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31.
Nicholas Achilleos Nicolas André Xochitl Blanco-Cano Pontus C. Brandt Peter A. Delamere Robert Winglee 《Space Science Reviews》2015,187(1-4):229-299
The rapid rotation of the gas giant planets, Jupiter and Saturn, leads to the formation of magnetodisc regions in their magnetospheric environments. In these regions, relatively cold plasma is confined towards the equatorial regions, and the magnetic field generated by the azimuthal (ring) current adds to the planetary dipole, forming radially distended field lines near the equatorial plane. The ensuing force balance in the equatorial magnetodisc is strongly influenced by centrifugal stress and by the thermal pressure of hot ion populations, whose thermal energy is large compared to the magnitude of their centrifugal potential energy. The sources of plasma for the Jovian and Kronian magnetospheres are the respective satellites Io (a volcanic moon) and Enceladus (an icy moon). The plasma produced by these sources is globally transported outwards through the respective magnetosphere, and ultimately lost from the system. One of the most studied mechanisms for this transport is flux tube interchange, a plasma instability which displaces mass but does not displace magnetic flux—an important observational constraint for any transport process. Pressure anisotropy is likely to play a role in the loss of plasma from these magnetospheres. This is especially the case for the Jovian system, which can harbour strong parallel pressures at the equatorial segments of rotating, expanding flux tubes, leading to these regions becoming unstable, blowing open and releasing their plasma. Plasma mass loss is also associated with magnetic reconnection events in the magnetotail regions. In this overview, we summarise some important observational and theoretical concepts associated with the production and transport of plasma in giant planet magnetodiscs. We begin by considering aspects of force balance in these systems, and their coupling with the ionospheres of their parent planets. We then describe the role of the interaction between neutral and ionized species, and how it determines the rate at which plasma mass and momentum are added to the magnetodisc. Following this, we describe the observational properties of plasma injections, and the consequent implications for the nature of global plasma transport and magnetodisc stability. The theory of the flux tube interchange instability is reviewed, and the influences of gravity and magnetic curvature on the instability are described. The interaction between simulated interchange plasma structures and Saturn’s moon Titan is discussed, and its relationship to observed periodic phenomena at Saturn is described. Finally, the observation, generation and evolution of plasma waves associated with mass loading in the magnetodisc regions is reviewed. 相似文献
32.
The measured D/H ratios in interstellar environments and in the solar system are reviewed. The two extreme D/H ratios in solar
system water - (720±120)×10−6 in clay minerals and (88±11)×10−6 in chondrules, both from LL3 chondritic meteorites - are interpreted as the result of a progressive isotopic exchange in
the solar nebula between deuterium-rich interstellar water and protosolar H2. According to a turbulent model describing the evolution of the nebula (Drouart et al., 1999), water in the solar system cannot be a product of thermal (neutral) reactions occurring in the solar nebula. Taking
720×10−6 as a face value for the isotopic composition of the interstellar water that predates the formation of the solar nebula, numerical
simulations show that the water D/H ratio decreases via an isotopic exchange with H2. During the course of this process, a D/H gradient was established in the nebula. This gradient was smoothed with time and
the isotopic homogenization of the solar nebula was completed in 106 years, reaching a D/H ratio of 88×10−6. In this model, cometary water should have also suffered a partial isotopic re-equilibration with H2. The isotopic heterogeneity observed in chondrites result from the turbulent mixing of grains, condensed at different epochs
and locations in the solar nebula. Recent isotopic determinations of water ice in cold interstellar clouds are in agreement
with these chondritic data and their interpretation (Texeira et al., 1999).
This revised version was published online in June 2006 with corrections to the Cover Date. 相似文献
33.
Infrared spectroscopic observations of planets and Saturn's satellite Titan with the Infrared Space Observatory led to many
significant discoveries that improved our understanding on the formation, physics and chemistry of these objects. The prime
results achieved by ISO are: (1) a new and consistent determination of the D/H ratios on the giant planets and Titan; (2)
the first precise measurement of the 15N/14N ratio in Jupiter, a valuable indicator of the protosolar nitrogen isotopic ratio; (3) the first detection of an external
oxygen flux for all giant planets and Titan; (4) the first detection of some stratospheric hydrocarbons (CH3, C2H4, CH3C2H, C4H2, C6H6); (5) the first detection of tropospheric water in Saturn; (6) the tentative detection of carbonate minerals on Mars; (7)
the first thermal lightcurve of Pluto.
Based on observations with ISO, an ESA project with instruments funded by ESA Member States (especially the PI countries:
France, Germany, The Netherlands, and the United Kingdom), and with the participation of ISAS and NASA. 相似文献
34.
Magnetic reconnection is a universal phenomenon where energy is efficiently converted from the magnetic field to charged particles
as a result of global magnetic topology changes during which earlier separated plasma regions become magnetically connected.
While the reconnection affects large volumes in space most of the topology changes and of the energization occur within small
localized regions. Regions of special importance are the X-region and the separatrix region. The understanding of the microphysics
of these regions is crucial for the overall understanding of the reconnection. The Earth magnetosphere is the best environment
where the details of these regions can be studied in situ. We summarize their main properties and discuss recent spacecraft observations. 相似文献
35.
K. Kecskeméty S. Pintér 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》1981,1(3):101-104
Flux variations of 1 – 5 MeV protons are studied in energetic storm particle events with respect to the preshock solar wind plasma parameters and to the thickness of the collisionless interplanetary shock wave. It is found that the peak intensity in ESP events depends on pre-shock plasma density and on the thickness of the transition region. These relations predict, in agreement with recent observations, the increase of ESP events at larger heliocentric distances. 相似文献
36.
Y. Malméjac 《Advances in Space Research (includes Cospar's Information Bulletin, Space Research Today)》1981,1(5):61-64
This report deals with the present plans and philosophy of the European Space Agency as regards the interest and the justification for a European Experimental Research Program in Space. It offers recommendations to be considered by both the space investigators and the E.S.A. executive in order to achieve the most urgent needs. 相似文献
37.
Artificial gravity: a possible countermeasure for post-flight orthostatic intolerance 总被引:1,自引:0,他引:1
Moore ST Diedrich A Biaggioni I Kaufmann H Raphan T Cohen B 《Acta Astronautica》2005,56(9-12):867-876
Four payload crewmembers were exposed to sustained linear acceleration in a centrifuge during the Neurolab (STS-90) flight. In contrast to previous studies, otolith–ocular reflexes were preserved during and after flight. This raised the possibility that artificial gravity may have acted as a countermeasure to the deconditioning of otolith–ocular reflexes. None of the astronauts who were centrifuged had orthostatic intolerance when tested with head-up passive tilt after flight. Thus, centrifugation may also have helped maintain post-flight hemodynamic responses to orthostasis by preserving the gain of the otolith–sympathetic reflex. A comparison with two fellow Neurolab orbiter crewmembers not exposed to artificial gravity provided some support for this hypothesis. One of the two had hemodynamic changes in response to post-flight tilt similar to orthostatically intolerant subjects from previous missions. More data is necessary to evaluate this hypothesis, but if it were proven correct, in-flight short-radius centrifugation may help counteract orthostatic intolerance after space flight. 相似文献
38.
The occurrence of geometric orientation illusions and the perception of ambiguous figures were analyzed in 24 subjects during static body tilt relative to gravity on Earth. Results showed that illusions such as the Rock's diamond/square, the Ponzo illusion, and orientation contrast illusions occurred less frequently, and that depth reversal of ambiguous figures took more time when subjects were lying on their side or supine compared to upright, thus suggesting that the gravitational reference plays a significant role in these “visual” illusions. The structure of images, our representation of the environment, and orientation relative to gravity are all integral parts in interpreting visual images. In a weightless environment where no gravitational reference can be used, it is expected that similar alterations in visual perception will occur. 相似文献
39.
40.
Roddy M Grieves Éléonore Duvelle Paul A Dudchenko 《Spatial Cognition & Computation》2018,18(3):217-256
Hippocampal place cells are thought to form the neural substrate of a global cognitive map. However, in multicompartment mazes, these cells exhibit locally repeating representations, undermining the global cognitive map view of place cells. This phenomenon appears to be related to the repetitive layout of these mazes, but still no hypothesis adequately explains it. Here, we use a boundary vector cell (BVC) model of place cell firing to model the activity of place cells in numerous multicompartment environments. The activity of modeled place cells bears a striking resemblance to experimental data, replicating virtually every major experimental result. Our results support the BVC model and indicate that locally repeating place cell firing could result purely from local geometry. 相似文献