全文获取类型
收费全文 | 57378篇 |
免费 | 868篇 |
国内免费 | 335篇 |
专业分类
测绘学 | 1530篇 |
大气科学 | 4441篇 |
地球物理 | 10840篇 |
地质学 | 23805篇 |
海洋学 | 4285篇 |
天文学 | 10771篇 |
综合类 | 250篇 |
自然地理 | 2659篇 |
出版年
2022年 | 206篇 |
2021年 | 392篇 |
2020年 | 446篇 |
2019年 | 411篇 |
2018年 | 3894篇 |
2017年 | 3667篇 |
2016年 | 2719篇 |
2015年 | 867篇 |
2014年 | 1167篇 |
2013年 | 2041篇 |
2012年 | 2320篇 |
2011年 | 4172篇 |
2010年 | 3802篇 |
2009年 | 4168篇 |
2008年 | 3317篇 |
2007年 | 3897篇 |
2006年 | 1388篇 |
2005年 | 1455篇 |
2004年 | 1360篇 |
2003年 | 1335篇 |
2002年 | 1151篇 |
2001年 | 845篇 |
2000年 | 817篇 |
1999年 | 623篇 |
1998年 | 590篇 |
1997年 | 695篇 |
1996年 | 520篇 |
1995年 | 538篇 |
1994年 | 536篇 |
1993年 | 428篇 |
1992年 | 419篇 |
1991年 | 392篇 |
1990年 | 429篇 |
1989年 | 370篇 |
1988年 | 363篇 |
1987年 | 412篇 |
1986年 | 355篇 |
1985年 | 448篇 |
1984年 | 451篇 |
1983年 | 473篇 |
1982年 | 452篇 |
1981年 | 417篇 |
1980年 | 440篇 |
1979年 | 357篇 |
1978年 | 321篇 |
1977年 | 323篇 |
1976年 | 285篇 |
1975年 | 282篇 |
1974年 | 285篇 |
1973年 | 279篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
961.
The planetary radio astronomy experiment on the Voyager spacecraft observed several type II solar radiobursts at frequencies below 1.3 MHz; these correspond to shock waves at distances between 20R
and 1 AU from the Sun. We study the characteristics of these bursts and discuss the information that they give on shock waves in the interplanetary medium and on the origin of the high energy electrons which give rise to the radioemission. The relatively frequent occurence of type II bursts at large distances from the Sun favors the hypothesis of the emission by a longitudinal shockwave. The observed spectral characteristics reveal that the source of emission is restricted to only a small portion of the shock. From the relation between type II bursts, type III bursts and optical flares, we suggest that some of the type II bursts could be excited by type III burst fast electrons which catch up the shock and are then trapped. 相似文献
962.
This article is an English translation of the work cited in 46: 3092. The authors review the conclusions of Pivovarov (see 44: Title 3118) concerning the nature of the urbanization process and optimal city size in the USSR. The relationship between urbanization and the characteristics of a given socioeconomic system is examined, and the possibility of determining optimal size for cities of different functional types is evaluated 相似文献
963.
In the first part, the authors review a group of studies dealing with the identification of regional differences in Italian migratory patterns, especially using the concept of migration field. The second part is devoted to a study of the hierarchical clustering of Italian provinces using 1973 inter-provincial migratory flow data. (summary in ENG, FRE) 相似文献
964.
965.
Dennis B. Ward 《Icarus》1977,32(4):437-442
The spectrum of Saturn and its rings between 45 and 115 μm has been measured at an average resolving power of 14 from the NASA Lear Jet. The combined brightness temperature of the rings and planetary disk decreases beyond 65 μm, in disagreement with previous results. A brightness temperature of 65 ± 10°K is obtained for the planetary disk in the 80–110-μm wavelength range if a large-particle, constant-emissivity model is assumed for the rings. The possible effects of small particles in the rings are briefly considered. 相似文献
966.
B. Schmieder 《Solar physics》1977,54(2):269-288
A method coupling the hydrodynamical equations and radiative transfer in a realistic solar model atmosphere is described. The influence of the temperature gradient of the model and the radiative dissipation is pointed out.The effect of the large temperature gradient is important in the layers where the optical depth 5000 is greater than 0.5; the ratio between the amplitude of the temperature and the velocity fluctuations decreases with the altitude by a factor 2 between = 1 and = 0.5 and in the case of the acoustic waves, the phase shift between these fluctuations is small.The radiative energy loss in the thick layers (
5000 = 1) leads to a decrease of the vertical phase velocity of the waves and to a damping of their amplitudes in the layers of intermediate optical depth (10-2 < 5000 < 0.5). The effect of the dissipation is negligible in the thin layers (5000 < 10-2). 相似文献
967.
Radiative recombination of N and O provides a significant source for auroral emission in the γ and δ bands of NO with selective population of vibrational levels in the A2Σ+ and C2Π states. This mechanism may account for emissions detected near 2150 Å. Models are derived for the auroral ionosphere and include estimates for the concentrations of N and NO. The concentration of NO is estimated to have a value of about 108 cm?1 near 140 km in an IBC III aurora. The corresponding density for N is about 5 × 107cm?3 and the concentration ratio has a value of about 5.5. 相似文献
968.
M. Hénon 《Celestial Mechanics and Dynamical Astronomy》1977,15(1):99-105
We show the existence of a general relation between the parameters of periodic solutions in dynamical systems with ignorable coordinates. In particular, for time-independent systems with an axis of symmetry, the relation takes the form T/A=–/E, whereT is the period,A is the angular momentum, is the angle through which the system has rotated after one period, andE is the energy. 相似文献
969.
The north-south incidence has been studied of 31 white-light flares observed since 1859 and of 1669 events meeting the criteria for major flares of Dodson and Hedeman (1971) for the period 1955–1974. The asymmetry in favor of the northern hemisphere increases strikingly with the importance of the events. Similarly, magnetically complex sunspot groups (Mt. Wilson classes, and) display a more pronounced asymmetry in favor of the north than non-complex groups for 1962–1970. Contrary to the flare asymmetry, the spottedness asymmetry is independent of the size of sunspots. 相似文献
970.
J. B. Eades Jr. 《Celestial Mechanics and Dynamical Astronomy》1977,16(3):315-342
In this paper station-keeping is regarded as the specialization of relative motion referred to a moving, orbiting origin. For this study a well-known linear solution to this problem serves as a base formulation from which the investigation begins.Even though the overall problem could be separated into three parts—necessitating two distinct studies for coverage—this paper examines only the last phase of the operation. That is, it is the continuation, or retention, of a relative motion sub-orbit which is of interest in this analysis.The situation becomes one of determining what is needed to maintain the sub-orbit, and/or to adjust it. In addition, the consequences of these actions, and the results predicted from theory are established, examined, and commented upon. In the presentation of information gathered from this study, various graphs have been prepared; however, these are expressed in a dimensionless format so that the results apply equally to all reasonable reference orbits.For this analysis the separate and combined influences, due to both the initial relative motion state and to a thrusting condition, are examined. Also, the consequences of referencing the acquired sub-orbits to an inertially aligned frame of reference (rather than the familiar rotating one) are illustrated, discussed, and commented upon.Formerly: Sr. Analyst and Manager, Seabrook Office, AMA, Inc., Seabrook, Md., U.S.A. 相似文献