全文获取类型
收费全文 | 409篇 |
免费 | 9篇 |
国内免费 | 7篇 |
专业分类
测绘学 | 29篇 |
大气科学 | 18篇 |
地球物理 | 68篇 |
地质学 | 171篇 |
海洋学 | 9篇 |
天文学 | 113篇 |
综合类 | 7篇 |
自然地理 | 10篇 |
出版年
2023年 | 2篇 |
2022年 | 13篇 |
2021年 | 13篇 |
2020年 | 9篇 |
2019年 | 10篇 |
2018年 | 18篇 |
2017年 | 22篇 |
2016年 | 23篇 |
2015年 | 20篇 |
2014年 | 24篇 |
2013年 | 27篇 |
2012年 | 21篇 |
2011年 | 20篇 |
2010年 | 14篇 |
2009年 | 13篇 |
2008年 | 7篇 |
2007年 | 11篇 |
2006年 | 13篇 |
2004年 | 6篇 |
2001年 | 2篇 |
2000年 | 11篇 |
1999年 | 4篇 |
1998年 | 3篇 |
1997年 | 2篇 |
1996年 | 5篇 |
1995年 | 2篇 |
1994年 | 4篇 |
1993年 | 6篇 |
1992年 | 4篇 |
1991年 | 2篇 |
1988年 | 6篇 |
1987年 | 5篇 |
1986年 | 6篇 |
1985年 | 3篇 |
1984年 | 5篇 |
1983年 | 6篇 |
1982年 | 3篇 |
1981年 | 3篇 |
1980年 | 5篇 |
1979年 | 6篇 |
1978年 | 7篇 |
1976年 | 3篇 |
1975年 | 4篇 |
1974年 | 3篇 |
1973年 | 4篇 |
1972年 | 4篇 |
1971年 | 6篇 |
1970年 | 4篇 |
1954年 | 1篇 |
1953年 | 1篇 |
排序方式: 共有425条查询结果,搜索用时 62 毫秒
31.
Efficient, robust simulation of groundwater flow in the unsaturated zone remains computationally expensive, especially for problems characterized by sharp fronts in both space and time. Standard approaches that employ uniform spatial and temporal discretizations for the numerical solution of these problems lead to inefficient and expensive simulations. In this work, we solve Richards’ equation using adaptive methods in both space and time. Spatial adaption is based upon a coarse grid solve and a gradient error indicator using a fixed-order approximation. Temporal adaption is accomplished using variable order, variable step size approximations based upon the backward difference formulas up to fifth order. Since the advantages of similar adaptive methods in time are now established, we evaluate our method by comparison with a uniform spatial discretization that is adaptive in time for four different one-dimensional test problems. The numerical results demonstrate that the proposed method provides a robust and efficient alternative to standard approaches for simulating variably saturated flow in one spatial dimension. 相似文献
32.
Umesh Chandra 《Physics of the Earth and Planetary Interiors》1979,20(1):33-41
Three basic tectonic units, the Eurasian continent, the Rigid Boundary Zone and the Indian continent, are recognized. The Rigid Boundary Zone is identified with aseismic lineaments on an epicenter map of the Himalayas and their vicinity. The origin of the Himalayan syntaxes, their re-entrant character, and the observation that most of the Himalayas are made up of rocks belonging geologically to the Indian peninsular shield, are explained by deformation of the Indian continent within this Rigid Boundary Zone since the continental collision. The focal mechanism solutions of six earthquakes in the Indian Peninsula support the inference derived from the model that the high stresses generated by the continental collision may be very extensive spatially, and that the entire Indian Peninsula may be under a state of left lateral shear along NNE vertical planes. 相似文献
33.
Dr. Ashok Sahni Mr. Harish Chandra Mitra 《International Journal of Earth Sciences》1980,69(3):824-848
The paper analytically discusses certain palaeoecological criteria in the reconstruction of Lower Miocene (Aquitanian-Burdigalian) palaeobiogeography of the Indian subcontinent. The period is characterised by major marine transgressions and a prolific marine invertebrate and terrestrial vertebrate faunas. Faunal affinities of molluscs from western (Sind-Baluchistan) and eastern (Garo Hills) sectors are distinct and different. Molluscs of Garo Hills show affinity to Burmese species, whereas Sind-Baluchistan species shows similarity to that of the Mediterranean region. Marine molluscs resemble with the present-day taxa of the Arabian Sea and Bay of Bengal, where more than 35% species are common. This similarity is also evidenced by marine elasmobranchs, where approximately 33% species and 86% genera are similar to that of the Recent taxa. Terrestrial mammals show an intermingling of southern U.S.S.R. and African forms along with the presence of probably some groups belonging to the Indian subcontinent.Angiosperms are the dominant group. The Lower Miocene floras of the eastern and the western sectors are entirely different. The palm,Sabal major andPinus are reported from various localities. The climate in the western sector and South India was tropical, whereas in the eastern sector, it was variable.The Lower Miocene period is also marked by the contraction of Sindhu Sea, Bangla Sea and the Burmese Gulf southwards resulting in the final disappearance of the sea by the Pliocene.
Zusammenfassung Die Paläobiogeographie des unteren Miocän (Aquitan-Burdigal) läßt sich auf dem indischen Subkontinent aus den palökologischen Daten rekonstruieren. Zu dieser Zeit gab es viele marine Invertebraten und terrestrische Vertebraten-Faunen, die große marine Transgression anzeigen. Die Mollusken des Westens (Find-Baluchistan) und Ostens (Garo-Gebirge) sind unterschiedlich. Die Mollusken des Garo-Gebirges sind mit den Spezies von Burma verwandt. Dagegen sind die Faunen von Sind-Baluchistan für die mediterrane Faunenprovinz typisch. Die marinen Mollusken sind verwandt mit den Faunen des Arabischen Meeres und der Bucht von Bengalen; etwa 35% der Spezies sind gleich. Von den marinen Elasmobranchien sind ungefähr 33 % Spezies und 86% Genera gleich wie man sie in den rezenten Faunen findet. Terrestrische Sänger zeigen eine Mischung von südlicher U.S.S.R. und Afrika mit einigen Teilen des Indischen Subkontinentes.In den Pflanzen-Gruppen herrschen die Angiospermen vor. Die Untermiocän-Flora der östlichen und westlichen Gebiete ist ganz unterschiedlich. Der Palme, dieSabal Major undPinus sind aus verschiedenen Lokalitäten bekannt. Das Klima im Westen und im Süden Indiens war tropisch, im Osten dagegen war es wechselfeucht.Das Untermiocän-Meer im Gebiet Sind, Bangladesh und Burma verschwindet vor dem Pliozän.
Résumé Cet article analyse certains critères paléoécologiques dans la reconstitution de la paléogéographie du Miocène inférieur (Aquitanien-Burdigalien) de l'Inde subcontinentale. La période est caractérisée par de grandes transgressions marines et par des faunes abondantes d'invertébrés marins et de vertébrés terrestres. Les affinités fauniques des mollusques sont différentes dans les secteurs occidentaux (Sind-Baluchistan) et orientaux (Garo Hills). Les mollusques des Garo Hills montrent des affinités avec les espèces de Burma, tandis que les espèces de Sind-Béloutchistan montrent des similitudes avec celles de la région méditerranéenne. Les mollusques marins ont des ressemblances avec les taxons actuels de la Mer d'Arabie et du Golfe du Bengal, où plus de 35% des espèces sont les mÊmes. Cette similarité vaut aussi pour les élasmobranches marin, dont 33% des espèces et 86% des genres ressemblent aux taxons récents. Les mammifères terrestres montrent un mélange de formes du sud de l'U.S.S.R. et de formes africaines, allant de pair avec la présence de certains groupes probables appartenant au sous-continent indien.Les angiospermes forment le groupe dominant. Les formes du Miocène inférieur des secteurs orientaux et occidentaux sont entièrement différentes. Le palmier, le grand Sabal et le pin, sont connus dans différentes localités. Le climat dans le secteur occidental et dans l'Inde méridionale était tropical, tandis que dans le secteur oriental, il était variable.La période du Miocène inférieur est également marquée par le rétrécissement de la Mer de Sindhu, de la Mer de Bangla et du Golfe de Burma vers le sud, avec comme résultat la disparition finale de la mer au Pliocène.
(Aquitanian Nurdigalian) . . (Sind- Beluchistan) (Garo Hills) . , . : 35% . Elasmobranchs 33 % 86 % . , . . , Sabal major . , , , , , — . Sindhu, .相似文献
34.
P. Kharb D. V. Lal V. Singh J. Bagchi C. H. Ishwara Chandra A. Hota C. Konar Y. Wadadekar P. Shastri M. Das K. Baliyan B. B. Nath M. Pandey-Pommier 《Journal of Astrophysics and Astronomy》2016,37(4):34
We present detailed science cases that a large fraction of the Indian AGN community is interested in pursuing with the upcoming Square Kilometre Array (SKA). These interests range from understanding low luminosity active galactic nuclei in the nearby Universe to powerful radio galaxies at high redshifts. Important unresolved science questions in AGN physics are discussed. Ongoing low-frequency surveys with the SKA pathfinder telescope GMRT, are highlighted. 相似文献
35.
Neeraj Singh Bankoti Navin Chandra Joshi Bimal Pande Seema Pande Wahab Uddin Kavita Pandey 《New Astronomy》2011,16(4):269-275
This paper presents the study of normalized north–south asymmetry, cumulative normalized north–south asymmetry and cumulative difference indices of sunspot areas, solar active prominences (at total, low (?40°) and high (?50°) latitudes) and Hα solar flares from 1964 to 2008 spanning the solar cycles 20–23. Three different statistical methods are used to obtain the asymmetric behavior of different solar activity features. Hemispherical distribution of activity features shows the dominance of activities in northern hemisphere for solar cycle 20 and in southern hemisphere for solar cycles 21–23 excluding solar active prominences at high latitudes. Cumulative difference index of solar activity features in each solar cycle is observed at the maximum of the respective solar cycle suggesting a cyclic behavior of approximately one solar cycle length. Asymmetric behavior of all activity features except solar active prominences at high latitudes hints at the long term periodic trend of eight solar cycles. North–south asymmetries of SAP (H) express the specific behavior of solar activity at high solar latitudes and its behavior in long-time scale is distinctly opposite to those of other activity features. Our results show that in most cases the asymmetry is statistically highly significant meaning thereby that the asymmetries are real features in the N–S distribution of solar activity features. 相似文献
36.
The transition 111 ? 110 at 4.829 GHz of formaldehyde (H2CO) was the first one showing the anomalous absorption, i.e., the absorption against the cosmic microwave background. Anomalous absorption is an unusual phenomena. Structure of H2CC is very similar to that of H2CO and H2CS. Both H2CO and H2CS have already been identified in a number of cosmic objects. Though H2CC is not yet identified in the cosmic objects, we propose that H2CC may be identified in cool cosmic objects through its transition 111 ? 110 at 4.85 GHz in anomalous absorption. 相似文献
37.
K. A. Firoz J. Hwang I. Dorotovič T. Pintér Subhash C. Kaushik 《Astrophysics and Space Science》2011,331(2):469-484
Cosmic rays registered by Neutron Monitor on the surface of the Earth are believed to originate from outer space, and sometimes
also from the exotic objects of the Sun. Whilst the intensities of the cosmic rays are observed to be enhanced with sudden,
sharp and short-lived increases, they are termed as ground level enhancements (GLEs). They are the occurrences in solar cosmic
ray intensity variations on short-term basis, so different solar factors erupted from the Sun can be responsible for causing
them. In this context, an attempt has been made to determine quantitative relationships of the GLEs having peak increase >5%
with simultaneous solar, interplanetary and geophysical factors from 1997 through 2006, thereby searching the responsible
factors which seem to cause the enhancements. Results suggest that GLE peaks might be caused by solar energetic particle fluxes
and solar flares. The proton fluxes which seemed to cause GLE peaks were also supported by their corresponding fluences. For
most of the flares, the time integrated rising portion of the flare emission refers to the strong portion of X-ray fluxes
which might be the concern to GLE peak. On an average, GLE peak associated X-ray flux (0.71×10−4 w/m2) is much stronger than GLE background associated X-ray flux (0.11×10−6 w/m2). It gives a general consent that the GLE peak is presumably caused by the solar flare. Coronal mass ejection alone does
not seem to cause GLE. Coronal mass ejection presumably causes geomagnetic disturbances characterized by geomagnetic indices
and polarities of interplanetary magnetic fields. 相似文献
38.
R. Chandra B. Schmieder C. H. Mandrini P. Démoulin E. Pariat T. Török W. Uddin 《Solar physics》2011,269(1):83-104
We present and interpret observations of two morphologically homologous flares that occurred in active region (AR) NOAA 10501
on 20 November 2003. Both flares displayed four homologous Hα ribbons and were both accompanied by coronal mass ejections
(CMEs). The central flare ribbons were located at the site of an emerging bipole in the centre of the active region. The negative
polarity of this bipole fragmented in two main pieces, one rotating around the positive polarity by ≈ 110° within 32 hours.
We model the coronal magnetic field and compute its topology, using as boundary condition the magnetogram closest in time
to each flare. In particular, we calculate the location of quasi-separatrix layers (QSLs) in order to understand the connectivity between the flare ribbons. Though several polarities were present in AR 10501,
the global magnetic field topology corresponds to a quadrupolar magnetic field distribution without magnetic null points.
For both flares, the photospheric traces of QSLs are similar and match well the locations of the four Hα ribbons. This globally
unchanged topology and the continuous shearing by the rotating bipole are two key factors responsible for the flare homology.
However, our analyses also indicate that different magnetic connectivity domains of the quadrupolar configuration become unstable
during each flare, so that magnetic reconnection proceeds differently in both events. 相似文献
39.
The geoeffective magnetic cloud (MC) of 20 November 2003 was associated with the 18 November 2003 solar active events in previous
studies. In some of these, it was estimated that the magnetic helicity carried by the MC had a positive sign, as did its solar
source, active region (AR) NOAA 10501. In this article we show that the large-scale magnetic field of AR 10501 has a negative
helicity sign. Since coronal mass ejections (CMEs) are one of the means by which the Sun ejects magnetic helicity excess into
interplanetary space, the signs of magnetic helicity in the AR and MC must agree. Therefore, this finding contradicts what
is expected from magnetic helicity conservation. However, using, for the first time, correct helicity density maps to determine
the spatial distribution of magnetic helicity injections, we show the existence of a localized flux of positive helicity in
the southern part of AR 10501. We conclude that positive helicity was ejected from this portion of the AR leading to the observed
positive helicity MC. 相似文献
40.
M. Chandra Singh P. Kundal R. A. S. Kushwaha 《Journal of the Geological Society of India》2010,76(6):573-586
Oligocene-Miocene deposits of Bhuban and Boka Bil Formations, Surma Group, Manipur Western Hill consist of well preserved
ichnofossil assemblages. These formations are represented by eight lithofacies such as Massive sandstone (Sm), Rippled marked
argillaceous sandstone (Sr), Wavy laminated sandstone-siltstone-silty shale (Sw), Laminated shale (Fl), Massive mudstone (Fm),
Trough cross-bedded sandstone (St), Lenticular laminated sandstonesiltstonesilty shale (Sll) and Laminated to massive sandstone-siltstone
(Ssc). Fifteen ichnospecies were identified, which further categories into Skolithos, Cruziana, and Skolithos/Cruziana ichnofacies. Overall distribution pattern and behavioural nature of the ichnoassemblage and sedimentological attributes suggests
that the sediments of Bhuban and Boka Bil Formations were deposited under frequent fluctuating sea level, moderate to strong
energy condition, subtidal to lower intertidal environment, rich in organic nutrients. 相似文献