全文获取类型
收费全文 | 1297篇 |
免费 | 38篇 |
国内免费 | 8篇 |
专业分类
测绘学 | 19篇 |
大气科学 | 95篇 |
地球物理 | 291篇 |
地质学 | 424篇 |
海洋学 | 138篇 |
天文学 | 211篇 |
综合类 | 4篇 |
自然地理 | 161篇 |
出版年
2022年 | 8篇 |
2021年 | 12篇 |
2020年 | 18篇 |
2019年 | 24篇 |
2018年 | 22篇 |
2017年 | 21篇 |
2016年 | 34篇 |
2015年 | 39篇 |
2014年 | 35篇 |
2013年 | 96篇 |
2012年 | 51篇 |
2011年 | 61篇 |
2010年 | 48篇 |
2009年 | 64篇 |
2008年 | 50篇 |
2007年 | 57篇 |
2006年 | 42篇 |
2005年 | 45篇 |
2004年 | 38篇 |
2003年 | 45篇 |
2002年 | 41篇 |
2001年 | 37篇 |
2000年 | 29篇 |
1999年 | 32篇 |
1998年 | 27篇 |
1997年 | 22篇 |
1996年 | 13篇 |
1995年 | 13篇 |
1994年 | 20篇 |
1993年 | 14篇 |
1992年 | 17篇 |
1991年 | 12篇 |
1990年 | 13篇 |
1989年 | 11篇 |
1988年 | 10篇 |
1987年 | 18篇 |
1986年 | 15篇 |
1985年 | 10篇 |
1984年 | 18篇 |
1983年 | 15篇 |
1982年 | 18篇 |
1981年 | 19篇 |
1980年 | 16篇 |
1979年 | 18篇 |
1978年 | 6篇 |
1977年 | 7篇 |
1976年 | 10篇 |
1975年 | 11篇 |
1974年 | 9篇 |
1973年 | 12篇 |
排序方式: 共有1343条查询结果,搜索用时 15 毫秒
151.
Small-scale spatially periodic distortions of auroral forms have been studied utilizing low-light level television observations made at various locations in the Northern and Southern Hemispheres. The most commonly observed features were folds and vortex-like curl formations. The curls, identified here with the Kelvin-Helmholtz instability due to fluid shear, invariably had a counterclockwise rotational shape and motion when viewed in a direction anti-parallel to the Earth's magnetic field. The typical measured wavelength (5 km) and measured growth rate (4.2 sec−1) were used to evaluate the Kelvin-Helmholtz dispersion relation for the apparent shear ωs = ∂ νx/ ∂y (28 sec−1). The apparent horizontal velocities of both folds (0–5 km/sec) and curls (0–22 km/sec) were invariably observed to be counterclockwise with respect to the multiple arc centre when viewed antiparallel to B. Consistent agreement between rotational shape and rotational motion suggests that the apparent growth rate and the apparent horizontal velocities closely approximate the actual values. If the shear results from E×B drifts in a space charge field, the calculated value for ωs, implies an unneutralized electron density 0–1 cm−3 and a ΔE across the arc element 500mV/m. The velocity measurements indicate that the ΔE values for individual elements can combine to produce transient electric fields at the edges of multiple arcs as high as 1000 mV/m. 相似文献
152.
Yun X. Bachmann E.R. McGhee R.B. Whalen R.H. Roberts R.L. Knapp R.G. Healey A.J. Zyda M.J. 《Oceanic Engineering, IEEE Journal of》1999,24(3):396-404
A Small Autonomous Underwater Vehicle Navigation System (SANS) is being developed at the Naval Postgraduate School. The SANS is an integrated Global Positioning System/Inertial Navigation System (GPS/INS) navigation system composed of low-cost and small-size components. It is designed to demonstrate the feasibility of using a low-cost strap-down inertial measurement unit (IMU) to navigate between intermittent GPS fixes. The present hardware consists of a GPS/DGPS receiver, IMU, compass, water speed sensor, water depth sensor, and a data processing computer. The software is based on a 12-state complementary filter that combines measurement data from all sensors to derive a vehicle position/orientation estimate. This paper describes hardware and software design and testing results of the SANS. It is shown that results from tilt table testing and bench testing provide an effective means for tuning filter gains. Ground vehicle testing verifies the overall functioning of the SANS and exhibits an encouraging degree of accuracy 相似文献
153.
The nucleus of Comet C/2001 A2 (LINEAR) split several times during its recent apparition, presenting an unusual opportunity to search for chemical differences in freshly exposed material. We conducted this search using NIRSPEC at the W.M. Keck Observatory on four dates in 2001: 9.5 and 10.5 July and 4.4 and 10.5 August. We detected the R0 and R1 lines of the ν3 vibrational band of CH4 near 3.3 μm on all dates. The R2 line was detected on 4.4 and 10.5 August. When we compare production rates of CH4 to H2O, we find evidence of a significant enhancement in August relative to that found in July. H2CO was securely detected via its ν1 and ν5 bands on 9.5 July. On 10.5 July, H2CO emission was much weaker, and its mixing ratio had dropped by a factor of about four. The mixing ratios for other detected volatile species did not change significantly over the course of the observations. We discuss the implications of this evidence for chemical heterogeneity in the nucleus of Comet C/2001 A2. 相似文献
154.
155.
The Cassini spacecraft encountered Jupiter in late 2000. Within more than 1 AU of the gas giant the Cosmic Dust Analyser onboard the spacecraft recorded the first ever mass spectra of jovian stream particles. To determine the chemical composition of particles, a comprehensive statistical analysis of the dataset was performed. Our results imply that the vast majority (>95%) of the observed stream particles originate from the volcanic active jovian satellite Io from where they are sprinkled out far into the Solar System. Sodium chloride (NaCl) was identified as the major particle constituent, accompanied by sulphurous as well as potassium bearing components. This is in contrast to observations of gas in the ionian atmosphere, its co-rotating plasma torus, and the neutral cloud, where sulphur species are dominant while alkali and chlorine species are only minor components. Io has the largest active volcanoes of the Solar System with plumes reaching heights of more than 400 km above the moons surface. Our in situ measurements indicate that alkaline salt condensation of volcanic gases inside those plumes could be the dominant formation process for particles reaching the ionian exosphere. 相似文献
156.
I. Neill Reid F. van Wyk F. Marang G. Roberts D. Kilkenny S. Mahoney 《Monthly notices of the Royal Astronomical Society》2001,325(3):931-962
We have identified 317 stars included in the Hipparcos astrometric catalogue that have parallaxes measured to a precision of better than 15 per cent, and the location of which in the ( M V ,( B − V ) T ) diagram implies a metallicity comparable to or less than that of the intermediate-abundance globular cluster M5. We have undertaken an extensive literature search to locate Strömgren, Johnson/Cousins and Walraven photometry for over 120 stars. In addition, we present new UBV ( RI )C photometry of 201 of these candidate halo stars, together with similar data for a further 14 known metal-poor subdwarfs. These observations provide the first extensive data set of R C I C photometry of metal-poor, main-sequence stars with well-determined trigonometric parallaxes. Finally, we have obtained intermediate-resolution optical spectroscopy of 175 stars.
47 stars still lack sufficient supplementary observations for population classification; however, we are able to estimate abundances for 270 stars, or over 80 per cent of the sample. The overwhelming majority have near-solar abundance, with their inclusion in the present sample stemming from errors in the colours listed in the Hipparcos catalogue. Only 44 stars show consistent evidence of abundances below [Fe/H]=−1.0 . Nine are additions to the small sample of metal-poor subdwarfs with accurate photometry. We consider briefly the implication of these results for cluster main-sequence fitting. 相似文献
47 stars still lack sufficient supplementary observations for population classification; however, we are able to estimate abundances for 270 stars, or over 80 per cent of the sample. The overwhelming majority have near-solar abundance, with their inclusion in the present sample stemming from errors in the colours listed in the Hipparcos catalogue. Only 44 stars show consistent evidence of abundances below [Fe/H]=−1.0 . Nine are additions to the small sample of metal-poor subdwarfs with accurate photometry. We consider briefly the implication of these results for cluster main-sequence fitting. 相似文献
157.
158.
T.P. Roberts N.J. Schurch R.S. Warwick 《Monthly notices of the Royal Astronomical Society》2001,324(3):737-746
We use ROSAT HRI spatial data and ASCA spectral measurements for a sample of seven nearby, early-type spiral galaxies, to address the question of whether a low-luminosity active galactic nucleus (LLAGN) is present in galaxies that have a LINER 2 classification. The brightest discrete X-ray source in the ROSAT HRI observations is invariably found to be positionally coincident with the optical galactic nucleus, and in most cases its flux dominates the X-ray emission from the central region of the galaxy. All seven galaxies have X-ray spectra consistent with a two-component, soft thermal plus hard power-law, spectral form. If we exclude the two galaxies with relatively hard X-ray spectra, NGC 3628 and NGC 4594, for which there is supporting evidence for a LLAGN (or alternatively in the case of NGC 3628 a dominant ultraluminous X-ray binary), then the remaining galaxies show surprisingly similar X-ray spectral properties. Specifically the flux ratio F X (0.5–1)/ F X (2–5) , which measures the relative strengths of the thermal and non-thermal emission components, shows little scatter about a mean of 0.66, a value very similar to that measured in the classic starburst galaxy NGC 253. As there is no obvious reason why the luminosity of the hard power-law continuum emanating from a putative LLAGN should be very closely correlated with the thermal emission of the surrounding region, this suggests that that the broad-band (0.5–5 keV) X-ray emission from these LINER 2 galaxies may originate in a common set of processes probably associated with the starburst phenomenon. Conversely, it appears that in many LINER 2 galaxies and perhaps the majority, the nuclear X-ray luminosity does not derive directly from the presence of a LLAGN. 相似文献
159.
The magnetosphere, ionosphere and solar–terrestrial physics community gathered at the University of Edinburgh from 29 March – 1 April 2004, for a meeting including a day of joint sessions with UKSP. The four day programme reflected the breadth and depth of the science covered. Neil Arnold and Alan Thomson report; the UKSP meeting is covered in the following pages. 相似文献
160.