全文获取类型
收费全文 | 17709篇 |
免费 | 34篇 |
国内免费 | 121篇 |
专业分类
航空 | 9793篇 |
航天技术 | 5197篇 |
综合类 | 242篇 |
航天 | 2632篇 |
出版年
2021年 | 155篇 |
2018年 | 189篇 |
2016年 | 151篇 |
2014年 | 433篇 |
2013年 | 515篇 |
2012年 | 406篇 |
2011年 | 565篇 |
2010年 | 395篇 |
2009年 | 743篇 |
2008年 | 787篇 |
2007年 | 347篇 |
2006年 | 428篇 |
2005年 | 368篇 |
2004年 | 415篇 |
2003年 | 497篇 |
2002年 | 459篇 |
2001年 | 549篇 |
2000年 | 351篇 |
1999年 | 449篇 |
1998年 | 409篇 |
1997年 | 313篇 |
1996年 | 363篇 |
1995年 | 448篇 |
1994年 | 406篇 |
1993年 | 355篇 |
1992年 | 309篇 |
1991年 | 249篇 |
1990年 | 239篇 |
1989年 | 392篇 |
1988年 | 203篇 |
1987年 | 235篇 |
1986年 | 229篇 |
1985年 | 637篇 |
1984年 | 516篇 |
1983年 | 401篇 |
1982年 | 488篇 |
1981年 | 607篇 |
1980年 | 245篇 |
1979年 | 192篇 |
1978年 | 189篇 |
1977年 | 145篇 |
1976年 | 155篇 |
1975年 | 188篇 |
1974年 | 180篇 |
1973年 | 161篇 |
1972年 | 188篇 |
1971年 | 148篇 |
1970年 | 143篇 |
1969年 | 147篇 |
1967年 | 142篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
321.
Comparing the improved Di Giovanni/Radicella model with sounding-based electron density profiles and with the IRI model 总被引:1,自引:0,他引:1
W. Singer J. Weiss J. Bremer 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》1994,14(12):83-86
The Di Giovanni/Radicella model (DGR) /1/ determines a bottom side electron densty profile alone from the set of routinely scaled ionogram parameters foE, foF1, foF2 and M(3000)F2 and the total electron content; the smoothed sunspot number R12 appears in the calculation. Present designations are DGR2/2/ and DRR3 /3/ [see Appendix]; they are valid in the northern hemisphere. DGR is compared with electron density profiles derived from ionograms obtained at Juliusruh (54.6°N, 13.4°E), and with the (URSI-based) IRI90 at different conditiones. Experimental total electron content (TEC) data are compared to both models. At the considered station, the profiles obtained by both models are reasonably in agreement amongst themselves and with the experimental data.
The TEC derived from the DGR3 model is in good agreement with experimental TEC, whereas, at high solar activity, IRI90 gives too high TEC values, especially during daytime. 相似文献
322.
T.L. Gulyaeva 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》1994,14(12):149-152
A properly weighted measure of histogram distortion is proposed to evaluate data with different time scales for any index in any field. This approach is applied to produce a long-term data base of disturbance measure of the geomagnetic indices and ionospheric VI sounding characteristics. It is shown that the ionosphere disturbances detected by the proposed technique include all the storm periods defined by the statistical procedure used earlier at IRPL, NBS, Washington, D.C. 相似文献
323.
H. Ben Hadid B. Roux P. Laure P. Tison D. Camel J. J. Favier 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》1988,8(12):293-304
The problem of surface tension-driven flows in horizontal liquid layers has been studied experimentally, and theoretically by direct numerical simulation and small perturbation analysis. We focus our attention on situations in which the depth of the fluid (liquid tin; small Prandtl number, Pr=0.015) is small enough to ensure the predominance of the surface tension forces over those due to the buoyancy. The surface velocity has been experimentally obtained for liquid tin layer with various aspect ratio (length to height) in the range 5<A<83. The thermal gradients are ranged from 5 to 40°K/cm. In the numerical study, the Navier-Stokes and energy equations are solved by an efficient finite difference technique. The parameters governing the flow behaviour in the liquid are varied to determine their effects on thermocapillary convection: the Reynolds number 10<Re<2104 and the aspect ratio 2<A<25; with Pr kept constant at Pr=0.015. The linear eigenequation resulting from small spatial disturbances of the Couette flow solution is solved using an Tau-Chebyshev approximation. A notable feature of the theoretical study is the totally different end circulations. In the region near the cold wall a multicell structure is evident. This agrees with the eigensolution which is of complex type, indicating spatial periodicity. In the hot wall region the flow is accelerated to reach the velocity value for the fully-developed Couette flow which is reached under conditions such as Re/A<20. The transition from viscous to boundary layer regime occurs for a critical value (Re/A)c of nearly about 200, as deduced from the numerical and experimental results. 相似文献
324.
B. Valnicek V. Vanysek 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》1985,5(12):137-141
A total of 3600 spectra of Comet Halley in the 275–710 nm were obtained on March, 8, 9, 10 and 11, 1986, from the VEGA 2 spacecraft. The emissions of OH, NH, CN, C3, CH, C2, NH2 and H2O+ are identified. From the OH intensity in the (0,0) band: 1.1 Megarayleigh at 5400 km from the nucleus, it can be inferred that the OH production rate was (1.4 ± 0.5)×1030 molecules s−1. The NH, C3, CH and NH2 bands became comparatively more intense at distances from the nucleus shorter than 3000km. At 06:40 U.T. when the instrument field of view was 6000×4500 km, two jets were observed. Spectra from the jets show significant differences with other spectra. Inside a jet NH, C3 and NH2 are comparatively more intense and the rotational distributions of OH, CN and C2 are strongly distorted. This shows that part of the observed emissions probably comes from radicals directly produced in the excited state during the initial process of photolysis of the parent molecules. 相似文献
325.
Modeling of “gradual” solar energeticparticle events using a stochastic differential equation method
M. Den T. Yoshida K. Yamashita 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2003,32(12):2685-2690
We have modeled “gradual” solar energetic particle events through numerical simulations using a StochasticDifferential Equation (SDE) method. We consider that energetic particle events are roughly divided into two groups: (1) where the shock was driven by coronal mass ejections (CMEs) associated with large solar flares, and (2) where they have no related solar events apart from the CMEs. (The detailed classification of energetic particle events was discussed in our previous paper.) What we call “gradual” solar energetic particle events belong to the former group. Particles with energies greater than 10 MeV are observed within several hours after the occurrence of flares and CMEs in many gradual events. By applying the SDE method coupled with particle splitting to diffusive acceleration, we found that an injection of high energy particles is necessary for early enhancement of such a high-energy proton flux and that it should not be presumed that the solar wind particles act as the seed population. 相似文献
326.
L. Dam M. Derrien 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2002,29(12):373-2073
This paper presents the scientific objectives of the Solar Physics and Interferometry Mission (SPI), describes succinctly the model payload and summarizes mission's issues. Novel instrumentation (interferometry) and clever mission design (small platform on low orbit with high telemetry and dedicated smaller platform on hexapod for permanently Sun-centered instruments) allow both spectral imaging and Helioseismology at very high spatial and temporal resolutions. Although not retained by ESA, this mission could become reality through NASA MIDEX and/or CNES PROTEUS opportunities as soon as 2007–2008. 相似文献
327.
K. Arai J. Sakakibara 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2006,37(12):2202-II
A method for estimation of sea surface temperature, ocean wind speed and water vapor with microwave radiometer data based on simulated annealing is proposed. The proposed method shows about 60% improvement of sea surface temperature estimation accuracy in comparison to the existing method using Newton’s iterative algorithm. 相似文献
328.
A. Aran B. Sanahuja D. Lario 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2005,36(12):2333-2338
We present a preliminary version of a potential tool for real time proton flux prediction which provides proton flux profiles and cumulative fluence profiles at 0.5 and 2 MeV of solar energetic particle events, from their onset up to the arrival of the interplanetary shock at the spacecraft position (located at 1 or 0.4 AU). Based on the proton transportation model by Lario et al. [Lario, D., Sanahuja, B., Heras, A.M. Energetic particle events: efficiency of interplanetary shocks as 50 keV E < 100 MeV proton accelerators. Astrophys. J. 509, 415–434, 1998] and the magnetohydrodynamic shock propagation model of Wu et al. [Wu, S.T., Dryer, M., Han, S.M. Non-planar MHD model for solar flare-generated disturbances in the Heliospheric equatorial plane. Sol. Phys. 84, 395–418, 1983], we have generated a database containing “synthetic” profiles of the proton fluxes and cumulative fluences of 384 solar energetic particle events. We are currently validating the applicability of this code for space weather forecasting by comparing the resulting “synthetic” flux profiles with those of several real events. 相似文献
329.
P.L Pritchett 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》1997,19(12):1807-1816
Recent developments regarding collisionless reconnection in current sheets with a finite normal magnetic field component (Bz) are reviewed. In 2-D x, z configurations the ion tearing mode is stabilized by the electron compressibility. When the y dependence is included, cross-field current instabilities can be excited. Of these, the drift kink mode appears to be particularly important. 3-D electromagnetic particle simulations indicate that this mode can act as the precursor to the growth of tearing modes and subsequent reconnection. 相似文献
330.
Some comparisons of orbit determination and parameter estimation based on LAGEOS 1 and LAGEOS 2 data
H. Montag 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》1995,16(12):149-154
The modelling accuracy of the LAGEOS 1 orbit was continously improved since its launch in 1976. In spite of these experiences the modelling accuracy of LAGEOS 2 is still about 20 per cent worse. Considering e. g. only the influence of different gravity field models it has been shown that the orbital fits for arc lengths of one month is generally about 25 mm for LAGEOS 1 and more than 30 mm for LAGEOS 2. This is mainly due to the fact that LAGEOS 2 has not yet been used for the determination of most gravity field models. The influences of different model parameters on the estimation of station coordinates, Earth rotation parameters, the geogravitational coefficient, the radiation pressure coefficient, and the empirical acceleration has been studied. The differences and peculiarities of both satellites are discussed. Although the analysis of LAGEOS 2 data still does not reach the high level of LAGEOS 1 combination solutions using both satellites allow new insights, higher accuracies, and a higher time resolution for the parameters and phenomena investigated. 相似文献