全文获取类型
收费全文 | 4365篇 |
免费 | 150篇 |
国内免费 | 10篇 |
专业分类
测绘学 | 171篇 |
大气科学 | 589篇 |
地球物理 | 1022篇 |
地质学 | 1768篇 |
海洋学 | 204篇 |
天文学 | 574篇 |
综合类 | 15篇 |
自然地理 | 182篇 |
出版年
2021年 | 64篇 |
2020年 | 60篇 |
2019年 | 47篇 |
2018年 | 123篇 |
2017年 | 140篇 |
2016年 | 213篇 |
2015年 | 132篇 |
2014年 | 189篇 |
2013年 | 247篇 |
2012年 | 102篇 |
2011年 | 162篇 |
2010年 | 181篇 |
2009年 | 216篇 |
2008年 | 148篇 |
2007年 | 133篇 |
2006年 | 112篇 |
2005年 | 95篇 |
2004年 | 68篇 |
2003年 | 73篇 |
2002年 | 102篇 |
2001年 | 88篇 |
2000年 | 74篇 |
1999年 | 74篇 |
1998年 | 75篇 |
1997年 | 65篇 |
1996年 | 57篇 |
1995年 | 65篇 |
1994年 | 79篇 |
1993年 | 35篇 |
1992年 | 38篇 |
1991年 | 38篇 |
1990年 | 45篇 |
1989年 | 38篇 |
1988年 | 31篇 |
1987年 | 35篇 |
1985年 | 47篇 |
1984年 | 48篇 |
1983年 | 57篇 |
1982年 | 46篇 |
1981年 | 47篇 |
1980年 | 41篇 |
1979年 | 34篇 |
1978年 | 63篇 |
1977年 | 35篇 |
1976年 | 36篇 |
1975年 | 44篇 |
1974年 | 53篇 |
1973年 | 51篇 |
1968年 | 27篇 |
1962年 | 26篇 |
排序方式: 共有4525条查询结果,搜索用时 218 毫秒
581.
The observational set-up for a detailed study of the velocity, intensity and magnetic-field fine structure in and around a sunspot is described. On highly resolved spectra we detected in the vicinity of a sunspot a large number of points with strong magnetic fields (magnetic knots). The magnetic field in these knots causes a striking decrease of the line depth (or a line gap after Sheeley, 1967). The properties of the magnetic knots are: (1) magnetic fields up to 1400 gauss; (2) diameter 1100 km; (3) coincidence with dark intergranular spaces; (4) generally downward material motion; (5) lifetime>30min; (6) estimated total number around an unipolar spot 2000; (7) combined magnetic flux comparable to the sunspot flux; (8) coincidence with Ca+ plages.For the smallest sunspots (pores) we obtained magnetic fields >1500 gauss. Hence a magnetic field of about 1400–1500 gauss appears to be a rather critical level for pore and spot formation.We found a large number of small areas producing line gaps without measurable magnetic field. These non-magnetic gap-regions coincide with bright continuum structures.Some aspects arising from the occurrence of hundreds of magnetic knots in an active region are discussed in the last section.Presently guest investigator at the Göttingen Observatory.Previously member of the High Altitude Observatory solar project at Sacramento Peak (Contract Nr. AF (628) - 4078). 相似文献
582.
583.
Karl-Heinz Böhm 《Astrophysics and Space Science》1968,2(3):375-383
The outer convection zone of the low-temperature white dwarf Van Maanen 2 has been studied for two different atmospheric models given byWeidemann (1960). A slight modification of the standard mixing length theory and the abundances derived by Weidemann have been used.The thickness of the convection zone is about 8 km for the atmospheric model withT
eff=5780 K,g=108 cm sec–2 and about 23 km forT
eff=5040,g=3.16×107K. In both cases the temperature at the lower boundary of the convection zone is about 9.8×105K. It is shown that this temperature corresponds approximately to the transition temperatureT
tr to the (almost) isothermal core of the white dwarf. This value is considerably lower than the values ofT
tr discussed in the literature until now.The outer convection zone consists of an upper completely non-degenerate part and a lower part with moderate degeneracy. In this lower part the degree of degeneracy is practically independent of depth. 相似文献
584.
H. Köhler 《Solar physics》1970,13(1):3-18
The law of rotation as well as the corresponding meridional circulations in the hydrogen convection zone (HCZ) are investigated by solving numerically the time independent Navier-Stokes equations. The HCZ is assumed to be a spherical layer of fluid with constant density and viscosity. It is assumed further that the viscosity is caused by unisotropic turbulent motions.The results show differential rotation together with circulations. The detailed behaviour depends on a parameters characterizing the nonisotropic friction and on the kinematic viscosityv. If the friction is larger in radial direction than in lateral directions (0 s < 1) the poles rotate faster than the equator and the circulation rises at the equator and falls at the poles; if friction is smaller in radial direction (s > 1) the equator rotates faster and the sense of the circulation is reversed. The differential rotation observed at the solar surface is obtained for the values = 1.2.For small values ofv the angular velocity is constant on cylindrical surfaces, for large values ofv it is constant on spherical surfaces. The solar law of rotation turns out to be very close to the first case.Based on the author's Thesis in Göttingen. 相似文献
585.
586.
Dan?HammarlundEmail author Svante?Bj?rck Bj?rn?Buchardt Charlotte T.?Thomsen 《Journal of Paleolimnology》2005,34(4):471-480
Highly resolved sediment stratigraphic and stable isotope records from a lake in south-central Sweden give evidence of relatively humid summers during the wide-spread cold event at c. 8200 cal. BP. A transient lake-level rise led to increased catchment erosion followed by enhanced phytoplankton production and disturbance of the Chara-dominated algal community. An associated increase in the deposition of organic material resulted in hypolimnetic oxygen deficit and methane formation as inferred from elevated sulphur content of the sediments and strong depletion in 13C of bulk organic material. In combination with dry and cold winter conditions as revealed by other proxy records, these data suggest the development of a strongly seasonal climate in northwestern Europe in response to altered atmospheric circulation at this stage. Enhanced zonal circulation with frequent cyclonic activity and increased effective humidity probably characterized the summer climate of southern and central Scandinavia, consistent with conditions inferred for continental Europe. In contrast, the winter climate was dominated by blocking high-pressure circulation, giving rise to cold and dry conditions. 相似文献
587.
Photoelectric polarisation measurements in a stable sunspot (type H) with a particularly dark umbra carried out with the Capri magnetograph have been evaluated in terms of Unno's (1956) theory to give the value and direction of the magnetic field vector. A linear increase of the inclination angle with distance from the spot centre up tor 1.2R
s results, as originally found by Hale and Nicholson (1938) with a different procedure. The field strength decreases from the maximum value (about 3300) to about 15% at the penumbral border, still continuously decreasing outside the spot. The projected field direction deviates considerably from the radial symmetry in several parts of the spot region, but it is in good agreement with that of the overlaying chromospheric structures.Mitteilungen aus dem Fraunhofer Institut Nr. 97. 相似文献
588.
H. Leitmeier F. Machatschki H. Haberlandt A. Preisinger H. Tertsch A. Köhler 《Mineralogy and Petrology》1954,5(1-2):137-152
Ohne Zusammenfassung 相似文献
589.
Erika Böhm-Vitense 《Astrophysics and Space Science》1969,4(2):233-239
For the case of isotropic coherent scattering plus absorption a simple expression is given (Equation 12) to compute the mean intensity of the radiationJ
v
(as a function of optical depth
v
) if the scattering coefficient
v
, the absorption coefficientK
v
and the Planck functionB
v are given as a function of depth. In general the accuracy of this approximation is of the order of a few percent.A fairly simple temperature-correction procedure for the case when scattering is important is described. 相似文献
590.
S. Rösch 《Mineralogy and Petrology》1962,8(1):36-48
Ohne ZusammenfassungMit 4 TextabbildungenHerrn Prof.K. H. Scheumann zum 80. Geburtstag gewidmet. 相似文献