全文获取类型
收费全文 | 647篇 |
免费 | 45篇 |
国内免费 | 74篇 |
专业分类
测绘学 | 84篇 |
大气科学 | 198篇 |
地球物理 | 32篇 |
地质学 | 4篇 |
海洋学 | 8篇 |
天文学 | 409篇 |
综合类 | 21篇 |
自然地理 | 10篇 |
出版年
2024年 | 1篇 |
2023年 | 2篇 |
2022年 | 12篇 |
2021年 | 16篇 |
2020年 | 13篇 |
2019年 | 17篇 |
2018年 | 9篇 |
2017年 | 18篇 |
2016年 | 14篇 |
2015年 | 19篇 |
2014年 | 23篇 |
2013年 | 27篇 |
2012年 | 21篇 |
2011年 | 21篇 |
2010年 | 15篇 |
2009年 | 46篇 |
2008年 | 32篇 |
2007年 | 48篇 |
2006年 | 52篇 |
2005年 | 54篇 |
2004年 | 42篇 |
2003年 | 38篇 |
2002年 | 33篇 |
2001年 | 35篇 |
2000年 | 34篇 |
1999年 | 28篇 |
1998年 | 33篇 |
1997年 | 1篇 |
1996年 | 8篇 |
1995年 | 13篇 |
1994年 | 14篇 |
1993年 | 1篇 |
1992年 | 1篇 |
1991年 | 2篇 |
1990年 | 1篇 |
1989年 | 8篇 |
1988年 | 4篇 |
1987年 | 1篇 |
1985年 | 3篇 |
1980年 | 4篇 |
1978年 | 2篇 |
排序方式: 共有766条查询结果,搜索用时 940 毫秒
691.
Stephen O'Sullivan Turlough P. Downes 《Monthly notices of the Royal Astronomical Society》2006,366(4):1329-1336
When modelling astrophysical fluid flows, it is often appropriate to discard the canonical magnetohydrodynamic approximation, thereby freeing the magnetic field to diffuse with respect to the bulk velocity field. As a consequence, however, the induction equation can become problematic to solve via standard explicit techniques. In particular, the Hall diffusion term admits fast-moving whistler waves which can impose a vanishing time-step limit.
Within an explicit differencing framework, a multifluid scheme for weakly ionized plasmas is presented which relies upon a new approach to integrating the induction equation efficiently. The first component of this approach is a relatively unknown method of accelerating the integration of parabolic systems by enforcing stability over large compound time-steps rather than over each of the constituent substeps. This method, Super Time-Stepping, proves to be very effective in applying a part of the Hall term up to a known critical value. The excess of the Hall term above this critical value is then included via a new scheme for pure Hall diffusion. 相似文献
Within an explicit differencing framework, a multifluid scheme for weakly ionized plasmas is presented which relies upon a new approach to integrating the induction equation efficiently. The first component of this approach is a relatively unknown method of accelerating the integration of parabolic systems by enforcing stability over large compound time-steps rather than over each of the constituent substeps. This method, Super Time-Stepping, proves to be very effective in applying a part of the Hall term up to a known critical value. The excess of the Hall term above this critical value is then included via a new scheme for pure Hall diffusion. 相似文献
692.
Michael D. Smith Alexander Rosen 《Monthly notices of the Royal Astronomical Society》2007,378(2):691-700
Molecular outflows and the jets which may drive them can be expected to display signatures associated with rotation if they are the channels through which angular momentum is extracted from material accreting on to protostars. Here, we determine some basic signatures of rapidly rotating flows through three-dimensional numerical simulations of hydrodynamic jets with molecular cooling and chemistry. We find that these rotating jets generate a broad advancing interface which is unstable and develops into a large swarm of small bow features. In comparison to precessing jets, there is no stagnation point along the axis. The greater the rotation rate, the greater the instability. On the other hand, velocity signatures are only significant close to the jet inlet since jet expansion rapidly reduces the rotation speed. We present predictions for atomic, H2 and CO submillimetre images and spectroscopy including velocity channel maps and position–velocity diagrams. We also include simulated images corresponding to Spitzer IRAC band images and CO emission, relevant for APEX and eventual ALMA observations. We conclude that protostellar jets often show signs of slow precession but only a few sources display properties which could indicate jet rotation. 相似文献
693.
Ryo Yamazaki Kazunori Kohri Aya Bamba Tatsuo Yoshida Toru Tsuribe Fumio Takahara 《Monthly notices of the Royal Astronomical Society》2006,371(4):1975-1982
We study the emission from an old supernova remnant (SNR) with an age of around 105 yr and that from a giant molecular cloud (GMC) encountered by the SNR. When the SNR age is around 105 yr, proton acceleration is efficient enough to emit TeV γ-rays both at the shock of the SNR and that in the GMC. The maximum energy of primarily accelerated electrons is so small that TeV γ-rays and X-rays are dominated by hadronic processes, π0 -decay and synchrotron radiation from secondary electrons, respectively. However, if the SNR is older than several 105 yr, there are few high-energy particles emitting TeV γ-rays because of the energy-loss effect and/or the wave-damping effect occurring at low-velocity isothermal shocks. For old SNRs or SNR–GMC interacting systems capable of generating TeV γ-ray emitting particles, we calculated the ratio of TeV γ-ray (1–10 TeV) to X-ray (2–10 keV) energy flux and found that it can be more than ∼102 . Such a source showing large flux ratio may be a possible origin of recently discovered unidentified TeV sources. 相似文献
694.
695.
New multifrequency spatial and spectral studies of the hot molecular core associated with the ultracompact HII region G34.3+0.15 have demonstrated an extremely rich chemistry in this archetypal hot core and revealed differing spatial structure between certain species which may be a dynamical effect of chemical evolution. The structure of the hot core has been studied with the JCMT in the high excitation J=19-18 and J=18-17 lines of CH3CN and with the Nobeyama Millimetre Array at 4 arc resolution in the J=6-5 transition. Comparison with a VLA NH3(3,3) map shows a displacement between peak emission in the two chemical species which is consistent with chemical processing on a time scale comparable to the dynamical time scale of 105 yrs.A 330-360 GHz spectral survey of the hot core with the JCMT has detected 358 spectral lines from at least 46 distinct chemical species, including many typical of shocked chemistry while other species indicate abundances that reflect the chemistry of a previous cold phase. The first unambiguous detection of ethanol in hot gas has been made. Observations of 14 rotational transitions of this molecule yield a temperature of 125 K and column density 2×1015 cm–2. This large abundance cannot be made by purely gas-phase processes and it is concluded that ethanol must have formed by grain surface chemistry. 相似文献
696.
青海省秋季一次对流云人工增雨的数值模拟 总被引:10,自引:3,他引:10
利用中国气象科学研究院的三维对流云数值模式,模拟了青海省2002年秋季一次对流云过程,分析了青海省秋季对流云云体发展的动力学特点和微物理特征。数值模拟结果表明,青海省秋季的对流云降水几乎全部是由霰的融化形成的,而自然云中冰晶含量少、冰霰自动转化形成霰胚的过程非常微弱。但采用合适的方法催化以后,冰晶的核化、繁生量增加,通过冰霰自动转化过程形成大量霰胚,霰胚再通过其他冷云微物理过程迅速增长,融化成降水。催化后各种微物理机制都比催化前活跃,同时催化改变了云体的动力场分布,在动力过程和微物理过程的相互促进下,使增雨取得了很好的效果。 相似文献
697.
698.
Maps are presented of 3 P 1 →3 P 0 [C i ] and J =2→1 C18 O line emission from the interstellar molecular cloud G35.2−0.74N. The maps are interpreted with reference to a previous model for the structure of the cloud in which opposing jets from a central object, embedded in a rotating interstellar disc, precess and drive a bipolar molecular outflow. The C18 O emission traces the rotating interstellar disc, but the [C i ] emission shows several features. An unresolved component is observed which probably results from dissociation of CO in the centre of the disc by UV radiation from the central source. Background [C i ] emission is also observed which shares the rotation of the disc on larger scales. The C i /CO ratio in these components is typically a few per cent. High-velocity [C i ] emission, where C i /CO is high (>0.1–0.4), is observed between the CO molecular outflow and the cavity exacavated by the jet. This material has probably been accelerated by the jet but dissociated by far-UV radiation propagating through the cavity. The C i /CO ratio falls as the shocked outflow later sweeps up CO. 相似文献
699.
D. P. Ruffle D. A. Williams & W. W. Duley 《Monthly notices of the Royal Astronomical Society》1998,296(3):746-748
The pure rotational spectrum of homonuclear diatomic molecules in the interstellar medium is strongly forbidden, and no such spectrum has been detected. In regions of high excitation, vibrational emission may occur, as is widely detected in the case of H2 in interstellar shocks and photon-dominated regions. However, it is of considerable interest to know the abundance of homonuclear diatomics in quiescent regions. We propose that vibrational emission from homonuclear diatomic molecules in cold clouds may be detectable, where the excitation is mainly through collisions with non-thermal electrons arising from the cosmic-ray ionization of H2 . As an example, we estimate the intensity of emission from N2 in cold, dark interstellar clouds. We show that such emission is at the limit of detectability with current technology. Other excitation mechanisms may also contribute and enhance this emission. 相似文献
700.
We have obtained a spectrum of the Na D lines of the Seyfert galaxy Mrk 595 with a signal-to-noise ratio (S/N) of about 30. The importance of this sightline is that this galaxy lies in a direction close to the highest column densities of the Cohen high-velocity (HV) stream, and hence represents the best opportunity of detecting this stream in interstellar absorption, as previous searches towards stellar targets up to a distance of 600 pc have not been successful. The H I column density in the direction of this galaxy implies that the HV gas should be detectable in absorption in the Na D lines. No such detections are made (at the 3σ level) and we consider possible explanations. Hence previous non-detections towards stars do not necessarily mean that the HV gas is more distant than the stars, and we emphasize that the distance of this high-velocity cloud (HVC), which is a key parameter to the understanding of the nature and origin of this feature, remains unknown and indeed may be difficult to establish. 相似文献