全文获取类型
收费全文 | 2826篇 |
免费 | 12篇 |
国内免费 | 5篇 |
专业分类
航空 | 1292篇 |
航天技术 | 1164篇 |
综合类 | 14篇 |
航天 | 373篇 |
出版年
2021年 | 23篇 |
2019年 | 27篇 |
2018年 | 58篇 |
2017年 | 26篇 |
2016年 | 40篇 |
2015年 | 12篇 |
2014年 | 62篇 |
2013年 | 77篇 |
2012年 | 68篇 |
2011年 | 92篇 |
2010年 | 66篇 |
2009年 | 119篇 |
2008年 | 190篇 |
2007年 | 83篇 |
2006年 | 70篇 |
2005年 | 68篇 |
2004年 | 76篇 |
2003年 | 93篇 |
2002年 | 64篇 |
2001年 | 87篇 |
2000年 | 55篇 |
1999年 | 60篇 |
1998年 | 77篇 |
1997年 | 52篇 |
1996年 | 60篇 |
1995年 | 82篇 |
1994年 | 86篇 |
1993年 | 51篇 |
1992年 | 70篇 |
1991年 | 36篇 |
1990年 | 30篇 |
1989年 | 68篇 |
1988年 | 24篇 |
1987年 | 30篇 |
1986年 | 28篇 |
1985年 | 102篇 |
1984年 | 73篇 |
1983年 | 66篇 |
1982年 | 77篇 |
1981年 | 88篇 |
1980年 | 29篇 |
1979年 | 39篇 |
1978年 | 25篇 |
1977年 | 17篇 |
1976年 | 12篇 |
1975年 | 11篇 |
1974年 | 14篇 |
1973年 | 15篇 |
1972年 | 14篇 |
1970年 | 12篇 |
排序方式: 共有2843条查询结果,搜索用时 0 毫秒
221.
Energetic solar proton events within the energy interval 1–48 MeV at the stage of their decay are considered over the period of 1974–2001. The dependence of the characteristic decay time on the proton energy in the assumed power-law representation τ(E) =E ?n is analyzed for the events with an exponential decay form. The dependence of n on the heliolongitude of the flare (the particles source on the Sun) is studied. 相似文献
222.
The necessary first-order conditions of strong local optimality (conditions of maximum principle) are considered for the problems of optimal control over a set of dynamic systems. To derive them a method is suggested based on the Lagrange principle of removing constraints in the problems on a conditional extremum in a functional space. An algorithm of conversion from the problem of optimal control of an aggregate of dynamic systems to a multipoint boundary value problem is suggested for a set of systems of ordinary differential equations with the complete set of conditions necessary for its solution. An example of application of the methods and algorithm proposed is considered: the solution of the problem of constructing the trajectories of a spacecraft flight at a constant altitude above a preset area (or above a preset point) of a planet's surface in a vacuum (for a planet with atmosphere beyond the atmosphere). The spacecraft is launched from a certain circular orbit of a planet's satellite. This orbit is to be determined (optimized). Then the satellite is injected to the desired trajectory segment (or desired point) of a flyby above the planet's surface at a specified altitude. After the flyby the satellite is returned to the initial circular orbit. A method is proposed of correct accounting for constraints imposed on overload (mixed restrictions of inequality type) and on the distance from the planet center: extended (nonpointlike) intermediate (phase) restrictions of the equality type. 相似文献
223.
A method of modeling the total electron content (TEC) based on the semi-empirical ionospheric model developed in Irkutsk State University is suggested. Comparison with the Klobuchar model has shown that the proposed method provides a more accurate presentation of TEC. A conclusion is drawn that the use of this method for compensation of the ionospheric error in single-frequency navigation receivers would lead to a substantial increase in the accuracy of their positioning. 相似文献
224.
Doppler ultrasound is frequently used for monitoring circulating microbubbles during decompression to assess the symptoms of Decompression Sickness (DCS). This analysis was carried out to evaluate its effectiveness for screening symptoms of DCS during simulated extravehicular activities (EVA). The information from various hypobaric chamber studies carried out at the NASA Johnson Space Center, Houston, TX was used in this analysis (n = 516). The circulating microbubbles were detected in the precordial area in 42% (218/516), and symptoms were reported in 16% (81/516) of these exposures. The accuracy of Doppler-detectable bubbles (Spencer grades) on all symptoms of DCS was examined by calculating measures of sensitivity and specificity. The efficacy of Doppler as a screening device was examined by calculating their positive predictive value (PPV) and negative predictive value (NPV). The results of these analyses indicated that the sensitivity of Doppler decreased, and the PPV increased with higher Spencer grades. However, the likelihood of detecting true negative cases (NPV) was consistently higher with all bubble grades. Due to the high false-positive rate and low prior probabilities of the risk of DCS, Doppler was found to be more useful to identify those who did not develop DCS, than to detect positive cases of DCS in the simulated EVA exposures. 相似文献
225.
The simple tilted dipole picture of Corotating Interaction Regions which prevailed during the first polar pass of Ulysses no longer applies since the Sun entered a more active phase. Recent observations show that CIRs still persist, though the
large polar coronal holes of solar minimum shrink to smaller areas and move to lower latitudes. We present 3-D simulations
for the cosmic-ray intensity variations in a model with non-polar high speed streams. Latitudinal and recurrent time-variations
are discussed, but more detailed and realistic simulations are required before quantitative comparisons with observations
can be made.
This revised version was published online in August 2006 with corrections to the Cover Date. 相似文献
226.
L. McDonald L.K. Harra-Murnion J.L. Culhane R.A. Schwartz 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》1997,20(12):2327-2331
Most solar flare observations show that intense hard X-ray bursts come from large flares that have a large GOES classification (large peak 1 – 8 Å flux). This correlation, known as the “Big Flare Syndrome”, suggests that more intense flares tend to have harder spectra. We have observed 7 flares that are exceptions to this. These flares have small GOES classifications ranging from B1.4 to C5.5 and peak hard X-ray count rates similar to those often observed from M class flares. This paper examines the cause of this anomoly using the Yohkoh Soft X-Ray Telescope, Hard X-Ray Telescope, and Bragg Crystal Spectrometer. Two hypotheses are proposed for the exceptions: (1) flares with multiple magnetic loops and common footpoints, producing multiple hard X-ray emission regions and low density thermal plasma distributed over a large volume, and (2) high densities in the magnetic loops restricting the propagation of the non-thermal electrons in the loop after magnetic reconnection has occurred and suppressing chromospheric evaporation. Two of the flares support the first hypothesis. The other flares either have data missing or are too small to be properly analysed by the Yohkoh instruments. 相似文献
227.
Chao-Song Huang J.C. Foster K. Yumoto J.L. Chau O. Veliz 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2005,36(12):2407-2412
It is well known that the solar wind can significantly affect high-latitude ionospheric dynamics. However, the effects of the solar wind on the middle- and low-latitude ionosphere are much less studied. In this paper, we report observations that large perturbations in the middle- and low-latitude ionosphere are well correlated with solar wind variations. In one event, a significant (20–30%) decrease of the midlatitude ionospheric electron density over a large latitudinal range was related to a sudden drop in the solar wind pressure and a northward turning of the interplanetary magnetic field, and the density decrease became larger at lower latitudes. In another event, periodic perturbations in the dayside equatorial ionospheric E × B drift and electrojet were closely associated with variations in the interplanetary electric field. Since the solar wind is always changing with time, it can be a very important and common source of ionospheric perturbations at middle- and low-latitudes. The relationship between solar wind variations and significant ionospheric perturbations has important applications in space weather. 相似文献
228.
The theoretical framework and experimental methodology used to interpret observations of ionospheric scintillations in terms of geophysical processes are reviewed and recent experimental observations of ionospheric scintillations are discussed in this paper. During the past 15 years significant progress has been made in several areas. In particular, significant advances have been made in theoretical understanding of the strong scintillation regime and the effects of short-term temporal variations of the scintillation producing irregularities on observations made with spaced-receiver geometries in both weak and strong scintillations. This improved understanding of the scintillation process has significantly increased the utility of the technique particularly in the equatorial latitudes where geometrical effects are least important. 相似文献
229.
P. Bobik K. Kudela M. Boschini D. Grandi M. Gervasi P.G. Rancoita 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》2008,41(2):339-342
We developed a one dimensional model of particle transport in the heliosphere. As opposite to widely used models, we apply a method where a quasi-particle is traced back in time. The model gives us the possibility to work on the possible existence of reentrant particles in the heliosphere that can be hardly solved by the traditional forward tracking method. Particles escape from the heliosphere and may reenter back. We estimate how these particles affect the modulation process in the heliosphere. Presented here are the results for different values of particles mean free path in the interstellar space and for different interstellar magnetic field values. 相似文献
230.
O. Alexandrova C. H. K. Chen L. Sorriso-Valvo T. S. Horbury S. D. Bale 《Space Science Reviews》2013,178(2-4):101-139
Solar wind is probably the best laboratory to study turbulence in astrophysical plasmas. In addition to the presence of magnetic field, the differences with neutral fluid isotropic turbulence are: (i) weakness of collisional dissipation and (ii) presence of several characteristic space and time scales. In this paper we discuss observational properties of solar wind turbulence in a large range from the MHD to the electron scales. At MHD scales, within the inertial range, turbulence cascade of magnetic fluctuations develops mostly in the plane perpendicular to the mean field, with the Kolmogorov scaling $k_{\perp}^{-5/3}$ for the perpendicular cascade and $k_{\|}^{-2}$ for the parallel one. Solar wind turbulence is compressible in nature: density fluctuations at MHD scales have the Kolmogorov spectrum. Velocity fluctuations do not follow magnetic field ones: their spectrum is a power-law with a ?3/2 spectral index. Probability distribution functions of different plasma parameters are not Gaussian, indicating presence of intermittency. At the moment there is no global model taking into account all these observed properties of the inertial range. At ion scales, turbulent spectra have a break, compressibility increases and the density fluctuation spectrum has a local flattening. Around ion scales, magnetic spectra are variable and ion instabilities occur as a function of the local plasma parameters. Between ion and electron scales, a small scale turbulent cascade seems to be established. It is characterized by a well defined power-law spectrum in magnetic and density fluctuations with a spectral index close to ?2.8. Approaching electron scales, the fluctuations are no more self-similar: an exponential cut-off is usually observed (for time intervals without quasi-parallel whistlers) indicating an onset of dissipation. The small scale inertial range between ion and electron scales and the electron dissipation range can be together described by $\sim k_{\perp}^{-\alpha}\exp(-k_{\perp}\ell_{d})$ , with α?8/3 and the dissipation scale ? d close to the electron Larmor radius ? d ?ρ e . The nature of this small scale cascade and a possible dissipation mechanism are still under debate. 相似文献