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We present the first clear observations of meteor shower activity from meteor-head echoes detected by a high-power large-aperture radar (HPLAR). Such observations have been performed at the Jicamarca VHF radar using its interferometric capabilities allowing the discrimination of meteor shower echoes from the much more frequent sporadic meteors. Until now, HPLARs were unable to distinguish meteor shower from the much more common sporadic meteor ones. In this work we have been able to detect and characterize the η-Aquariids (ETA) as well as the Perseids (PER) showers. The shower activity is more conspicuous for the ETA than for the PER shower due to the more favorable geometry. Namely, PER meteors come from low elevation angles, experiencing more filtering due to the combined Earth-atmosphere-radar instrument. In both cases, there is an excellent agreement between the measured mean velocity of the shower echoes and their expected velocity, within a fraction of 1 km s−1. Besides the good agreement with expected visual results, HPLARs observe meteors with a variety of particles sizes and masses, not observed by any other technique. Taking into account the different viewing volumes, compare to optical observations Jicamarca observes more than 1000 times more ETA meteors. Our results indicate that Jicamarca and other HPLARs are able to detect the echoes from meteor showers, but without interferometric capabilities such populations are difficult to identify just from their velocity distributions, particularly if their velocity distributions are expected to be similar to the more dominant distributions of sporadic meteors. 相似文献
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Patricia Whitelock Freddy Marang 《Monthly notices of the Royal Astronomical Society》2001,323(1):L13-L16
The binary companion to the peculiar F supergiant HD 172481 is shown to be a Mira variable with a pulsation period of 312 d. Its characteristics are within the normal range found for solitary Miras of that period, although its pulsation amplitude and mass-loss rate ̇ ∼3×10−6 M⊙ yr−1 are higher than average. Reasons are given for suspecting that the F supergiant, which has L ∼104 L⊙ , is a white dwarf burning hydrogen accreted from its companion. 相似文献
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Lucy?E.?McGeeEmail authorView author&#;s OrcID profile Raimundo?Brahm Michael?C.?Rowe Heather?K.?Handley Eduardo?Morgado Luis?E.?Lara Michael?B.?Turner Nicolas?Vinet Miguel-ángel?Parada Pedro?Valdivia 《Contributions to Mineralogy and Petrology》2017,172(6):44
Small eruptive centres (SECs) representing short-lived, isolated eruptions are effective samples of mantle heterogeneity over a given area, as they are generally of basaltic composition and show evidence of little magmatic processing. This is particularly powerful in volcanic arcs where the original melting process generating stratovolcanoes is often obscured by additions from the down-going slab (fluids and sediments) and the overlying crust. The Pucón area of southern Chile contains active and dormant stratovolcanoes, Holocene, basaltic SECs and an arc-scale strike-slip fault (the Liquiñe Ofqui Fault System: LOFS). The SECs show unexpected compositional heterogeneity considering their spatial proximity. We present a detailed study of these SECs combining whole rock major and trace element concentrations, U-Th isotopes and olivine-hosted melt inclusion major element and volatile contents to highlight the complex inter-relations in this small but active area. We show that heterogeneity preserved at individual SECs relates to different processes: some start in the melting region with the input of slab-derived fluids, whilst others occur later in a centre’s magmatic history with the influence of crustal contamination prior to olivine crystallisation. These signals are deduced through the combination of the different geochemical tools used in this study. We show that there is no correlation between composition and distance from the arc front, whilst the local tectonic regime has an effect on melt composition: SECs aligned along the LOFS have either equilibrium U-Th ratios or small Th-excesses instead of the large—fluid influenced—U-excesses displayed by SECs situated away from this feature. One of the SECs is modelled as being generated from fluid-enriched depleted mantle, a source which it may share with the stratovolcano Villarrica, whilst another SEC with abundant evidence of crustal contamination may share its plumbing system with its neighbouring stratovolcano Quetrupillán, showing that polygenetic–monogenetic connections are unpredictable. Such marked preservation of individual magmatic histories highlights the isolation of individual melting events even in complex and highly volcanically active areas. 相似文献
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Freddy J. Gathorne-Hardy Egill Erlendsson Peter G. Langdon Kevin J. Edwards 《Journal of Paleolimnology》2009,42(3):413-426
Ecosystem variability must be assessed over a range of timescales in order to fully understand natural ecosystem processes.
Long-term climate change, at millennial and centennial scales, is a major driver of natural ecosystem variability, but identifying
evidence of past climate change is frequently confounded by human-induced impacts on the ecosystem. Iceland is a location
where it is possible to separate natural from anthropogenic change in environmental archives, as the date of settlement is
accepted to be around AD 874, prior to which the island was free from proven human impacts. We used a lake sediment core from
Breieavatn, near Reykholt, a major farm of the Norse period in western Iceland, to examine landscape development. A change
in pollen concentration in the sediments, especially the decline in Betula, indicated initial landscape degradation immediately post-settlement, whereas the chironomid fauna and reconstructed temperatures
were relatively complacent during this period. The pollen evidence is corroborated by 14C analyses, which indicate an increase in older carbon entering the lake, inferred to have been caused by increased erosion
following settlement. Further decreases in Betula pollen occurred around AD 1300, pre-dating a drop in chironomid-inferred temperatures (CI-T) of ~1°C over 100–200 years.
The CI-T reconstruction also shows a significant cooling after ~AD 1800, likely indicative of the coldest phase of the Little
Ice Age. The evidence suggests that the chironomid record was relatively unaffected by the increased landscape degradation
and hence reveals a temperature reconstruction independent of human impact. 相似文献
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Freddy Rey 《山地科学学报》2018,15(2):225-236
Improving knowledge on the ability of bioengineering structures made of willow cuttings to enhance efficient and sustainable sediment trapping in marly gullies in the Southern French Alps under a mountainous Mediterranean climate, to decreasesediment trapped per experiment, the mean annual values reached 0.25, 0.14 and 0.08 m3 yr-1 in experiments C, A and B, respectively. Maximum values for one structure reached 1.94 m3 per year in experiment C. The significance of the volumes ofsediment yield at their outlets, is a key issue today for the international scientific community working in geosciences and ecology. This study therefore aims to assess three real-size experiments(A, B and C) carried out between 2003 and 2013 in this environment. A total of 157 bioengineering structures using purple and white willow(Salix purpurea and Salix incana)cuttings – which have been shown to resprout and survive more than 2 years after their installation,corresponding to brush layers with brush mats on wooden sills(BLM), 1.2 m wide and 2 m long,installed on the floors of 33 experimental marly gullies, were monitored. The results showed that sediment trapping occurred upstream of the vegetation barriers from the 1 st year onwards.Considering the depth of sediment trapped per experiment, the mean annual values reached 11.2 cm yr-1 after 3 years in experiment A, 7.7 cm yr-1 after 2–4 years in experiment C and 5.1 cm yr-1 after 5 years in experiment B. Occasionally, BLMs showed that they could trap up to 16 and 15 cm yr-1 in experiments A and C, respectively. Considering the volumes oftrapped sediment and the sustainability of sediment trapping are discussed, and rules for bioengineering strategies are proposed. 相似文献
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Chris Koen Ruby Van Rooyen Francois Van Wyk Freddy Marang 《Monthly notices of the Royal Astronomical Society》1999,309(4):1051-1062
HD 23194, a member of the Pleiades, was found to pulsate with a period of about 30 min. The literature on the star is reviewed, and it is concluded that it may be a marginal Am star in a binary system. HD 95321 is an evolved Am ( ρ Puppis) star with a 5.1-h periodicity. Mode identification of its pulsation, based on multicolour photometry, suggests that the oscillation is probably non-radial with ℓ=2. We also report on the discovery of six other new δ Scuti stars, some of which may be pulsating in gravity modes. 相似文献
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This paper aims to analyse and compare the loss of life that occurred during two recent floods in France. The first flood
was due to a sea surge triggered by the storm called Xynthia that hit the Atlantic coast on 28 February 2010 (41 flood-related
deaths). The second was a flash flood that struck the Var Department in the French Mediterranean region on 15 June 2010 (26
fatalities). After detailing the assumptions and expected outcomes of the study of disaster-related fatalities, the paper
focuses on the characteristics of the victims and the circumstances of their deaths. In the first case, 71% of the victims
were people aged over 60 (mainly women) who were surprised while they were sleeping and who died in or near their homes. In
the case of the flash flood, the profiles of the victims were diverse as the flooding occurred in the afternoon and many people
were trapped in open. The paper also highlights the factors that explain mortality. Physical factors such as water depth were
determined. The relationship between the water depth and the age of victims was found to be relevant. In the case of the storm
surge Xynthia, the correlation between age and water depth is positive. For the flash flood, the correlation between age and
water depth was negative as male adults died in open. In the first case, the vulnerability of people was closely linked to
human features such as type of housing and age that people are not directly responsible for. During flash floods, dangerous
behaviour by people highlighted the role of risk-taking in loss of life. We also examined the particular case of the deaths
of the children. The paper concludes by discussing the factors of vulnerability on frail population such as elderly people
or marginalized. The lack of risk awareness and crisis preparedness were clearly a major factor of vulnerability. 相似文献