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41.
Boška J. Šauli P. Altadill D. German Solé J. Alberca L.F. 《Studia Geophysica et Geodaetica》2003,47(3):579-586
We investigate the short-term fluctuations in the period range from 15 to 180 minutes in the electron density variations of the F region ionosphere. Electron density profiles obtained at the ionospheric stations of Pruhonice (49.9° N, 14.5° E) and Ebro (40.8° N, 0.5° E) at five minute time sampling have been used for this analysis. The diurnal changes of the activity of the acoustic gravity wave fluctuations (AGW) show a clear enhancement during and several hours after sunrise. The periods of such AGW's are about 60 to 75 minutes and these waves propagates vertically through the ionosphere from a source located at an altitude of 180-220 km. The most likely source for these events seems to be passage of the Solar terminator. 相似文献
42.
This review describes developments major and in trace element determination using atomic absorption spectroscopy (AAS), inductively coupled plasma-atomic emission spectrometry (ICP-AES), instrumental neutron activation analysis (INAA), and X-ray fluorescence (XRF) spectrometry that were reported in 2008 and 2009. Publication levels were comparable to those of previous years, except for XRF which showed an increase in the number of articles published in the past 2 years. In terms of number of publications and impact, the most active field was AAS, while INAA was the least active. As expected for well-established and mature methods, novel developments for all four analytical techniques were relatively sparse. For AAS, the most notable publications concerned developments in sample introduction, particularly focussing on solid samples, increasing analyte sensitivity, and higher sample throughput. For ICP-AES, publication of developments in sample introduction, remediation of matrix effects and calibration continues. Compared with past years, there was a clear focus on sample preconcentration methods, but very few publications reporting new hyphenated speciation methods. For INAA, there were several publications exploring the accuracy and robustness of the method, as well as the requirements for INAA to meet criteria for a primary method of measurement. Two other related techniques, delayed neutron activation analysis, and prompt gamma activation analysis, were also described. Hyphenated XRF techniques showed interesting developments in enabling XRF and XRD analysis on the same spot, and further work characterising and calibrating three-dimensional micro-XRF shows promising results for investigating sample heterogeneity. 相似文献
43.
Laurent Millet Charline Giguet-Covex Valérie Verneaux Jean-Claude Druart Thierry Adatte Fabien Arnaud 《Journal of Paleolimnology》2010,44(4):963-978
This paper presents the recent history of a large prealpine lake (Lake Bourget) using chironomids, diatoms and organic matter
analysis, and deals with the ability of paleolimnological approach to define an ecological reference state for the lake in
the sense of the European Framework Directive. The study at low resolution of subfossil chironomids in a 4-m-long core shows
the remarkable stability over the last 2.5 kyrs of the profundal community dominated by a Micropsectra-association until the beginning of the twentieth century, when oxyphilous taxa disappeared. Focusing on this key recent period,
a high resolution and multiproxy study of two short cores reveals a progressive evolution of the lake’s ecological state.
Until AD 1880, Lake Bourget showed low organic matter content in the deep sediments (TOC less than 1%) and a well-oxygenated
hypolimnion that allowed the development of a profundal oxyphilous chironomid fauna (Micropsectra-association). Diatom communities were characteristic of oligotrophic conditions. Around AD 1880, a slight increase in the
TOC was the first sign of changes in lake conditions. This was followed by a first limited decline in oligotrophic diatom
taxa and the disappearance of two oxyphilous chironomid taxa at the beginning of the twentieth century. The 1940s were a major
turning point in recent lake history. Diatom assemblages and accumulation of well preserved planktonic organic matter in the
sediment provide evidence of strong eutrophication. The absence of profundal chironomid communities reveals permanent hypolimnetic
anoxia. From AD 1995 to 2006, the diatom assemblages suggest a reduction in nutrients, and a return to mesotrophic conditions,
a result of improved wastewater management. However, no change in hypolimnion benthic conditions has been shown by either
the organic matter or the subfossil chironomid profundal community. Our results emphasize the relevance of the paleolimnological
approach for the assessment of reference conditions for modern lakes. Before AD 1900, the profundal Micropsectra-association and the Cyclotella dominated diatom community can be considered as the Lake Bourget reference community, which reflects the reference ecological
state of the lake. 相似文献
44.
Assessing landslide movements in volcanic islands using near-shore marine geophysical data: south Pico island,Azores 总被引:1,自引:0,他引:1
Marine geophysical data from around the submarine flanks of volcanic islands can potentially help to resolve whether large-scale
instability of an edifice has been geologically recently active. We use geophysical data to investigate part of the coast
of Pico Island of the Azores where, above sea-level, a major slump of Topo volcano has been interpreted previously from arcuate
escarpments and a rugged irregular topography seaward of them. Multibeam echo-sounder data collected offshore of this feature
show remarkably little evidence for slump fault movements in the island's submarine slope. Mid-slope benches, like those associated
with the Hilina Slump of Kilauea, are absent. The high-resolution data extends onto the island's shelf, allowing us to evaluate
evidence for continuing activity there. In particular, as the shelf's rock platform will have been last modified by surf erosion
during the postglacial period of sea-level transgression, it provides a reference surface of intermediate age (7–19 ka) that
can potentially reveal whether any movements occurred in pre-historic times. Where the arcuate escarpments are continued offshore,
the modern seabed shows no bathymetric evidence for active faults where the shelf rock platform crops out in the multibeam
data. Elsewhere, mobile shelf sediments could be disguising evidence for active faulting so we examined boomer profiles able
to image the rock platform beneath them. The data reveal a platform that is steep (6.6°) compared with the dips of platforms
that we have studied previously around the coast of adjacent Faial Island and steeper than the platform outside the proposed
slump. This suggests that it was created by coastal erosion over a shorter period and hence is consistent with a younger age
of the coastline. As with the multibeam data, where escarpments are continued offshore onto the shelf, the rock surface imaged
with these boomer data also shows no clear evidence of major slump-related fault displacements. This study therefore illustrates
how high-resolution boomer seismic and multibeam data could usefully contribute to hazard assessment of volcanic islands,
by helping to evaluate areas with no historical movements. Explanations to reconcile the onshore and offshore data here are
also put forward. 相似文献
45.
The results of DSDP Legs 67 and 84, during which a transect of holes was drilled across the Middle America Trench off Guatemala, show no accretion at the front of that subduction zone since early Eocene time. The tectonic evolution of the trench includes extensional structure on the landward as well as the seaward trench slopes. The Guatemalan margin is proposed as the model of a new type of active margin, the convergent-extensional active margin (CE margin). 相似文献
46.
Soil feedback drives the mid-Holocene North African monsoon northward in fully coupled CCSM2 simulations with a dynamic vegetation model 总被引:1,自引:0,他引:1
We explore climate-vegetation interactions in mid-Holocene North Africa with a suite of community climate system model (CCSM2) simulations. The CCSM includes synchronously coupled atmosphere, ocean, sea ice, land, and vegetation models. The CCSMs present-day precipitation for North Africa compares well with simulations of other models and observations. Mid-Holocene data reveal a wetter and greener Sahara compared to the present. The CCSM exhibits a greater, closer to the expected, precipitation increase than other models, and in response, grasses advance from 18.75° to 22.5°N in much of North Africa. Precipitation is enhanced locally by the northward advance of grasses, but suppressed regionally mainly due to an insufficient albedo decrease with the expansion of vegetation. Prior studies have always lowered the surface albedo with the expansion of vegetation in North Africa. In the CCSMs mid-Holocene simulations, the albedo decreases more because wetter soils are simulated darker than drier soils than due to expanding vegetation. These results isolate albedo as the key ingredient in obtaining a positive precipitation-vegetation feedback in North Africa. Two additional simulations support this conclusion. In the first simulation, the deserts sandy soil textures are changed to loam to represent increased organic matter. Soil water retention and grass cover increase; albedo decreases somewhat. Precipitation responds with a small, yet widespread, increase. In the second simulation, a darker soil color is prescribed for this region. Now the monsoon advances north about 4°. These results illustrate a North African monsoon highly sensitive to changes in surface albedo and less sensitive to changes in evapotranspiration. 相似文献
47.
Isabel Pérez Francisco Martín Romero Olivia Zamora Margarita Eugenia Gutiérrez-Ruiz 《Environmental Earth Sciences》2014,72(5):1521-1531
A total of 113 samples of waste and soil were collected from a site in the state of San Luis Potosi, Mexico, that was occupied for several years by the metallurgical industry. Specific magnetic susceptibility (MS), electrical conductivity (EC) and pH were determined, as well as the total and available concentrations of potentially toxic elements (PTEs) such as As, Cd, Cu, Fe, Pb and Zn, which may cause a health risk for humans, animals and ecosystems, and the concentrations of major ions in aqueous extracts of soils and wastes. The solid phases of the samples were also characterized. The results revealed that the soils and wastes exhibited elevated values of PTEs, MS and EC. For soils these values decreased with increasing distance from the waste storage sites. The MS values were elevated primarily due to the presence of Fe-oxyhydroxides, such as magnetite, hematite and goethite, which contain PTEs in their structure leading to a high correlation between the value of MS and the As, Cd, Fe and Pb contents (r = 0.57–0.91) as well as between the PTEs values (r = 0.68–0.92). The elevated EC values measured in the metallurgical wastes were the result of presence of the sulfate minerals of Ca, Mg and Fe. The pollution index, which indicates the levels of simultaneous toxicity from elements such as As, Cd and Pb, was determined, with extreme hazard zones corresponding to areas that exhibit high MS values (0.91 correlation). In conclusion, MS measurements can be used as an indirect indicator to evaluate the PTE contamination in metallurgical areas, and EC measurements can aid in the identification of pollution sources. 相似文献
48.
Mount Pinatubo volcano erupted in June 1991 in the main island of Luzon belonging to the Philippines archipelago. Huge economic
losses and population exodus have followed. This major crisis has been relayed with other crises due to rain-fed lahars which
have been supplied with eruption deposits. These lahars have occurred every year since 1991 during the rainy season. They
will probably last until 2005. After a brief presentation of the Philippine official response system to disasters, this paper
draws up a critical analysis of the different kinds of institutional and social responses deployed to manage the different
crisis and post-crisis phases of this event. Based on three viewpoints: from population, media and other actors, this analysis
attempts to point out the strengths and weaknesses of the official management system, especially by studying the efficiency
and the range of the solutions taken. So, it appears that the management of the June 1991 main crisis (eruption) was a success.
On the other hand, difficulties have occurred with lahars risk management. Indeed, these lahars have obliged the authorities
to protect and relocate thousands of people. In spite of persistent problems, the management system (monitoring/warning/evacuation)
of lahar crises improves year after year. Failures appear especially within the rehabilitation program (protection/rehousing).
Many direct (lack of means, preparedness, coordination, dialog, etc.) and indirect (politico-administrative, socio-economic,
cultural contexts) factors come together to lock the wheels of the institutional response system. They defer the socio-economic
start of this vital northern Philippines area.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
49.
C. P. Goff L. van Driel-Gesztelyi P. Démoulin J. L. Culhane S. A. Matthews L. K. Harra C. H. Mandrini K. L. Klein H. Kurokawa 《Solar physics》2007,240(2):283-299
A series of flares (GOES class M, M and C) and a CME were observed in close succession on 20 January 2004 in NOAA 10540. Radio
observations, which took the form of types II, III and N bursts, were associated with these events. We use the combined observations
from TRACE, EIT, Hα images from Kwasan, MDI magnetograms and GOES to understand the complex development of this event. Contrary
to a standard interpretation, we conclude that the first two impulsive flares are part of the CME launch process while the
following long-duration event flare represents simply the recovery phase. Observations show that the flare ribbons not only
separate but also shift along the magnetic inversion line so that magnetic reconnection progresses stepwise to neighboring
flux tubes. We conclude that “tether cutting” reconnection in the sheared arcade progressively transforms it to a twisted
flux tube, which becomes unstable, leading to a CME. We interpret the third flare, a long-duration event, as a combination
of the classical two-ribbon flare with the relaxation process following forced reconnection between the expanding CME structure
and neighboring magnetic fields.
Electronic Supplementary Material The online version of this article () contains supplementary material, which is available to authorized users. 相似文献
50.
There is an emerging requirement for processing global navigation satellite system (GNSS) signals indoor where the signal
is very weak and subjected to spatial fading. Typically, longer coherent integration intervals provide the additional processing
gain required for the detection and processing of such weak signals. However, the arbitrary physical motion of the handset
imputed by the user limits the effectiveness of longer coherent integration intervals due to the spatial decorrelation of
the multipath-faded GNSS signal. In this paper, limits of coherent integration due to spatial decorrelation are derived and
corroborated with experimental verification. A general result is that the processing gain resulting from direct coherent integration
saturates after the antenna has moved through a certain distance, which for typical indoor propagation, is about half a carrier
wavelength. However, a refined Doppler search coupled with a prolonged coherent integration interval extends this limit, which
is effectively a manifestation of selective diversity. 相似文献