全文获取类型
收费全文 | 123篇 |
免费 | 5篇 |
国内免费 | 1篇 |
专业分类
测绘学 | 1篇 |
大气科学 | 22篇 |
地球物理 | 35篇 |
地质学 | 32篇 |
海洋学 | 15篇 |
天文学 | 22篇 |
自然地理 | 2篇 |
出版年
2021年 | 4篇 |
2020年 | 2篇 |
2018年 | 2篇 |
2017年 | 5篇 |
2016年 | 14篇 |
2015年 | 6篇 |
2014年 | 5篇 |
2013年 | 7篇 |
2012年 | 6篇 |
2011年 | 13篇 |
2010年 | 9篇 |
2009年 | 10篇 |
2008年 | 6篇 |
2007年 | 5篇 |
2006年 | 9篇 |
2005年 | 4篇 |
2004年 | 7篇 |
2003年 | 1篇 |
2002年 | 1篇 |
2001年 | 1篇 |
2000年 | 2篇 |
1999年 | 1篇 |
1998年 | 3篇 |
1996年 | 1篇 |
1995年 | 1篇 |
1993年 | 1篇 |
1985年 | 1篇 |
1982年 | 1篇 |
1969年 | 1篇 |
排序方式: 共有129条查询结果,搜索用时 15 毫秒
51.
Irradiance variability has been monitored from space for more than two decades. Even though data come from different sources, it is well established that a temporal variability exists ≈0.1%, in phase with the solar cycle. Today, one of the best explanations for such an irradiance variability is provided by the evolution of the solar surface magnetic fields. But if some 90–95% can be reproduced, what would be the origin of the 10–5% left? Non-magnetic effects are conceivable. In this paper we will consider temporal variations of the diameter of the Sun as a possible contributor for the remaining part. Such an approach imposes strong constraints on the solar radius variability. We will show that over a solar cycle, variations of no more than 20 mas of amplitude can be considered. Such a variability – far from what is reported by observers conducting measurements by means of ground-based solar astrolabes – may explain a little part of the irradiance changes not explained by magnetic features. Further, requirements are needed that may help to reach a conclusion. Dedicated space missions are necessary (for example PICARD, GOLF-NG or SDO, scheduled for a launch around 2008); it is also proposed to reactivate SDS flights for such a purpose. 相似文献
52.
Sandrine Vinatier Bruno Bézard Nick A. Teanby Patrick G.J. Irwin Conor A. Nixon F. Michael Flasar 《Icarus》2007,188(1):120-138
Limb spectra recorded by the Composite InfraRed Spectrometer (CIRS) on Cassini provide information on abundance vertical profiles of C2H2, C2H4, C2H6, CH3C2H, C3H8, C4H2, C6H6 and HCN, along with the temperature profiles in Titan's atmosphere. We analyzed two sets of spectra, one at 15° S (Tb flyby) and the other one at 80° N (T3 flyby). The spectral range 600-1400 cm−1, recorded at a resolution of 0.5 cm−1, was used to determine molecular abundances and temperatures in the stratosphere in the altitude range 100-460 km for Tb and 170-495 km for T3. Both temperature profiles show a well defined stratopause, at around 310 km (0.07 mbar) and 183 K at 13° S, and 380 km (0.01 mbar) with 207 K at 80° N. Near the north pole, stratospheric temperatures are colder and mesospheric temperatures are warmer than near the equator. C2H2, C2H6, C3H8 and HCN display vertical mixing ratio profiles that increase with height at 15° S and 80° N, consistent with their formation in the upper atmosphere, diffusion downwards and condensation in the lower stratosphere, as expected from photochemical models. The CH3C2H and C4H2 mixing ratios also increase with height at 15° S. But near the north pole, their profiles present an unexpected minimum around 300 km, observed for the first time thanks to the high vertical resolution of the CIRS limb data. C2H4 is the only molecule having a vertical abundance profile that decreases with height at 15° S. At 80° N, it also displays a minimum of its mixing ratio around the 0.1-mbar level. For C6H6, an upper limit of 1.1 ppb (in the 0.3-10 mbar range) is derived at 15° S, whereas a constant mixing ratio profile of is inferred near the north pole. At 15° S, the vertical profile of HCN exhibits a steeper gradient than other molecules, which suggests that a sink for this molecule exists in the stratosphere, possibly due to haze formation. All molecules display a more or less pronounced enrichment towards the north pole, probably due, in part, to subsidence of air at the north (winter) pole that brings air enriched in photochemical compounds from the upper atmosphere to lower levels. 相似文献
53.
We report the detection of H13CN and HC15N in mid-infrared spectra recorded by the Composite Infrared Spectrometer (CIRS) aboard Cassini, along with the determination of the 12C/13C and 14N/15N isotopic ratios. We analyzed two sets of limb spectra recorded near 13-15° S (Tb flyby) and 83° N (T4 flyby) at 0.5 cm−1 resolution. The spectral range 1210-1310 cm−1 was used to retrieve the temperature profile in the range 145-490 km at 13° S and 165-300 km at 83° N. These two temperature profiles were then incorporated in the atmospheric model to retrieve the abundance profile of H12C14N, H13CN and HC15N from their bands at 713, 706 and 711 cm−1, respectively. The HCN abundance profile was retrieved in the range 90-460 km at 15° S and 165-305 km at 83° N. There is no evidence for vertical variations of the isotopic ratios. Constraining the isotopic abundance profiles to be proportional to the HCN one, we find at 15° S, and at 83° N, two values that are statistically consistent. A combination of these results yields a 12C/13C value equal to 75±12. This global result, as well as the 15° S one, envelop the value in Titan's methane (82.3±1) [Niemann, H.B., and 17 colleagues, 2005. Nature 438, 779-784] measured at 10° S and is slightly lower than the terrestrial inorganic standard value (89). The 14N/15N isotopic ratio is found equal to at 15° S and at 83° N. Combining the two values yields 14N/15N = 56 ± 8, which corresponds to an enrichment in 15N of about 4.9 compared with the terrestrial ratio. These results agree with the values obtained from previous ground-based millimeter observations [Hidayat, T., Marten, A., Bézard, B., Gautier, D., Owen, T., Matthews, H.E., Paubert, G., 1997. Icarus 126, 170-182; Marten, A., Hidayat, T., Biraud, Y., Moreno, R., 2002. Icarus 158, 532-544]. The 15N/14N ratio found in HCN is ∼3 times higher than in N2 [Niemann, H.B., and 17 colleagues, 2005. Nature 438, 779-784], which implies a large fractionation process in the HCN photochemistry. 相似文献
54.
Sandrine D’Hoedt Anne Lemaitre Nicolas Rambaux 《Celestial Mechanics and Dynamical Astronomy》2006,96(3-4):253-258
Mercury is observed in a stable Cassini’s state, close to a 3:2 spin-orbit resonance, and a 1:1 node resonance. This present situation is not the only possible mathematical stable state, as it is shown here through a simple model limited to the second-order in harmonics and where Mercury is considered as a rigid body. In this framework, using a Hamiltonian formalism, four different sets of resonant angles are computed from the differential Hamiltonian equations, and each of them corresponds to four values of the obliquity; thanks to the calculation of the corresponding eigenvalues, their linear stability is analyzed. In this simplified model, two equilibria (one of which corresponding to the present state of Mercury) are stable, one is unstable, and the fourth one is degenerate. This degenerate status disappears with the introduction of the orbit (node and pericenter) precessions. The influence of these precession rates on the proper frequencies of the rotation is also analyzed and quantified, for different planetary models. 相似文献
55.
Atoll lagoons display a high diversity of trophic states due mainly to their specific geomorphology, and probably to their level and mode of human exploitation. We investigated the functioning of the Ahe atoll lagoon, utilized for pearl oyster farming, through estimations of photosynthetic parameters (pulse amplitude modulation fluorometry) and primary production ((13)C incorporation) measurements of the size structured phytoplankton biomass (<2μm and >2μm). Spatial and temporal scales of variability were surveyed during four seasons, over 16months, at four sites within the lagoon. While primary production (P) was dominated by the picophytoplankton, its biomass specific primary productivity (P(B)) was lower than in other atoll lagoons. The variables size fraction of the phytoplankton, water temperature, season, the interaction term station*fraction and site, explained significantly the variance of the data set using redundancy analysis. No significant trends over depth were observed in the range of 0-20m. A clear spatial pattern was found which was persistent over the seasons: south and north sites were different from the two central stations for most of the measured variables. This pattern could possibly be explained by the existence of water cells showing different water residence time within the lagoon. Photoacclimation strategies of the two size fractions differed through their light saturation coefficient (higher for picophytoplankton), but not through their maximum photosynthetic capacity (ETR(max)). Positive linear relationships between photosynthetic parameters indicated that their dynamic was independent of light availability in this ecosystem, but most probably dependent on nutrient availability and/or rapid changes in the community structure. Spatial and temporal patterns of the measured processes are then further discussed in the context of nutrient availability and the possible role of cultured oysters in nutrient recycling. 相似文献
56.
A new method to detect active features at the solar limb 总被引:1,自引:0,他引:1
57.
Samantha Murphy Thomas Ouellon Jean-Marc Ballard Ren�� Lefebvre Ian D. Clark 《Hydrogeology Journal》2011,19(1):195-207
Tritium?Chelium groundwater dating was carried out in a trichloroethylene (TCE)-contaminated valley-fill aquifer system in Quebec, Canada, where a numerical groundwater flow model was developed. Forty seven discrete groundwater and dissolved gas samples were obtained along two flow paths originating from known TCE source zones whose related plumes converge down gradient to form a single plume. Sampling points in monitoring wells were projected onto vertical sections showing particle tracks along the two flow paths. At these points, simulated advective ages obtained from particle tracking were matched to tritium?Chelium ages using different porosity values; the best match was 0.35. Ages were also obtained above and below a prodeltaic silty aquitard in a portion of the aquifer where some source zones are located, which provide groundwater and TCE transit times through the aquitard as well as a mean vertical hydraulic conductivity that agrees with previous estimates used in the model. In certain locations, anomalously old ages associated with high terrigenic 4He indicate areas where groundwater from the underlying proglacial unit flows upward into the deltaic sand aquifer through aquitard windows. Upflow locations correspond with increased TCE concentrations, suggesting significant TCE provenance through the proglacial unit originating from a previously unrecognized TCE source zone. 相似文献
58.
Sandrine Meresse Franois Costard Nicolas Mangold David Baratoux Joseph M. Boyce 《Meteoritics & planetary science》2006,41(10):1647-1658
Abstract— The northern lowland plains, such as those found in Acidalia and Utopia Planitia, have high percentages of impact craters with fluidized ejecta. In both regions, the analysis of crater geometry from Mars Orbiter Laser Altimeter (MOLA) data has revealed large ejecta volumes, some exceeding the volume of excavation. Moreover, some of the crater cavities and fluidized ejecta blankets of these craters are topographically perched above the surrounding plains. These perched craters are concentrated between 40 and 70°N in the northern plains. The atypical high volumes of the ejecta and the perched craters suggest that the northern lowlands have experienced one or more episodes of resurfacing that involved deposition and erosion. The removal of material, most likely caused by the sublimation of ice in the materials and their subsequent erosion and transport by the wind, is more rapid on the plains than on the ejecta blankets. The thermal inertia difference between the ejecta and the surrounding plains suggests that ejecta, characterized by a lower thermal inertia, protect the underneath terrain from sublimation. This results in a decreased elevation of the plains relative to the ejecta blankets. Sublimation and eolian erosion can be particularly high during periods of high obliquity. 相似文献
59.
Sandrine Vinatier Pascal Rannou Carrie M. Anderson Bruno Bézard Remco de Kok Robert E. Samuelson 《Icarus》2012,219(1):5-12
We utilized aerosol extinction coefficient inferred from Cassini/CIRS spectra in the far and mid infrared region to derive the extinction cross-section near an altitude of 190 km at 15°S (from far-IR) and 20°S (from mid-IR). By comparing the extinction cross section that are derived from observations with theoretical calculations for a fractal aggregate of 3000 monomers, each having a radius of 0.05 μm, and a fractal dimension of 2, we are able to constrain the refractive index of Titan’s aerosol between 70 and 1500 cm?1 (143 and 6.7 μm). As the real and imaginary parts of the refractive index are related by the Kramers–Kronig equation, we apply an iterative process to determine the optical constants in the thermal infrared. The resulting spectral dependence of the imaginary index displays several spectral signatures, some of which are also seen for some Titan’s aerosol analogues (tholins) produced in laboratory experiments. We find that Titan’s aerosols are less absorbent than tholins in the thermal infrared. The most prominent emission bands observed in the mid-infrared are due to CH bending vibrations in methyl and methylene groups. It appears that Titan’s aerosols predominantly display vibrations implying carbon and hydrogen atoms and perhaps marginally nitrogen. In the mid infrared, all the aerosol spectral signatures are observed at three additional latitudes (56°S, 5°N and 30°N) and in the 193–274 km altitude range, which implies that Titan’s aerosols exhibit the same chemical composition in all investigated latitude and altitude regions. 相似文献
60.
Sandrine Vinatier Bruno Bézard Andrei Mamoutkine Ronald C. Carlson Donald E. Jennings Nick A. Teanby F. Michael Flasar 《Icarus》2010,205(2):559-570
Observations of the Composite InfraRed Spectrometer (CIRS) during the entire nominal Cassini mission (2004-2008) provide us with an accurate global view of composition and temperature in the middle atmosphere of Titan (between 100 and 500 km). We investigated limb spectra acquired at resolution at nine different latitudes between 56°S and 80°N, with a better sampling in the northern hemisphere where molecular abundances and temperature present strong latitudinal variations. From this limb data acquired between February 2005 and May 2008, we retrieved the vertical mixing ratio profiles of C2H2, C2H4, C2H6, C3H8, CH3C2H, C4H2, C6H6, HCN, HC3N and CO2. We present here for the first time, the latitudinal variations of the C2H6, C3H8, CO2, C2H4 and C6H6 vertical mixing ratios profiles. Some molecules, such as C2H6 or C3H8 present little variations above their condensation level. The other molecules (except CO2) show a significant enhancement of their mixing ratios poleward of 50°N. C2H4 is the only molecule whose mixing ratio decreases with height at latitudes below 46°N. Regions depleted in C2H2, HCN and C4H2 are observed around 400 km (0.01 mbar) and 55°N. We also inferred a region enriched in CO2 located between 30 and 40°N in the 2-0.7 mbar pressure range. At 80°N, almost all molecules studied here present a local minimum of their mixing ratio profiles near 300 km (∼0.07 mbar), which is in contradiction with Global Circulation Models that predict constant-with-height vertical profiles due to subsidence at the north pole. 相似文献