首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   2559篇
  免费   15篇
  国内免费   11篇
航空   1317篇
航天技术   857篇
综合类   16篇
航天   395篇
  2021年   24篇
  2019年   15篇
  2018年   95篇
  2017年   53篇
  2016年   30篇
  2015年   15篇
  2014年   49篇
  2013年   70篇
  2012年   57篇
  2011年   107篇
  2010年   73篇
  2009年   100篇
  2008年   102篇
  2007年   67篇
  2006年   51篇
  2005年   70篇
  2004年   76篇
  2003年   72篇
  2002年   37篇
  2001年   67篇
  2000年   49篇
  1999年   54篇
  1998年   68篇
  1997年   43篇
  1996年   70篇
  1995年   84篇
  1994年   58篇
  1993年   52篇
  1992年   69篇
  1991年   31篇
  1990年   19篇
  1989年   49篇
  1988年   23篇
  1987年   20篇
  1986年   21篇
  1985年   96篇
  1984年   62篇
  1983年   54篇
  1982年   63篇
  1981年   86篇
  1980年   22篇
  1979年   26篇
  1978年   26篇
  1977年   27篇
  1976年   20篇
  1975年   21篇
  1974年   20篇
  1972年   15篇
  1970年   18篇
  1969年   21篇
排序方式: 共有2585条查询结果,搜索用时 15 毫秒
951.
Numerical simulations of magnetic reconnection with uniform resistivity show that the length of the current sheet increases for increasing magnetic Reynolds number. In order to prevent the current sheet from growing in general the resistivity is assumed to be localized. For uniform resistivity the reconnection proceeds much slower than for localized resistivity. In this paper analytical solutions of the hydromagnetic equation are presented for localized and uniform resistivity. It is shown that there exists an essential singularity in the behaviour of a solution of magnetic reconnection in the limit of large magnetic Reynolds number: For prescribed boundary conditions the solution for localized resistivity does not approach the solution for uniform resistivity in the limit of large magnetic Reynolds number.  相似文献   
952.
We investigate the role of gravity in a linear stability analysis of the onset of coupled convective and morphological instabilities during directional solidification at constant velocity of a dilute alloy of tin in lead. For solidification vertically upwards, the temperature gradient alone would cause a negative density gradient and the solute gradient alone would cause a positive density gradient. Two types of instability are found, a convective type that occurs for long wavelengths and a morphological type that occurs for short wavelengths. In general, these are coupled but the morphological instabilities are practically independent of gravity and thus correspond to the predictions of previous morphological stability theory in which density changes and convection are neglected. The convective instabilities depend strongly on gravity; for a growth velocity of V = 1 μm/s and a temperature gradient in the liquid of GL = 200 K/cm, the critical concentrations for convective instabilities are 3.1 × 10?4, 3.1 × 10?2 and 2.39 wt. % for ge = 980 cm/s2, 10?4 ge and 10?6 ge, respectively. For low velocities, the convective instabilities occur at much lower solute concentrations than the morphological instabilities whereas at high velocities, the reverse is true. At intermediate velocities where the changeover takes place, there are oscillitory instabilities of mixed character whose periods range from 60 s at ge to 6 × 104 s at 10?6 ge.  相似文献   
953.
Pfeffermann  E.  Aschenbach  B.  Bräuninger  H.  Trümper  J. 《Space Science Reviews》1981,30(1-4):251-251
Space Science Reviews -  相似文献   
954.
Most, but not all, theoretical models of X-ray bursters require a binary system consisting of a mass donating star and a neutron star. The observational evidence in support of this model, however, is both indirect and meager. We have detected absorption dips in the X-ray spectrum of the Burster MXB 1916-05 with the IPC and the MPC on the Einstein Observatory which occur with a binary period of 2985 seconds. These dips are shown to be the result of a gas stream emanating from a companion star and hence this data represents the first direct evidence of the binary nature of X-ray bursters. Detailed models of the interaction of the gas stream with the accretion disk are presented. A 22nd mag. optical candidate for the system has been found.  相似文献   
955.
We present the results of the spectral and timing analysis of an observation of GX9+1/4U1758-205 performed with the Medium Energy Experiment aboard EXOSAT. During our observation the source flux varied irregularly in time scales from minutes to hours. No periodic emission in the period range from 16 msec to 2000 sec was found with an upper limit of around 1% (3 ) for the pulsed fraction. The hardness ratio shows a correlated change with the flux intensity (Sco X-1 behaviour). The spectrum could be fitted by a double component model, a black body component (kT=1.16–1.26 keV) together with a thermal bremsstrahlung law (kT=13–15keV). The black-body temperature-black-body flux relation follows a Stefan Boltzmann law with RBB=15.3 km*D/10 kpc. No iron line was detected. The upper limit for the line equivalent width of a 6.7 keV iron emission line is 40 eV (1). The X-ray spectral behaviour of GX9+1 indicates, that this source belongs to the class of Low-Mass X-ray Binaries (LMXB).  相似文献   
956.
The visible and near infrared channels, Ch1 and CH2 respectively, on the Advanced Very High Resolution Radiometer (AVHRR) provide daily information for monitoring changes in vegetation and crops. Data from these channels are used to create a normalized vegetation index (NVI) that is sensitive to changes in green leaf biomass and is represented mathematically by:
NVI = CH2 ? CH1CH2 + CH1
Operational products generated at NOAA include full-scale 1-km resolution images of the NVI covering areas viewed in a single swath of the polar-orbiting NOAA satellite. Global scale NVI images are also produced by compositing over a seven-day period, saving the maximum NVI created daily for each local array (resolution of 15 km at the equator to 30 km at the poles). Such seven-day mapping reduces the effect of cloud contamination. The global vegetation indices are used by foreign and U.S. government agencies for operational and experimental purposes such as assessment of crop conditions, monitoring potential desert locust breeding grounds, forest fire danger models, and monitoring range lands for forage availability. Examples include changes in the NVI in the Lake Chad vicinity, 1981–1982 and 1984; western United States NVI; and seasonal variations of the NVI in the Sahel using the global operational data base, 1982–1983.  相似文献   
957.
The optimum (maximum likelihood criterion) bit synchronizer for phase-noncoherent reception of binary FSK signals is found and a suboptimum implementation of it is derived. The performance of this circuit is analyzed in the presence of thermal noise, and the expression of the timing jitter variance is obtained in the case of relatively large signal-to-noise ratios.  相似文献   
958.
LORAN, which stands for long range navigation, was first brought into service during World War II. Its earliest form, LORAN-A, served us well up to the time the last station was taken off the air in December 1980. It was succeeded by LORAN-C as part of the Department of Defense's program to develop a new generation of radionavigation aids. LORAN-C not only has a greater range than LORAN-A but also provides better accuracy. With the declining cost of integrated circuits, LORAN-C receivers are becoming more affordable. This, along with good repeatable accuracy and excellent coverage, is increasing the worldwide popularity of navigation with LORAN-C. Fishermen, recreational boaters and the general aviation community make up a good percentage of this dramatic increase (an additional 75,000 marine and 10,000 aviation users the past year alone).  相似文献   
959.
Stüeken  E. E.  Som  S. M.  Claire  M.  Rugheimer  S.  Scherf  M.  Sproß  L.  Tosi  N.  Ueno  Y.  Lammer  H. 《Space Science Reviews》2020,216(3):1-17
Space Science Reviews - The Ionospheric Connection Explorer (ICON) mission makes measurements in near-Earth space that provide knowledge of the state of the ionosphere. From the vantage of...  相似文献   
960.
The HOPE mass spectrometer of the Radiation Belt Storm Probes (RBSP) mission (renamed the Van Allen Probes) is designed to measure the in situ plasma ion and electron fluxes over 4π sr at each RBSP spacecraft within the terrestrial radiation belts. The scientific goal is to understand the underlying physical processes that govern the radiation belt structure and dynamics. Spectral measurements for both ions and electrons are acquired over 1 eV to 50 keV in 36 log-spaced steps at an energy resolution ΔE FWHM/E≈15 %. The dominant ion species (H+, He+, and O+) of the magnetosphere are identified using foil-based time-of-flight (TOF) mass spectrometry with channel electron multiplier (CEM) detectors. Angular measurements are derived using five polar pixels coplanar with the spacecraft spin axis, and up to 16 azimuthal bins are acquired for each polar pixel over time as the spacecraft spins. Ion and electron measurements are acquired on alternate spacecraft spins. HOPE incorporates several new methods to minimize and monitor the background induced by penetrating particles in the harsh environment of the radiation belts. The absolute efficiencies of detection are continuously monitored, enabling precise, quantitative measurements of electron and ion fluxes and ion species abundances throughout the mission. We describe the engineering approaches for plasma measurements in the radiation belts and present summaries of HOPE measurement strategy and performance.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号