissolving in the melt as a sulphide species and the weak inverse dependence of Pt solubility on fO2 to the diluting effect of increasing O in the melt at high fO2. These results, coupled with measurements of Pt solubility in silicate melts taken from the literature, allow the calculation of Pt matte/silicate-melt partition coefficients () for a range of conditions pertinent to the formation of Pt-rich horizons in layered intrusions. The calculated values range between 107 and 1011, depending on fO2 and fS2, several orders of magnitude higher than previously published values. Our preferred value for for conditions appropriate to the Merensky Reef is 107 and for the Stillwater Pt-rich horizon 108. The new results are consistent with the magmatic hypothesis for Pt-rich horizons in layered intrusions. 相似文献
74.
Sediment waves are commonly observed on the sea floor and often vary in morphology and geometry according to factors such as seabed slope, density and discharge of turbidity currents, and the presence of persistent contour currents. This paper documents the morphology, internal geometry and distribution of deep‐water (4000 to 5000 m) bedforms observed on the sea floor offshore eastern Canada using high‐resolution multibeam bathymetry data and seismic stratigraphy. The bedforms have wavelengths of >1 km but fundamentally vary in terms of morphology and internal stratigraphy, and are distinguished into three main types. The first type, characterized by their long‐wavelength crescentic shape, is interpreted as net‐erosional cyclic steps. These cyclic steps were formed by turbidity currents flowing through canyons and overtopping and breaching levées. The second type, characterized by their linear shape and presence on levées, is interpreted as net‐depositional cyclic steps. These upslope migrating bedforms are strongly aggradational, indicating high sediment deposition from turbidity currents. The third type, characterized by their obliqueness to canyons, is observed on an open slope and is interpreted as antidunes. These antidunes were formed by the deflection of the upper dilute, low‐density parts of turbidity currents by contour currents. The modelling of the behaviour of these different types of turbidity currents reveals that fast‐flowing flows form cyclic steps while their upper parts overspill and are entrained westward by contour currents. The interaction between turbidity currents and contour currents results in flow thickening and reduced sediment concentration, which leads to lower flow velocities. Lower velocities, in turn, allow the formation of antidunes instead of cyclic steps because the densiometric Froude number (Fr′) decreases. Therefore, this study shows that both net‐erosional and net‐depositional cyclic steps are distributed along channels where turbidity currents prevail whereas antidunes form on open slopes, in a mixed turbidite/contourite system. This study provides insights into the influence of turbidity currents versus contour currents on the morphology, geometry and distribution of bedforms in a mixed turbidite–contourite system. 相似文献
75.
Previous “fraction of young water” (Fyw) estimates based on relative annual isotopic amplitudes in precipitation (Ap) and streamflow (As) produced low Fyw values in mountain catchments, which is contrary to extensive research that reports rapid water transmission in mountains. This study investigated this discrepancy by testing the effect of snow accumulation on the model that underpins the Fyw method. A Monte-Carlo analysis of simulations for 20,000 randomly-generated catchment model configurations used 10 years of precipitation inputs for the Upper Elbow River catchment in the Rocky Mountains (Alberta, Canada) to model discharge with and without snowpack storage of winter precipitation. Neither direct nor modified precipitation input produced a 1:1 relationship between As/Ap and Fyw, undermining the applicability of the original Fyw method in mountain watersheds with large seasonal snow accumulation. With snowpack-modified input a given As/Ap ratio corresponds to a range of Fyw values, which can still provide semi-quantitative information. In the small (435 km2) Elbow River catchment a Fyw range of 7–23% supports previous findings of rapid transmission in mountain catchments. Further analysis showed that the improved discharge prediction (Nash–Sutcliffe efficiency > 0.9) correlates with higher Fyw values and demonstrated that the interannual shifts in δ18O can be used to estimate of new water (<1 year) fraction in winter streamflow, and the estimate of 20% for the Elbow River further supports rapid transmission in mountain catchments. 相似文献
76.
77.
C. G. Campbell 《Monthly notices of the Royal Astronomical Society》1999,310(4):1175-1184
The problem of magnetic field generation and advection in accretion discs is considered, in the context of wind launching and angular momentum extraction. A dipole-symmetry solution of the dynamo equations is found, with force-free boundary conditions appropriate for matching to a wind solution. Consideration of the curved field geometry and diffusive nature of the disc enables the position of the sonic point to be calculated and related to the field inclination at the disc surface. A critical inclination of 20° to the horizontal results, for which the sonic point lies in the disc surface and there is no potential barrier to wind launching. Hence the wind mass-loss rate will only become excessive, leading to disc disruption, for large field bending. The compressional effect of the horizontal magnetic field enhances the wind mass flux. 相似文献
78.
A regime shift in sediment export from a coastal watershed during a record wet winter,California: Implications for landscape response to hydroclimatic extremes 下载免费PDF全文
Amy E. East Andrew W. Stevens Andrew C. Ritchie Patrick L. Barnard Pamela Campbell‐Swarzenski Brian D. Collins Christopher H. Conaway 《地球表面变化过程与地形》2018,43(12):2562-2577
Small, steep watersheds are prolific sediment sources from which sediment flux is highly sensitive to climatic changes. Storm intensity and frequency are widely expected to increase during the 21st century, and so assessing the response of small, steep watersheds to extreme rainfall is essential to understanding landscape response to climate change. During record winter rainfall in 2016–2017, the San Lorenzo River, coastal California, had nine flow peaks representing 2–10‐year flood magnitudes. By the third flood, fluvial suspended sediment showed a regime shift to greater and coarser sediment supply, coincident with numerous landslides in the watershed. Even with no singular catastrophic flood, these flows exported more than half as much sediment as had a 100‐year flood 35 years earlier, substantially enlarging the nearshore delta. Annual sediment load in 2017 was an order of magnitude greater than during an average‐rainfall year, and 500‐fold greater than in a recent drought. These anomalous sediment inputs are critical to the coastal littoral system, delivering enough sediment, sometimes over only a few days, to maintain beaches for several years. Future projections of megadroughts punctuated by major atmospheric‐river storm activity suggest that interannual sediment‐yield variations will become more extreme than today in the western USA, with potential consequences for coastal management, ecosystems, and water‐storage capacity. The occurrence of two years with major sediment export over the past 35 years that were not associated with extremes of the El Niño Southern Oscillation or Pacific Decadal Oscillation suggests caution in interpreting climatic signals from marine sedimentary deposits derived from small, steep, coastal watersheds, to avoid misinterpreting the frequencies of those cycles. Published 2018. This article is a U.S. Government work and is in the public domain in the USA. 相似文献
79.
Sean W. Campbell 《地球表面变化过程与地形》1999,24(3):271-278
Papago Park, Arizona, is a pediment-inselberg complex that hosts a variety of well developed tafoni and alveolar weathering forms. The purpose of this paper is to analyse the nature of chemical weathering associated with the tafoni using backscatter electron microscopy (BSE) and quantitative wavelength dispersive X-ray analysis (WDS). Calcium-rich and iron-rich coatings occur on the outer shells of the tafoni. Calcium carbonate precipitation within mineral microfractures occurs on the underside of the tafoni. Chemical weathering of primary mineral grains provides a source of material found in the coatings. The WDS analyses show a near-complete lack of salt-forming elements. Copyright © 1999 John Wiley & Sons, Ltd. 相似文献
80.
C. G. Campbell 《地球物理与天体物理流体动力学》2014,108(3):333-349
An accretion curtain forms when a strongly magnetic star disrupts the inner region of its surrounding disc. It was previously shown that the disc expands vertically due to rapidly growing thermal pressure caused by magnetic heating over a narrow radial transition region inside the corotation radius. This allows material to flow from the disc into a magnetically channelled curtain through which it is transferred to the star. The curtain flow is trans-sonic and sub-Alfvénic, with small distortions of the stellar magnetic field. In the present paper, the disc and curtain flows are matched across the upper boundary of the disc transition region, and this is shown to determine the width of this region as a function of the stellar rotation rate. The sonic point position can adjust to allow steady mass transfer from the disc to the curtain flow. An upper limit can be defined for the rotation period of the star below which a strong magnetic channelling regime applies, with the outer edge of the disruption region lying inside a spherical Alfvén radius. The picture of a thin, magnetically channelled curtain flow fed from a thermally disrupted disc is self-consistent in this regime. A lower limit arises for the stellar angular velocity below which the sonic point merges with the curtain base, resulting in excessive mass loss from the disc which would be inconsistent with a steady solution. This corresponds to a lower limit on the disruption radius as a fraction of the corotation radius. It is noted that the spin-up timescale of the accreting star is significantly less than the lifetime of the system so that typical observed systems should lie in the strong magnetic regime. 相似文献