全文获取类型
收费全文 | 41036篇 |
免费 | 1202篇 |
国内免费 | 793篇 |
专业分类
测绘学 | 922篇 |
大气科学 | 3016篇 |
地球物理 | 8261篇 |
地质学 | 15275篇 |
海洋学 | 3840篇 |
天文学 | 8795篇 |
综合类 | 324篇 |
自然地理 | 2598篇 |
出版年
2022年 | 381篇 |
2021年 | 581篇 |
2020年 | 595篇 |
2019年 | 691篇 |
2018年 | 1159篇 |
2017年 | 1136篇 |
2016年 | 1219篇 |
2015年 | 712篇 |
2014年 | 1152篇 |
2013年 | 1995篇 |
2012年 | 1321篇 |
2011年 | 1772篇 |
2010年 | 1581篇 |
2009年 | 1922篇 |
2008年 | 1727篇 |
2007年 | 1797篇 |
2006年 | 1625篇 |
2005年 | 1140篇 |
2004年 | 1129篇 |
2003年 | 1171篇 |
2002年 | 1052篇 |
2001年 | 931篇 |
2000年 | 880篇 |
1999年 | 846篇 |
1998年 | 785篇 |
1997年 | 830篇 |
1996年 | 671篇 |
1995年 | 625篇 |
1994年 | 556篇 |
1993年 | 528篇 |
1992年 | 456篇 |
1991年 | 464篇 |
1990年 | 461篇 |
1989年 | 406篇 |
1988年 | 384篇 |
1987年 | 413篇 |
1986年 | 427篇 |
1985年 | 515篇 |
1984年 | 553篇 |
1983年 | 546篇 |
1982年 | 499篇 |
1981年 | 461篇 |
1980年 | 431篇 |
1979年 | 410篇 |
1978年 | 376篇 |
1977年 | 385篇 |
1976年 | 344篇 |
1975年 | 354篇 |
1974年 | 340篇 |
1973年 | 372篇 |
排序方式: 共有10000条查询结果,搜索用时 437 毫秒
851.
L. E. Gurevich 《Astrophysics and Space Science》1975,38(1):67-78
An attempt is made to explain two properties of the metagalaxy: its expansion and the absence of causal connection of its distant regions during a large part of its initial history. It is postulated that the gravitating matter was born from the cosmological field, i.e., a medium with equation of statep=–.It is shown that such a postulate explains the expansion of metagalaxy and leads to a correct estimate of the entropy per baryon. The problem of causal connection can also be solved on this basis. 相似文献
852.
E. P. Mazets S. V. Golenetskii V. N. Il'inskii Yu. A. Gur'yan T. V. Kharitonova 《Astrophysics and Space Science》1975,33(2):347-357
Diffuse cosmic background and atmospheric gamma-radiation in the range 28 keV-4.1 MeV were studied with a scintillation spectrometer on board of the Kosmos 461 satellite. Separation of the cosmic and atmospheric components was made possible through a reliable determination of the geomagnetic dependences of albedo gamma-radiation: The spectrum of diffuse background in the energy range covered cannot be fitted with a common law. At energies below 400 keV the spectrum follows a power-law $$I = (5.6 \pm 0.5) \times 10^{ - 3} E^{ - (2.80 \pm 0.05)} cm^{ - 2} s^{ - 1} sr^{ - 1} MeV^{ - 1} .$$ Starting from 400 keV, this power-law breaks down; the spectrum revealing a clearly pronounced shoulder. Extrapolation of the power-law spectrum to higher energies shows that the gamma-ray component responsible for the change in the shape of the spectrum is quite strong, becoming predominant in the diffuse background in the range 1–100 MeV. The intensity of excess radiation is maximum in the region of 700–800 keV reaching ~1.8×10?2 cm?2s?1sr?1 MeV?1. The shape of the high energy component spectrum of the diffuse background constructed using the data of Kosmos 461 and SAS-2 is in agreement with the hypotheses of the cosmological origin of the radiation. 相似文献
853.
P. Kaufmann P. Iacomo Jr. E. H. Koppe P. Marques Dos Santos R. E. Schaal J. R. Blakey 《Solar physics》1975,45(1):189-197
The active region McMath 10433 was the source of several flares and radio outbursts during the early part of July 1974. This region was tracked continuously, for several periods during the month at 22.2 GHz using a telescope with a 4 beam. Comparison with the results obtained simultaneously with a normal 7 GHz solar patrol instrument indicate that there is important burst activity occurring at levels below the detection limit of normal solar patrol instruments. The time-development morphology of these bursts is similar to those normally observed and has enabled the simple events to be re-interpreted. A completely new type of event - the fast absorption - has also been recognized. The correlation of the microwave events with SPA events observed on VLF propagation is also discussed.Spending a sabbatical year at CRAAM, São Paulo, Brazil. 相似文献
854.
The radio emission of a selected number of solar active regions has been investigated with high angular resolution at two frequencies: 10 and 17 GHz. By comparing the results of the two observations the following conclusions can be drawn:
- The brightness temperature distribution of an active region is often composed of very bright cores of small dimension (angular extent θ?20″) imbedded in extended halos of lower brightness.
- The radio emission of such structures as well as the degree of polarization can be explained with a thermal process. The halos can originate by pure thermal bremsstrahlung while in the case of the very bright cores found at 10 GHz (brightness temperature T b?1–9 × 106K) the emission at the harmonics of the gyrofrequency is needed.
855.
J. A. Vorpahl E. G. Gibson P. B. Landecker D. L. McKenzie J. H. Underwood 《Solar physics》1975,45(1):199-216
132 soft X-ray flare events have been observed with The Aerospace Corporation/Marshall Space Flight Center S-056 X-ray telescope that was part of the ATM complement of instruments aboard Skylab. Analyses of these data are reported in this paper. The observations are summarized and a detailed discussion of the X-ray flare structures is presented. The data indicated that soft X-rays emitted by a flare come primarily from an intense well-defined core surrounded by a region of fainter, more diffuse emission. Loop structures are found to constitute a fundamental characteristic of flare cores and arcades of loops are found to play a more important role in the flare phenomena than previously thought. Size distributions of these core features are presented and a classification scheme describing the brightest flare X-ray features is proposed. The data show no correlations between the size of core features and: (1) the peak X-ray intensity, as indicated by detectors on the SOLRAD satellite; (2) the rise time of the X-ray flare event, or (3) the presence of a nonthermal X-ray component. An analysis of flare evolution indicates evidence for preliminary heating and energy release prior to the main phase of the flare. Core features are found to be remarkably stable and retain their shape throughout a flare. Most changes in the overall configuration seem to be the result of the appearance, disappearance or change in brightness of individual features, rather than the restructuring or re-orientation of these features. Brief comparisons with several theories are presented. 相似文献
856.
The origin of the material which is ejected during a white light coronal transient has not been determined heretofore. Study of a disturbance on 26 and 27 August 1973, during which a slowly ascending prominence and a more rapid accompanying coronal transient were simultaneously observed, helps to resolve this question. Prominence images obtained in Hα 6563 Å and in He II 304 Å are nearly identical. The mass ejection transient observed in white light (3700–7000 Å) appeared to be a loop about 1 R⊙ higher than the top of the ascending prominence; it accelerated away from the prominence below it. These observations imply: (1) the bulk of the ejected material did not originate in the ascending prominence; (2) therefore, most of the material must have come from the low corona above the prominence, (and was at coronal temperatures during its outward passage); and (3) the total event - ascending prominence accompanied by coronal mass ejection - was far larger, more energetic, and longer lasting than would be inferred from the prominence observations alone. The transient of 26–27 August was slow and of atypical shape compared to other mass ejection transients, but we believe that these three conclusions apply to most, if not all, of the more than 60 loop-shaped coronal transients observed by the High Altitude Observatory's coronagraph during the nine-month flight of Skylab. 相似文献
857.
N. R. Sheeley Jr. J. D. Bohlin G. E. Brueckner J. D. Purcell V. E. Scherrer R. Tousey J. B. Smith Jr. D. M. Speich E. Tandberg-Hanssen R. M. Wilson A. C. De Loach R. B. Hoover J. P. Mc Guire 《Solar physics》1975,45(2):377-392
This paper describes Skylab/ATM observations of the events associated with a disappearing filament near the center of the solar disk on January 18, 1974. As the filament disappeared, the nearby coronal plasma was heated to a temperature in excess of 6 × 106K. A change in the pattern of coronal emission occurred during the 11/3 hr period that the soft X-ray flux was increasing. This change seemed to consist of the formation and apparent expansion of a loop-like coronal structure which remained visible until its passage around the west limb several days later. The time history of the X-ray and microwave radio flux displayed the well-known gradual-rise-and-fall (GRF) signature, suggesting that this January 18 event may have properties characteristic of a wide class of X-ray and radio events.In pursuit of this idea, we examined other spatially-resolved Skylab/ATM observations of long-duration X-ray events to see what characteristics they may have in common. Nineteen similar long-lived SOLRAD X-ray events having either the GRF or post-burst radio classification occurred during the nine-month Skylab mission. Sixteen of these occurred during HAO/ATM coronagraph observations, and 7 of these 16 events occurred during observations with both the NRL/ATM slitless spectrograph and the MSFC-A/ATM X-ray telescope. The tabulation of these events suggests that all long-lived SOLRAD X-ray bursts involve transients in the outer corona and that at least two-thirds of the bursts involve either the eruption or major activation of a prominence. Also, these observations indicate that long-lived SOLARD events are characterized by the appearance of new loops of emission in the lower corona during the declining phase of the X-ray emission. However, sometimes these loops disappear after the X-ray event (like the post-flare loops associated with a sporadic coronal condensation), and sometimes the loops remain indefinitely (like the emission from a permanent coronal condensation).Visiting Scientist, Kitt Peak National Observatory, Tucson, Ariz. 85726, U.S.A. operated by the Association of Universities for Research in Astronomy, Inc., under contract with the National Science Foundation.Presently located at NASA/MSFC, Space Sciences Laboratory, Marshall Space Flight Center, Ala. 35812, U. S.A. 相似文献
858.
库姆塔格沙漠"羽毛状沙丘"形态的示量特征 总被引:10,自引:2,他引:10
通过卫星照片判读和实地观测,库姆塔格沙漠的“羽毛状沙丘”由两种风沙地貌构成:东北-西南走向的新月形“沙垄”为“羽轴”;垄间分布的波状微起伏的“大沙波”为“羽枝”,“大沙波”与沙垄的夹角为75°~103°。二者组合成类似“羽毛”的风沙地貌。新月形“沙垄”由单个新月形沙丘的前一沙丘的迎风坡与后一沙丘东翼相连构成“沙垄”。组成“沙垄”的新月形沙丘的两翼平均长37.5m,翼间距30~66m,沙丘高3~19m;在所观测区域内,沙丘沙的分选性由南向北变差。新月形“沙垄”长为3~22km,垄间距为1~3km,其问分布浅色和暗色相间的波状微地貌,暗色部分平均宽24.3m,浅色部分的平均宽11.6m。暗色部分表层沙粒的粒径有60%在1.00~0.25mm之间,以暗色矿物为主,而浅色部分表层沙粒的粒径的90%在1.00~0.25mm之间,分选性相对暗色部分较好,以石英等浅色矿物为主。暗色和浅色微地貌成对出现,相对高差约7cm。这种波状微地貌在库姆塔格沙漠中北部重复出现,类似于风沙地貌分类的沙波,暂称其为“大沙波”。 相似文献
859.
860.