全文获取类型
收费全文 | 41833篇 |
免费 | 723篇 |
国内免费 | 316篇 |
专业分类
测绘学 | 980篇 |
大气科学 | 2962篇 |
地球物理 | 8095篇 |
地质学 | 14395篇 |
海洋学 | 3915篇 |
天文学 | 9836篇 |
综合类 | 74篇 |
自然地理 | 2615篇 |
出版年
2021年 | 405篇 |
2020年 | 407篇 |
2019年 | 458篇 |
2018年 | 946篇 |
2017年 | 851篇 |
2016年 | 1096篇 |
2015年 | 621篇 |
2014年 | 1012篇 |
2013年 | 2103篇 |
2012年 | 1143篇 |
2011年 | 1637篇 |
2010年 | 1489篇 |
2009年 | 1998篇 |
2008年 | 1662篇 |
2007年 | 1748篇 |
2006年 | 1605篇 |
2005年 | 1324篇 |
2004年 | 1287篇 |
2003年 | 1252篇 |
2002年 | 1204篇 |
2001年 | 1064篇 |
2000年 | 995篇 |
1999年 | 819篇 |
1998年 | 828篇 |
1997年 | 843篇 |
1996年 | 677篇 |
1995年 | 666篇 |
1994年 | 625篇 |
1993年 | 574篇 |
1992年 | 536篇 |
1991年 | 502篇 |
1990年 | 513篇 |
1989年 | 505篇 |
1988年 | 482篇 |
1987年 | 562篇 |
1986年 | 498篇 |
1985年 | 616篇 |
1984年 | 660篇 |
1983年 | 604篇 |
1982年 | 534篇 |
1981年 | 591篇 |
1980年 | 479篇 |
1979年 | 467篇 |
1978年 | 443篇 |
1977年 | 439篇 |
1976年 | 386篇 |
1975年 | 385篇 |
1974年 | 383篇 |
1973年 | 392篇 |
1971年 | 230篇 |
排序方式: 共有10000条查询结果,搜索用时 250 毫秒
871.
Zaitsev and Stepanov (1991, 1992) proposed a mechanism for energy release in solar flares that involves the intrusion of dense prominence material into a coronal loop. The resulting non-steady state conditions are claimed to increase the resistance of the loop by 8–10 orders of magnitude. It is shown here that the dramatic increase in resistance calculated by Zaitsev and Stepanov depends on a gross overestimate of the of the magnitude of the magnetic force in the loop prior to the flare trigger. A more realistic estimate of the increase due to the mechanism suggests that it is by no more than about four orders of magnitude. As a consequence, the prominence-loading mechanism does not provide a tenable flare model. 相似文献
872.
Results of observations of the long-period variable Y Ori are presented. The photoelectric observations in UBVR were made at the 60-cm telescope of the high-altitude Maydanak station of the Tashkent Astronomical Institute during the autumn of 1989. The UBVR lightcurves as well as the variations in the color indices U-B, B-V, and V-R are presented. The photographic observations were made at the 40 Schmidt telescope of the Byurakan Observatory and at the 70-cm Maksutov telescope of the Abastumani Observatory. A nebulosity was discovered around Y Ori in red light near the brightness maximum. Such a formation is observed for the first time, not only for Y Ori but also for long-period variables in general. The obtained results are discussed in this work.Translated fromAstrofizika, Vol. 38, No. 1, pp. 5–15, January–March, 1995. 相似文献
873.
We give the results of photographic, photoelectric, and spectral observations of the flare star PP Ori. The 109 photographic observations used, which were obtained on the 40 Schmidt telescope of the Byurakan Observatory over a period of about 20 years, and four spectra obtained on the same telescope with a4° objective prism show no variation in brightness. Photoelectric observations in the UBVR bands using the 50cm and 60cm telescopes of the high-altitude Maidanak station of the Tashkent Astronomical Institute in 1987 and 1989 give grounds for suspecting a variation in brightness much larger than observational errors. Spectral observations of the star PP Ori made on the 2.6m telescope of the Byurakan Observatory show weak H emission. The results of all these observations show that PP Ori is an Orion variable of spectral class K7-M0 with absolute visual magnitude7
m
5–8
m
5.Translated fromAstrofizika, Vol. 38, No. 2, 1995. 相似文献
874.
Image intensified video detection systems were used to observe the 1991 Perseid meteor shower from two locations in eastern Canada. In 29.6 hours of total observing time a total of 668 meteors were detected, of which 403 were Perseids. We derived a profile of TVZHR (television zenithal hourly rate) values for the 1991 Perseid shower over the solar longitude (epoch 2000) interval 138°51 to 141°01. The apparent limiting stellar magnitudes of the observing systems were +9.4 and +8.8 (corresponding to limiting meteor magnitudes for our geometry ranging from +8.7 to +7.0). Within the observing period, the maximum TVZHR rate was approximately 1600, and occurred at solar longitude 139.9°. This is in good agreement with the second peak observed by visual observers. The data suggest that TVZHR values should be divided by a factor of approximately 5 to compare TVZHR and ZHR values. 相似文献
875.
Harry A. Taylor Jr. Leonard Kramer Paul A. Cloutier Shannon S. Walker 《Earth, Moon, and Planets》1995,69(2):173-199
Plasma and field relationships observed across the nightside of Venus evidence a chaotic variety of interactions between the ionosphere and the combined effect of the solar wind and interplanetary magnetic field draped about the planet. Close examination of these data reveal within the chaos a number of repeatable signatures key to understanding fundamental field-plasma interactions. Observed from the Pioneer Venus Orbiter, (PVO), nightside conditions range from extensive, full-up ionospheres with little evidence of dynamic or energetic perturbations, to an almost full depletion, sometimes described as disappearing ionospheres. Between these extremes, the ionospheric structure is often irregular, sometimes exhibiting well-defined density troughs, at other times complex intervals of either abundant or minimal plasma concentration. Consistently, large B-fields (typically exceeding 5–10 nanoteslas) coincide with plasma decreases, whereas stable, abundant plasma distributions are associated with very low-level field. We examine hundreds of nightside orbits, identifying close correlations between regions of elevated magnetic fields featuring polarity reversals, and (a) exclusive low-frequency or distinctive broadband noise, or both, in the electric field data, (b) turbulent, superthermal behavior of the the ions and electrons. We review extensive studies of nightside fields to show that the correlations observed are consistent with theoretical arguments that the presence of strong magnetic fields within normal ionospheric heights indicates the intrusion of magnetosheath fields and plasma within such regions. We find abundant evidence that the ionosphere is frequently disrupted by such events, exhibiting a chaotic, auroral-like complexity appearing over a wide range of altitude and local time. We show that field-plasma disturbances, widely suggested to be similar to conditions in the Earth's auroral regions, are tightly linked to the electric field noise otherwise attributed to lightning. Owing to the coincidence inherent in this relationship, we suggest that natural, predictable plasma instabilities associated with the plasma gradients and current sheets evident within these events produce the E-field noise. The data relationships argue for a more detailed investigation of solar wind induced E-field noise mechanisms as the appropriate scientific procedure for invoking sources for the noise previously attributed to lightning. Consistent with these views, we note that independent analyses have offered alternative explanations of the noise as arising from ionospheric disturbances, that repeated searches for optical evidence of lightning have found no such evidence, and that no accepted theoretical work has yet surfaced to support the inference of lightning at Venus. 相似文献
876.
M. Barsony D. Sasselov S. Rucinski E. Bloemhof L. -A. Nyman 《Astrophysics and Space Science》1995,233(1-2):51-54
We present new images of the well-known molecular outflow and Herbig-Haro complex L 1551-IRS 5. Deep, high-resolution images of the central region of the flow in [SII] 6716,6731 and H (6565 Å) are complemented by a mosaic of much of the CO outflow in H2 v=1-0 S(1). While the optical data trace the intermediate-to-high excitation shocks in the flow (v
shock
> 30 – 50 km s–1), the near-IR data reveal the lower-excitation, molecular shocks (v
shock
10–50 km s–1). In particular, the H2 data highlight the regions where the flow impacts and shocks ambient molecular gas. 相似文献
877.
The radial velocity and profile variations of UV lines of the shell star Tau have been examined in the IUE spectra obtained in 1978–1982. The neutral atoms, and once or twice-ionized ions (exceptCii, Aliii, Siiii resonance lines) follow the same velocity variations as in the visual spectra, while the Siiv andCiv resonance lines show a constant negative velocity (–50 km s–1 at the core). The Aliii,Cii resonance lines and probably Feiii (mult. No. 34) are formed in both regions, i.e., in lowly-ionized and highly-ionized regions and the Siiii resonance line is formed in a highly-ionized region.Paper presented at the Lembang-Bamberg IAU Colloquium No. 80 on Double Stars: Physical Properties and Generic Relations, held at Bandung, Indonesia, 3–7 June, 1983. 相似文献
878.
A statistical analysis is used to determine the properties of metre-wavelength events which are associated with interplanetary type II bursts. It is found that the likelihood of an interplanetary type II burst is greatly increased if: (a) an associated metre-wavelength type II has a starting frequency less than 45 MHz; (b) a strong metre-wavelength continuum is present; (c) the type II contains herringbone fine structure; and (d) the metre-wavelength activity is accompanied by strong, long-lasting H and soft X-ray events.Visiting scientist at Division of Radiophysics, January 1983; previous address - NASA/Goddard Space Flight Center, Greenbelt, Maryland. 相似文献
879.
An energy balance model has been developed to investigate how the Martian atmospheric environment could influence a community of photosynthetic microorganisms with properties similar to those of a cyanophyte (blue-green algal mat) and a lichen. Surface moisture and soil nutrients are assumed to be available. The model was developed to approximate equatorial equinox conditions and includes parameters for solar and thermal radiation, convective and conductive energy transport, and evaporative cooling. Calculations include the diurnal variation of organism temperature and transpiration and photosynthetic rates. The influences of different wind speeds and organism size and resistivity are also studied. The temperature of organisms in mats less than a few millimeters thick will not differ from the ground temperature by more than 10°K. Water loss is actually retarded at higher wind speeds, since the organism temperature is lowered, thus reducing the saturation vapor pressure. Typical photosynthetic rates lead to the production of 10?6 to 10?7 mole O2 cm?2 day?1. 相似文献
880.