全文获取类型
收费全文 | 170篇 |
免费 | 8篇 |
国内免费 | 1篇 |
专业分类
测绘学 | 3篇 |
大气科学 | 9篇 |
地球物理 | 51篇 |
地质学 | 49篇 |
海洋学 | 19篇 |
天文学 | 34篇 |
自然地理 | 14篇 |
出版年
2023年 | 2篇 |
2022年 | 2篇 |
2021年 | 4篇 |
2020年 | 2篇 |
2019年 | 1篇 |
2018年 | 4篇 |
2017年 | 2篇 |
2016年 | 10篇 |
2015年 | 2篇 |
2014年 | 6篇 |
2013年 | 9篇 |
2012年 | 7篇 |
2011年 | 7篇 |
2010年 | 7篇 |
2009年 | 6篇 |
2008年 | 10篇 |
2007年 | 3篇 |
2006年 | 11篇 |
2005年 | 13篇 |
2004年 | 5篇 |
2003年 | 7篇 |
2002年 | 5篇 |
2001年 | 3篇 |
2000年 | 4篇 |
1999年 | 6篇 |
1998年 | 3篇 |
1996年 | 2篇 |
1995年 | 2篇 |
1994年 | 2篇 |
1993年 | 1篇 |
1992年 | 1篇 |
1991年 | 1篇 |
1990年 | 3篇 |
1984年 | 2篇 |
1982年 | 1篇 |
1981年 | 2篇 |
1980年 | 1篇 |
1979年 | 1篇 |
1978年 | 3篇 |
1977年 | 2篇 |
1976年 | 2篇 |
1975年 | 3篇 |
1974年 | 2篇 |
1973年 | 6篇 |
1972年 | 1篇 |
排序方式: 共有179条查询结果,搜索用时 15 毫秒
21.
Observations of low frequency solar type III radio bursts and the associated fast solar electrons show that the total path length traveled by the particles between the Sun and the Earth is significantly greater than the length of the smooth Archimedean spiral trajectory followed by the centroid of the type III exciter (Alvarez et al., 1975). Here we assume that the ratio of electron path length and the spiral length increases approximately as r
n, where r is heliocentric distance, and then compute the radio bursts arrival time at 1 AU for different values of n. A comparison with the radio observations indicates that the best fit occurs for n = 1.5 ± 1.0. We interpret these results in terms of the variation of electron scattering with heliocentric distance. 相似文献
22.
IMP-6 spacecraft observations of low frequency radio emission, fast electrons, and solar wind plasma are used to examine the dynamics of the fast electron streams which generate solar type-III radio bursts. Of twenty solar electron events observed between April, 1971 and August, 1972, four were found to be amenable to detailed analysis. Observations of the direction of arrival of the radio emission at different frequencies were combined with the solar wind density and velocity measurements at 1 AU to define an Archimedean spiral trajectory for the radio burst exciter. The propagation characteristics of the exciter and of the fast electrons observed at 1 AU were then conpared. We find that: (1) the fast electrons excite the radio emission at the second harmonic; (2) the total distance travelled by the electrons was between 30 and 70% longer than the length of the smooth spiral defined by the radio observations; (3) this additional distance travelled is the result of scattering of the electrons in the interplanetary medium; (4) the observations are consistent with negligible true energy loss by the fast electrons. 相似文献
23.
Martha Alvarez Josep Maria Cors Joaquin Delgado 《Celestial Mechanics and Dynamical Astronomy》2006,95(1-4):173-200
In this paper, we prove the existence of special type of motions in the restricted planar parabolic three-body problem, of the type exchange, emission–capture, and emission–escape with close passages to collinear and equilateral triangle configuration, among others. The proof is based on a gradient-like property of the Jacobian function when equations of motion are written in a rotating–pulsating reference frame, and the extended phase space is compactified in the time direction. Thus a phase space diffeomorphic to
-coordinates (θ, ζ, ζ′) is obtained with the boundary manifolds θ = ± π/2 corresponding to escapes of the binaries when time tends to ± ∞. It is shown there exists exactly five critical points on each boundary, corresponding to classic homographic solutions. The connections of the invariant manifolds associated to the collinear configurations, and stable/unstable sets associated to binary collision on the boundary manifolds, are obtained for arbitrary masses of the primaries. For equal masses extra connections are obtained, which include equilateral configurations. Based on the gradient-like property, a geometric criterion for capture is proposed and is compared with a criterion introduced by Merman (1953b) in the fifties, and an example studied numerically by Kocina (1954). 相似文献
24.
25.
Contrasting rainfall generated debris flows from adjacent watersheds at Forest Falls, southern California, USA 总被引:1,自引:0,他引:1
Debris flows are widespread and common in many steeply sloping areas of southern California. The San Bernardino Mountains community of Forest Falls is probably subject to the most frequently documented debris flows in southern California. Debris flows at Forest Falls are generated during short-duration high-intensity rains that mobilize surface material. Except for debris flows on two consecutive days in November 1965, all the documented historic debris flows have occurred during high-intensity summer rainfall, locally referred to as ‘monsoon’ or ‘cloudburst’ rains. Velocities of the moving debris range from about 5 km/h to about 90 km/h. Velocity of a moving flow appears to be essentially a function of the water content of the flow. Low velocity debris flows are characterized by steep snouts that, when stopped, have only small amounts of water draining from the flow. In marked contrast are high-velocity debris flows whose deposits more resemble fluvial deposits. In the Forest Falls area two adjacent drainage basins, Snow Creek and Rattlesnake Creek, have considerably different histories of debris flows. Snow Creek basin, with an area about three times as large as Rattlesnake Creek basin, has a well developed debris flow channel with broad levees. Most of the debris flows in Snow Creek have greater water content and attain higher velocities than those of Rattlesnake Creek. Most debris flows are in relative equilibrium with the geometry of the channel morphology. Exceptionally high-velocity flows, however, overshoot the channel walls at particularly tight channel curves. After overshooting the channel, the flows degrade the adjacent levee surface and remove trees and structures in the immediate path, before spreading out with decreasing velocity. As the velocity decreases the clasts in the debris flows pulverize the up-slope side of the trees and often imbed clasts in them. Debris flows in Rattlesnake Creek are relatively slow moving and commonly stop in the channel. After the channel is blocked, subsequent debris flows cut a new channel upstream from the blockage that results in the deposition of new debris-flow deposits on the lower part of the fan. Shifting the location of debris flows on the Rattlesnake Creek fan tends to prevent trees from becoming mature. Dense growths of conifer seedlings sprout in the spring on the late summer debris flow deposits. This repeated process results in stands of even-aged trees whose age records the age of the debris flows. 相似文献
26.
27.
28.
Nadir Benhamiche Lamia Sahi Sabrina Tahar Hassiba Bir Khodir Madani Benoit Laignel 《水文科学杂志》2013,58(4):775-792
ABSTRACTFive-year monitoring of physicochemical parameters was performed with two campaigns in low and high water periods of the Lower Soummam catchment. Data from 18 wells were processed by multivariate statistical tools in order to identify the principal factors influencing groundwater chemistry. Two matrices of 14 and 8 physicochemical parameters with 18 groundwater samples collected in wells were obtained. The correlation matrix showed strong associations between nine variables: K+, Ca2+, Na+, SO42?, Cl?, Mg2+, NO2?, Zn2+ and Sr2+. Principal component analysis and factor analysis showed that the cumulated variance of high and low water periods was of 83.19% and 78.55%, respectively. The variables assigned to the mineralization effect or to pollution indicators were presented by the factor analysis. The bivariate plots confirmed a mineralization model, ascribed to dissolution of geological materials, and to high levels of saline contamination attributed to leakages from sanitary systems. They also showed an increase “upstream to downstream” of the mineralization, visualization of temporal variations, and a dilution process identification of the natural mineralization during the recharge of the aquifer.
EDITOR D. Koutsoyiannis; ASSOCIATE EDITOR X. Chen 相似文献
29.
Víctor Alcaraz‐González Fabián Azael Fregoso‐Sanchez Hugo Oscar Mendez‐Acosta Victor Gonzalez‐Alvarez 《洁净——土壤、空气、水》2013,41(12):1157-1164
In this paper, a multiple‐input multiple‐output (MIMO) model‐based robust control scheme is proposed for the indirect control of both total alkalinity and the ratio (intermediate alkalinity)/(total alkalinity) by regulating volatile fatty acid concentrations and strong ions concentration, while guaranteeing the so‐called operational stability in anaerobic digestion (AD) processes. The proposed MIMO regulator is an adaptive controller derived from an AD model which incorporates the physicochemical equilibrium of the system as well as the use of a robust interval observer to estimate key process bounds that are used in the computation of the control efforts. Numerical simulations were carried out for a number of operating conditions under the most uncertain scenarios. Results showed that the proposed multivariable control law is able to recover the system stability around a pre‐determined set point in the face of parameter uncertainty and load disturbances. 相似文献
30.
Groundwater flow model, recharge estimation and sustainability in an arid region of Patagonia, Argentina 总被引:2,自引:2,他引:0
María del Pilar Alvarez María Marta Trovatto Mario Alberto Hernández Nilda González 《Environmental Earth Sciences》2012,66(7):2097-2108
The Península Valdés, in northeastern Patagonia, Argentina, is characterised by its arid climate and the lack of perennial watercourses; thus, all economic activities depend on the groundwater resources. Water demand is mainly associated with tourism, which is centralised in Puerto Pirámides and supplied by a water desalination plant, and to sheep farming, supplied by the local aquifer. Due to the exponential growth of tourism, the government is planning to exploit groundwater and convey it by aqueduct to the abovementioned locality. The objectives of this study were to corroborate the conceptual geohydrological model, to develop a mathematical model to simulate the response of the aquifer to different scenarios, and to assess the incidence of water input into the system as a variable—a function that poses difficulties in the models for arid regions. The Visual Modflow 4.1 code was used, calibrating it in trial-and-error mode, changing the recharge and hydraulic conductivity parameters with different variants in the recharge zone and in the inclusion or exclusion of the evapotranspiration module. Results indicate the importance of the recharge analysis by treating rainfall at daily time steps. The adjusted model was exposed to four scenarios with variations in water input and in output by pumping. It can be concluded that under different input conditions, but with a controlled extraction, the system responds in a sustainable manner. 相似文献