全文获取类型
收费全文 | 1089篇 |
免费 | 159篇 |
国内免费 | 34篇 |
专业分类
测绘学 | 14篇 |
大气科学 | 115篇 |
地球物理 | 288篇 |
地质学 | 52篇 |
海洋学 | 15篇 |
天文学 | 743篇 |
综合类 | 7篇 |
自然地理 | 48篇 |
出版年
2024年 | 1篇 |
2023年 | 1篇 |
2022年 | 4篇 |
2021年 | 8篇 |
2020年 | 7篇 |
2019年 | 16篇 |
2018年 | 14篇 |
2017年 | 12篇 |
2016年 | 12篇 |
2015年 | 11篇 |
2014年 | 24篇 |
2013年 | 30篇 |
2012年 | 11篇 |
2011年 | 48篇 |
2010年 | 56篇 |
2009年 | 103篇 |
2008年 | 117篇 |
2007年 | 91篇 |
2006年 | 81篇 |
2005年 | 58篇 |
2004年 | 68篇 |
2003年 | 51篇 |
2002年 | 33篇 |
2001年 | 29篇 |
2000年 | 39篇 |
1999年 | 41篇 |
1998年 | 47篇 |
1997年 | 31篇 |
1996年 | 55篇 |
1995年 | 42篇 |
1994年 | 34篇 |
1993年 | 21篇 |
1992年 | 20篇 |
1991年 | 12篇 |
1990年 | 8篇 |
1989年 | 5篇 |
1988年 | 9篇 |
1987年 | 9篇 |
1986年 | 3篇 |
1985年 | 1篇 |
1984年 | 4篇 |
1983年 | 1篇 |
1982年 | 3篇 |
1981年 | 1篇 |
1980年 | 7篇 |
1978年 | 1篇 |
1977年 | 1篇 |
1954年 | 1篇 |
排序方式: 共有1282条查询结果,搜索用时 328 毫秒
81.
An extensive series of incoherent scatter studies of the ionospheric D-region was carried out at the Arecibo radar facility during 1978 and 1979. They included several full-day sequences of electron density measurements over a range of altitudes, and also included a sequence during the serendipitous occurrence of a large solar flare. For the solar flare event simultaneous data on solar X-ray fluxes in several wavelength bands were available from the GOES-2 and ISEE-3 satellites.
In the course of development of a large ionospheric computer model at Los Alamos we have used the solar flare data as a reality check. The solar X-ray flux data were used as inputs for computing ionization rates. The model computer includes 999 chemical reactions, and also includes diffusion and transport processes. In the course of the flare studies we used the data comparisons to adjust the values of three chemical rate coefficients that were poorly known. With those adjustments the model computations fitted the data quite well.
Subsequent to the flare analysis we have been using the same model with some minor updates to compute the expected diurnal variations of the ambient D-region under conditions chosen to match those existing at the times of the incoherent scatter measurements. Comparisons of the computations and the data will be shown, and the relative importance of the several separate ionization processes will be discussed. We also compare model results with experimental data on concentrations of NO. 相似文献
82.
Direct observations or deduced analysis indicate clearly that formation of intense fluxes of relativistic electrons is an important ingredient in the evolution of numerous active magnetized plasma systems. Examples of relativistic electron energization include the recovery phase of a planetary magnetic storm, post solar flare coronal activity and the afterglow of gamma ray bursts. It is suggested that there exists a universal mechanism, which may explain electron energization at the vastly different magnetized plasma environments. The favorite configuration consists of an inhomogeneous magnetic field anchored at a given magnetic structure and excitation of whistler waves due to external injection of low-energy non-isotropic electrons. The energization proceeds as a bootstrap process due to interaction with the propagating whistler waves along the inhomogeneous magnetic field. 相似文献
83.
Quantitative estimation of helium-3 spatial distribution in the lunar regolith layer 总被引:2,自引:0,他引:2
3He (helium-3) in the lunar regolith implanted by the solar wind is one of the most valuable resources because of its potential as a fusion fuel. The abundance of 3He in the lunar regolith is related to solar wind flux, lunar surface maturity and TiO2 content, etc. A model of solar wind flux, which takes account of variations due to shielding of the nearside when the Moon is in the Earth's magnetotail, is used to present a global distribution of relative solar wind flux over the lunar surface. Using Clementine UV/VIS multispectral data, the global distribution of lunar surface optical maturity (OMAT) and the TiO2 content in the lunar regolith are calculated. Based on Apollo regolith samples, a linear relation between 3He abundance and normalized solar wind flux, optical maturity, and TiO2 content is presented. To simulate the brightness temperature of the lunar surface, which is the mission of the Chinese Chang-E project's multichannel radiometers, a global distribution of regolith layer thickness is first empirically constructed from lunar digital elevation mapping (DEM). Then an inversion approach is presented to retrieve the global regolith layer thickness. It finally yields the total amount of 3He per unit area in the lunar regolith layer, which is related to the regolith layer thickness, solar wind flux, optical maturity and TiO2 content, etc. The global inventory of 3He is estimated as 6.50×108 kg, where 3.72×108 kg is for the lunar nearside and 2.78×108 kg is for the lunar farside. 相似文献
84.
Peter Bochsler 《Astronomy and Astrophysics Review》2007,14(1):1-40
Ions heavier than 4He are treated as “minors” in the solar wind. This is justified for many applications since minor ions have no significant
influence on the dynamics of the interplanetary plasma. However, minor ions carry information on many aspects of the formation,
on the acceleration and on the transfer of solar plasma from the corona into the interplanetary space. This review concentrates
on various aspects of minor ions as diagnostic tracers. The elemental abundance patterns of the solar wind are shaped in the
chromosphere and in the lower transition region by processes, which are not fully understood at this moment. Despite this
lack of detailed understanding, observed abundance patterns have been classified and are now commonly used to characterize
the sources, and to trace back solar-wind flows to their origins in the solar atmosphere. Furthermore, the solar wind is the
most important source of information for solar isotopic abundances and for solar abundances of volatile elements. In order
to fully exploit this information, a comprehensive understanding of elemental and isotopic fractionation processes is required.
We provide observational clues to distinguish different processes at work. 相似文献
85.
怀柔观测基地的多通道太阳望远镜是通过数十个电机调节晶体偏转角度实现多波带同时观测的自动控制。原有的计算机是通过一个串行接口控制若干个调波带的电机,响应时间长,速度慢且技术比较落后。本文利用当今流行的USB2.0芯片CYPRESS EZ-USB改造原有的串口通信控制系统,大大提高了响应的速度,同时,为解决一个USB应用程序控制多个电机转动的问题,提出了一种"编号"的方法。这样每个USB设备不论其插入顺序,PC主机都能通过识别其固定编号而加以区分,PC主机可以通过4个USB接口(16个电机)控制多通道太阳望远镜滤光嚣的调制角度,实现自动控制。这为大规模改造其他计算机接口提供了研制基础。 相似文献
86.
R. P. Kane 《Solar physics》2007,246(2):471-485
Many methods of predictions of sunspot maximum number use data before or at the preceding sunspot minimum to correlate with
the following sunspot maximum of the same cycle, which occurs a few years later. Kane and Trivedi (Solar Phys. 68, 135, 1980) found that correlations of R
z(max) (the maximum in the 12-month running means of sunspot number R
z) with R
z(min) (the minimum in the 12-month running means of sunspot number R
z) in the solar latitude belt 20° – 40°, particularly in the southern hemisphere, exceeded 0.6 and was still higher (0.86)
for the narrower belt > 30° S. Recently, Javaraiah (Mon. Not. Roy. Astron. Soc.
377, L34, 2007) studied the relationship of sunspot areas at different solar latitudes and reported correlations 0.95 – 0.97 between minima and maxima of sunspot areas at low latitudes
and sunspot maxima of the next cycle, and predictions could be made with an antecedence of more than 11 years. For the present
study, we selected another parameter, namely, SGN, the sunspot group number (irrespective of their areas) and found that SGN(min) during a sunspot minimum year at latitudes > 30° S had a correlation
+0.78±0.11 with the sunspot number R
z(max) of the same cycle. Also, the SGN during a sunspot minimum year in the latitude belt (10° – 30° N) had a correlation +0.87±0.07 with the
sunspot number R
z(max) of the next cycle. We obtain an appropriate regression equation, from which our prediction for the coming cycle 24 is R
z(max )=129.7±16.3. 相似文献
87.
The solar wind conditions at one astronomical unit (AU) can be strongly disturbed by interplanetary coronal mass ejections
(ICMEs). A subset, called magnetic clouds (MCs), is formed by twisted flux ropes that transport an important amount of magnetic
flux and helicity, which is released in CMEs. At 1 AU from the Sun, the magnetic structure of MCs is generally modeled by
neglecting their expansion during the spacecraft crossing. However, in some cases, MCs present a significant expansion. We
present here an analysis of the huge and significantly expanding MC observed by the Wind spacecraft during 9 – 10 November 2004. This MC was embedded in an ICME. After determining an approximate orientation for
the flux rope using the minimum variance method, we obtain a precise orientation of the cloud axis by relating its front and
rear magnetic discontinuities using a direct method. This method takes into account the conservation of the azimuthal magnetic
flux between the inbound and outbound branches and is valid for a finite impact parameter (i.e., not necessarily a small distance between the spacecraft trajectory and the cloud axis). The MC is also studied using dynamic
models with isotropic expansion. We have found (6.2±1.5)×1020 Mx for the axial flux and (78±18)×1020 Mx for the azimuthal flux. Moreover, using the direct method, we find that the ICME is formed by a flux rope (MC) followed
by an extended coherent magnetic region. These observations are interpreted by considering the existence of a previously larger
flux rope, which partially reconnected with its environment in the front. We estimate that the reconnection process started
close to the Sun. These findings imply that the ejected flux rope is progressively peeled by reconnection and transformed
to the observed ICME (with a remnant flux rope in the front part). 相似文献
88.
David Alexander 《Astrophysics and Space Science》2007,307(1-3):197-202
The solar atmosphere displays a wide variety of dynamic phenomena driven by the interaction of magnetic fields and plasma.
In particular, plasma jets in the solar chromosphere and corona, coronal heating, solar flares and coronal mass ejections
all point to the presence of magnetic phenomena such as reconnection, flux cancellation, the formation of magnetic islands,
and plasmoids. While we can observe the signatures and gross features of such phenomena we cannot probe the essential physics
driving them, given the spatial resolution of current instrumentation. Flexible and well-controlled laboratory experiments,
scaled to solar parameters, open unique opportunities to reproduce the relevant unsteady phenomena under various simulated
solar conditions. The ability to carefully control these parameters in the laboratory allows one to diagnose the dynamical
processes which occur and to apply the knowledge gained to the understanding of similar processes on the Sun, in addition
directing future solar observations and models. This talk introduces the solar phenomena and reviews the contributions made
by laboratory experimentation. 相似文献
89.
90.