全文获取类型
收费全文 | 8939篇 |
免费 | 19篇 |
国内免费 | 26篇 |
专业分类
航空 | 4095篇 |
航天技术 | 3088篇 |
综合类 | 34篇 |
航天 | 1767篇 |
出版年
2021年 | 83篇 |
2019年 | 62篇 |
2018年 | 237篇 |
2017年 | 163篇 |
2016年 | 138篇 |
2015年 | 64篇 |
2014年 | 209篇 |
2013年 | 253篇 |
2012年 | 257篇 |
2011年 | 418篇 |
2010年 | 309篇 |
2009年 | 439篇 |
2008年 | 486篇 |
2007年 | 323篇 |
2006年 | 204篇 |
2005年 | 251篇 |
2004年 | 219篇 |
2003年 | 266篇 |
2002年 | 182篇 |
2001年 | 281篇 |
2000年 | 160篇 |
1999年 | 200篇 |
1998年 | 236篇 |
1997年 | 143篇 |
1996年 | 207篇 |
1995年 | 243篇 |
1994年 | 247篇 |
1993年 | 158篇 |
1992年 | 194篇 |
1991年 | 77篇 |
1990年 | 75篇 |
1989年 | 187篇 |
1988年 | 77篇 |
1987年 | 83篇 |
1986年 | 85篇 |
1985年 | 248篇 |
1984年 | 195篇 |
1983年 | 156篇 |
1982年 | 175篇 |
1981年 | 261篇 |
1980年 | 71篇 |
1979年 | 77篇 |
1978年 | 67篇 |
1977年 | 51篇 |
1975年 | 58篇 |
1974年 | 45篇 |
1973年 | 41篇 |
1972年 | 43篇 |
1971年 | 42篇 |
1970年 | 45篇 |
排序方式: 共有8984条查询结果,搜索用时 390 毫秒
501.
502.
503.
504.
Cascaded detector for multiple high-PRF pulse Doppler radars 总被引:1,自引:0,他引:1
A postdetection design methodology for a multiple high-pulse-repetition frequency (PRF) pulse Doppler radar has been developed. The postdetection processor consists of an M out of N detector where range and target ambiguities are resolved, followed by a square-law detector which enhances the minimum signal-to-noise (S/N) power-ratio per pulse burst performance. For given probabilities of false alarm and detection, formulas are derived from which the three thresholds associated with the cascaded detector can be found. Fundamental tradeoffs between the minimum S/N required, number of ghosts, and the number of operations (NOPs) that the cascaded detector must perform are identified. It is shown that the NOPs and the number of ghosts increase and the minimum S/N required decreases as the binary M out of N detector passes more detections to the square-law detector 相似文献
505.
H. Kunow N. U. Crooker J. A. Linker R. Schwenn R. von Steiger 《Space Science Reviews》2006,123(1-3):1-2
506.
A. G. Yahnin I. V. Despirak A. A. Lubchich B. V. Kozelov N. P. Dmitrieva M. A. Shukhtina H. K Biernat 《Space Science Reviews》2006,122(1-4):97-106
Although the auroral substorm has been long regarded as a manifestation of the magnetospheric substorm, a direct relation
of active auroras to certain magnetospheric processes is still debatable. To investigate the relationship, we combine the
data of the UV imager onboard the Polar satellite with plasma and magnetic field measurements by the Geotail spacecraft. The
poleward edge of the auroral bulge, as determined from the images obtained at the LHBL passband, is found to be conjugated
with the region where the oppositely directed fast plasma flows observed in the near-Earth plasma sheet during substorms are
generated. We conclude that the auroras forming the bulge are due to the near-Earth reconnection process. This implies that
the magnetic flux through the auroral bulge is equal to the flux dissipated in the magnetotail during the substorm. Comparison
of the magnetic flux through the auroral bulge with the magnetic flux accumulated in the tail lobe during the growth phase
shows that these parameters have the comparable values. This is a clear evidence of the loading–unloading scheme of substorm
development. It is shown that the area of the auroral bulge developing during substorm is proportional to the total (magnetic
plus plasma) pressure decrease in the magnetotail. These findings stress the importance of auroral bulge observations for
monitoring of substorm intensity in terms of the magnetic flux and energy dissipation. 相似文献
507.
Y. Futaana S. Barabash A. Grigoriev D. Winningham R. Frahm M. Yamauchi R. Lundin 《Space Science Reviews》2006,126(1-4):315-332
As a part of the global plasma environment study of Mars and its response to the solar wind, we have analyzed a peculiar case
of the subsolar energetic neutral atom (ENA) jet observed on June 7, 2004 by the Neutral Particle Detector (NPD) on board
the Mars Express satellite. The “subsolar ENA jet” is generated by the interaction between the solar wind and the Martian
exosphere, and is one of the most intense sources of ENA flux observed in the vicinity of Mars. On June 7, 2004 (orbit 485
of Mars Express), the NPD observed a very intense subsolar ENA jet, which then abruptly decreased within ∼10 sec followed
by quasi-periodic (∼1 min) flux variations. Simultaneously, the plasma sensors detected a solar wind structure, which was
most likely an interplanetary shock surface. The abrupt decrease of the ENA flux and the quasi-periodic flux variations can
be understood in the framework of the global response of the Martian plasma obstacle to the interplanetary shock. The generation
region of the subsolar ENA jet was pushed towards the planet by the interplanetary shock; and therefore, Mars Express went
out of the ENA jet region. Associated global vibrations of the Martian plasma obstacle may have been the cause of the quasi-periodic
flux variations of the ENA flux at the spacecraft location. 相似文献
508.
Coronal mass ejections and post-shock streams driven by them are the most efficient drivers of strong magnetospheric activity,
magnetic storms. For this reason there is considerable interest in trying to make reliable forecasts for the effects of CMEs
as much in advance as possible. To succeed this requires understanding of all aspects related to CMEs, starting from their
emergence on the Sun to their propagation to the vicinity of the Earth and to effects within the magnetosphere. In this article
we discuss some recent results on the geoeffectivity of different types of CME/shock structures. A particularly intriguing
observation is that smoothly rotating magnetic fields within CMEs are most efficient in driving storm activity seen in the
inner magnetosphere due to enhanced ring current, whereas the sheath regions between the shock and the ejecta tend to favour
high-latitude activity. 相似文献
509.
510.
We present a brief introduction to the essential physics of coronal mass ejections as well as a review of theory and models
of CME initiation, solar energetic particle (SEP) acceleration, and shock propagation. A brief review of the history of CME
models demonstrates steady progress toward an understanding of CME initiation, but it is clear that the question of what initiates
CMEs has still not been solved. For illustration, we focus on the flux cancellation model and the breakout model. We contrast
the similarities and differences between these models, and we examine how their essential features compare with observations.
We review the generation of shocks by CMEs. We also outline the theoretical ideas behind the origin of a gradual SEP event
at the evolving CME-driven coronal/interplanetary shock and the origin of “impulsive” SEP events at flare sites of magnetic
reconnection below CMEs. We argue that future developments in models require focused study of “campaign events” to best utilize
the wealth of available CME and SEP observations. 相似文献