全文获取类型
收费全文 | 67521篇 |
免费 | 1133篇 |
国内免费 | 642篇 |
专业分类
测绘学 | 2010篇 |
大气科学 | 4922篇 |
地球物理 | 13277篇 |
地质学 | 23661篇 |
海洋学 | 5841篇 |
天文学 | 15632篇 |
综合类 | 204篇 |
自然地理 | 3749篇 |
出版年
2021年 | 627篇 |
2020年 | 623篇 |
2019年 | 726篇 |
2018年 | 1601篇 |
2017年 | 1474篇 |
2016年 | 1950篇 |
2015年 | 1103篇 |
2014年 | 1798篇 |
2013年 | 3466篇 |
2012年 | 1974篇 |
2011年 | 2661篇 |
2010年 | 2338篇 |
2009年 | 3056篇 |
2008年 | 2601篇 |
2007年 | 2632篇 |
2006年 | 2529篇 |
2005年 | 2065篇 |
2004年 | 1970篇 |
2003年 | 1948篇 |
2002年 | 1855篇 |
2001年 | 1646篇 |
2000年 | 1589篇 |
1999年 | 1364篇 |
1998年 | 1330篇 |
1997年 | 1326篇 |
1996年 | 1143篇 |
1995年 | 1096篇 |
1994年 | 1035篇 |
1993年 | 931篇 |
1992年 | 847篇 |
1991年 | 842篇 |
1990年 | 848篇 |
1989年 | 819篇 |
1988年 | 769篇 |
1987年 | 904篇 |
1986年 | 798篇 |
1985年 | 986篇 |
1984年 | 1044篇 |
1983年 | 987篇 |
1982年 | 907篇 |
1981年 | 882篇 |
1980年 | 785篇 |
1979年 | 723篇 |
1978年 | 692篇 |
1977年 | 693篇 |
1976年 | 612篇 |
1975年 | 593篇 |
1974年 | 621篇 |
1973年 | 657篇 |
1972年 | 384篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
361.
During major geomagnetic storms, the interplanetary magnetic field angle φ (phi) changes often abruptly, either from 135° to 315° or 315° to 135°, suggesting that the heliospheric current sheet is pushed upward or downward by disturbed solar wind. The distortion of the heliospheric current sheet by three successive solar flares is simulated to show that such a flapping motion can occur. 相似文献
362.
363.
Coincident seismic reflection/refraction studies of the continental lithosphere: a global review 总被引:5,自引:0,他引:5
Summary. Vertical-incidence reflection profiling has identified several characteristic features of the continental Iithosphere including a generally transparent upper crust, a reflective lower crust, reflections from the crust-mantle boundary, and a commonly transparent upper mantle. The underlying physical causes of these characteristic features remain poorly understood. This review summarizes additional information brought to bear on the physical properties of these characteristic crustal structures through the use of coincident wide-angle refraction profiling. 相似文献
364.
In this paper, the currently accepted correlation of the Early Pleistocene Ludhamian stage of England with the Tiglian‐A sub‐stage of the Netherlands is challenged. Recent investigations of Early Pleistocene marine North Sea deposits from a borehole near Noordwijk (the Netherlands) yielded evidence from molluscs, dinoflagellate cysts and sporomorphs for an alternation of warm‐temperate and arctic intervals within the Praetiglian and Tiglian stages. Marine equivalents of the terrestrial‐based pollen sub‐stages Tiglian A and B have been recognised in the upper part of the sequence. A Praetiglian age can be assigned to the lower part of the sequence on the basis of mollusc analysis. Within the Praetiglian, an alternation of warm and cold phases has been recognised from both the dinoflagellate cyst and molluscan records. Three cold phases within the Praetiglian are tentatively correlated with marine isotope stages (MIS) 96–100. The molluscan assemblages provide evidence for climate forcing of the sea level: highest sea levels are reached in the warm‐temperate intervals. Within the Praetiglian, an interval with an acme zone of the dinoflagellate cyst Impagidinium multiplexum, is correlated with the Ludhamian and tentatively linked to MIS 97 and/or MIS 96. The cold molluscan assemblages from the Noordwijk borehole include an acme zone of Megayoldia thraciaeformis, the first and only occurrence of this North Pacific bivalve in the North Sea Basin. Copyright © 2006 John Wiley & Sons, Ltd. 相似文献
365.
Abstract— Primary minerals in calcium‐aluminum‐rich inclusions (CAIs), Al‐rich and ferromagnesian chondrules in each chondrite group have δ18O values that typically range from ?50 to +5%0. Neglecting effects due to minor mass fractionations, the oxygen isotopic data for each chondrite group and for micrometeorites define lines on the three‐isotope plot with slopes of 1.01 ± 0.06 and intercepts of ?2 ± 1. This suggests that the same kind of nebular process produced the 16O variations among chondrules and CAIs in all groups. Chemical and isotopic properties of some CAIs and chondrules strongly suggest that they formed from solar nebula condensates. This is incompatible with the existing two‐component model for oxygen isotopes in which chondrules and CAIs were derived from heated and melted 16O‐rich presolar dust that exchanged oxygen with 16O‐poor nebular gas. Some FUN CAIs (inclusions with isotope anomalies due to fractionation and unknown nuclear effects) have chemical and isotopic compositions indicating they are evaporative residues of presolar material, which is incompatible with 16O fractionation during mass‐independent gas phase reactions in the solar nebula. There is only one plausible reason why solar nebula condensates and evaporative residues of presolar materials are both enriched in 16O. Condensation must have occurred in a nebular region where the oxygen was largely derived from evaporated 16O‐rich dust. A simple model suggests that dust was enriched (or gas was depleted) relative to cosmic proportions by factors of ~10 to >50 prior to condensation for most CAIs and factors of 1–5 for chondrule precursor material. We infer that dust‐gas fractionation prior to evaporation and condensation was more important in establishing the oxygen isotopic composition of CAIs and chondrules than any subsequent exchange with nebular gases. Dust‐gas fractionation may have occurred near the inner edge of the disk where nebular gases accreted into the protosun and Shu and colleagues suggest that CAIs formed. 相似文献
366.
The self-starter is improved using the operator of the split-step Pade solution. In addition to providing greater stability and being applicable closer to the source, the improved self-starter is an efficient forward model for geoacoustic inversion. It is necessary to solve only O(10) tridiagonal systems of equations to obtain the acoustic field on a vertical array located O(10) wavelengths from a source. This experimental configuration is effective for geoacoustic inverse problems involving unknown parameters deep in the ocean bottom. For problems involving depth-dependent acoustic parameters, the improved self-starter can be used to solve nonlinear inverse problems involving O(10) unknown sediment parameters in less than a minute on the current generation of workstations 相似文献
367.
368.
369.
We found the equilibrium conditions for a self-gravitating toroidal vortex by taking thermal pressure into account. These conditions are shown to significantly differ from those for a disk or a sphere. The evolution of a thin vortex turns it into a compact vortex that loses mechanical stability for low masses at a polytropic index γ<4/3 but retains stability for sufficiently high masses and densities determined by the velocity circulation in the vortex. 相似文献
370.
Cooper John F. Christian Eric R. Richardson John D. Wang Chi 《Earth, Moon, and Planets》2003,92(1-4):261-277
Times for accumulation of chemically significant dosages on icy surfaces of Centaur, Kuiper Belt, and Oort Cloud objects from plasma and energetic ions depend on irradiation position within or outside the heliosphere. Principal irradiation components include solar wind plasma ions, pickup ions from solar UV ionization of interstellar neutral gas, energetic ions accelerated by solar and interplanetary shocks, including the putative solar wind termination shock, and galactic cosmic ray ions from the Local Interstellar Medium (LISM). We present model flux spectra derived from spacecraft data and models for eV to GeV protons at 40 AU, a termination shock position at 85 AU, and in the LISM. Times in years to accumulate dosages ~100 eV per molecule are computed from the spectra as functions of sensible surface depth less than one centimeter at unit density.The collisional resurfacing model of Luu and Jewitt is reconsidered in thecontext of depth-dependent dosage rates from plasma, suprathermal,and higher energy protons, and global exposure, by micrometeoroiddust grain impacts, of moderately irradiated red material below athin crust of heavily irradiated neutral material. This material should be more visible on dynamically `cold’ objects in the ~40 AU region. 相似文献