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51.
Alessandro Mura Peter Wurz Helmold Schleicher Dominique Delcourt Stefano Orsini Maxim L. Khodachenko 《Icarus》2009,200(1):1-11
In this study we compare the sodium exosphere observations made by Schleicher et al. [Schleicher, H., and 4 colleagues, 2004. Astron. Astrophys. 425, 1119-1124] with the result of a detailed numerical simulation. The observations, made during the transit of Mercury across the solar disk on 7 May 2003, show a maximum of sodium emission near the polar regions, with north prevalence, and the presence of a dawn-dusk asymmetry. We interpret this distribution as the resulting effect of two combined processes: the solar wind proton precipitation causing chemical alteration of the surface, freeing the sodium atoms from their bounds in the crystalline structure on the surface, and the subsequent photon-stimulated and thermal desorption of the sodium atoms. While we find that the velocity distribution of photon desorbed sodium can explain the observed exosphere population, thermal desorption seems to play a minor role only causing a smearing at the locations where Na atoms are released on the dayside. The observed and simulated distributions agree very well with this hypothesis and indicate that the combination of the proposed processes is able to explain the observed features. 相似文献
52.
Serguei S. Komissarov Maxim V. Barkov 《Monthly notices of the Royal Astronomical Society》2009,397(3):1153-1168
The collapse of massive stars may result in the formation of accreting black holes in their interiors. The accreting stellar matter may advect substantial magnetic flux on to the black hole and promote the release of its rotational energy via magnetic stresses (the Blandford–Znajek mechanism). In this paper we explore whether this process can explain the stellar explosions and relativistic jets associated with long gamma-ray bursts. In particular, we show that the Blandford–Znajek mechanism is activated when the rest mass–energy density of matter drops below the energy density of the magnetic field in the near vicinity of the black hole (within its ergosphere). We also discuss whether such a strong magnetic field is in conflict with the rapid rotation of the stellar core required in the collapsar model, and suggest that the conflict can be avoided if the progenitor star is a component of a close binary. In this case the stellar rotation can be sustained via spin-orbital interaction. In an alternative scenario the magnetic field is generated in the accretion disc, but in this case the magnetic flux through the black hole ergosphere is not expected to be sufficiently high to explain the energetics of hypernovae by the BZ mechanism alone. However, this energy deficit can be recovered via the additional power provided by the disc. 相似文献
53.
Miles D. Lamare Michael P. Lesser Mike F. Barker Thomas M. Barry Kate B. Schimanski 《新西兰海洋与淡水研究杂志》2013,47(5):775-793
We examined the response of four species of New Zealand marine algae (Ecklonia radiata, Apophlaea lyallii, Rhodymenia spp., Ulva lactuca) and a sea urchin (Evechinus chloroticus) to spatial variation in ultraviolet radiation (UV‐R) by examining the concentration of UV‐R absorbing compounds known as mycosporine‐like amino acids (MAAs). The purpose was to understand how, and the degree to which, local marine species could potentially respond to any future increases in incident UV‐R in the New Zealand marine environment. The research was undertaken in Doubtful Sound, where we observed a gradient of water column UV‐R transmission along the 40 km length of the fiord. We examined spatial differences in MAAs along the UV‐B gradient in the macrophytes and temporal changes in MAAs in sea urchin gonads. Among the algae, thallus MAA concentrations (nmol mg–1 protein) ranged from 12.5 to 87.8 in E. radiata, from 433.1 to 1446.4 in A. lyallii, 12.7 to 103.4 in Rhodymenia spp., but were not detected in U. lactuca. For E. chloroticus, gonadal MAA concentrations ranged from 83.9 to 224.3 nmol mg–1 protein spatially, and over the year from 1.85 to 14.12 nmol mg–1 dry weight (DW) depending on site and gametogenic cycle. Laboratory manipulations indicated that concentrations of MAAs in E. chloroticus gonads and eggs are influenced by diet. MAA concentration could be correlated with UV‐B intensities in two of the algal species. E. chloroticus MAA concentrations could also be correlated with UV‐B transmission, which we concluded was a reflection of the greater ingestion and accumulation of MAA‐rich macrophytes at those sites where higher ambient UV‐R induced greater MAA concentrations to occur in the algae. Given this, we suggest that one response of marine species to increases in UV‐B would be an increase in the synthesis and/or accumulation of MAAs for photoautotrophs and a dietary accumulation of those MAAs in E. chloroticus, an important herbivore in this system. 相似文献
54.
Textural evolution of synthetic fluid inclusions in quartz during reequilibration,with applications to tectonic reconstruction 总被引:4,自引:0,他引:4
Experimental reequilibration of synthetic 10 wt% NaCl-H2O inclusions in natural quartz reveals that reequilibration textures show distinct differences depending upon the P-T path
followed by the inclusion after formation. These differences combined with other geological information may be used to determine
whether the sample (rock) followed a dominantly isothermal or isobaricP-T path following entrapment. The intensity and style of inclusion reequilibration features is related to the direction and
magnitude of the departure of theP-T path from the original isochore for the inclusion. Thus, fluid inclusion reequilibration textures not only permit inclusionists
to determine whether the rocks followed an isothermal or isobaric retrogradeP-T path, but also the magnitude of departure of this path from one that is isochoric.
Editorial responsibility: J. Touret 相似文献
55.
This paper shows how static failure loads can be used to predict impact failure of granules. A theoretical model is presented that gives the maximum force experienced during impact and equates this to experimentally measured static failure load to define a critical impact velocity for impact failure. A granule will fail if the predicted theoretical maximum force during impact due to the impact velocity is greater than the real force required to produce failure in that particular granule. 相似文献
56.
Maxim G. Ponomarjov 《Astrophysics and Space Science》2000,274(1-2):423-429
In this paper a method is concerned which makes it possible to describe numerically and analytically the most famous structures
in the non-equilibrium ionosphere, such as stratified and yacht sail like structures, flute jets, wakes and clouds. These
problems are of practical interest in space sciences, astrophysics and in turbulence theory, and also of fundamental interest
since they enable one to concentrate on the effects of the ambient electric and magnetic fields. Disturbances of charged particle
flows due to the ambient flow interactions with bodies are simulated with taking into account the ambient magnetic field effect.
The effects of interactions between solid surfaces and the flows was simulated by making use of an original image method.
The flow disturbances were described by the Boltzmann equation. In the case of the ambient homogeneous magnetic field the
Boltzmann equation is solved analytically. The case of diffuse reflection of particles by surface is considered in detail.
The disturbances of charged particle concentration are calculated in 3D space. The contours of constant particle concentration
obtained from numerical simulations illustrate the dynamics of developing stratifications and flute structures in charged
particle jets and feffect. The wakes under the ambient magnetic field effect. The basic goal of this paper is to present the
method and to demonstate its possibility for simulations of turbulence, plasma jets, wakes and clouds inthe ionosphere and
Space when effects of electric and magnetic fields are taken into account.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
57.
Investigation of active faults is crucial for the seismic hazard assessment and, in the case of volcanic belts, it provides a deeper understanding of the interactions between volcanism and tectonic faulting. In this study, we report the results of the first paleoseismological and tephrochronological investigation undertaken on Holocene faulting in Kamchatka's volcanic belts. The studied trenches and additional excavations are located along the axial fault zone of the Eastern Volcanic Front, where the earlier dated tephra layers provide a robust age control of the faulting events. Electron microprobe analysis of glass from 22 tephra samples permitted correlations among the disparate tephra profiles for constructing a summary tephra sequence. The latter, together with published geochronological data, allowed the construction of a Bayesian age model. Detailed examination of the tephra layers deformed by faulting allowed us to reconstruct and date six faulting events with the offsets of 1 to 20 cm indicating paleoearthquakes with magnitudes of Mw < 5.4. Holocene crustal seismicity of the Eastern Volcanic Front manifests temporal clustering rather than a uniform flux of events. However, no correlation between dated seismic events and the largest Holocene eruptions of proximal volcanoes was observed. 相似文献
58.
Polina Vakhrameeva Maxim Portnyagin Vera Ponomareva Peter M. Abbott Tatiana Repkina Anna Novikova Andreas Koutsodendris Jörg Pross 《第四纪科学杂志》2020,35(4):493-504
The generation of reliable age models for palaeoenvironmental and archaeological records in the Eurasian Arctic is often problematic when using conventional dating techniques. Tephrochronology can potentially improve the chronologies of such records and synchronise disparate sedimentary archives. However, to date, systematic tephra studies are lacking for this region. This paper presents the first cryptotephra data from the White Sea region (northwestern Russia) based on a peat core spanning the past ~1800 years. We identify seven geochemical glass populations that derive from six Icelandic volcanoes and correlate four of them to north European tephra isochrons; these include Askja ad 1875, the basaltic component of the ad 877 Landnám tephra, and tephras BTD-15 (c. ad 1750–1650) and SL-2/SB-2 (ad 803–767) from unknown eruptions of Katla and Snæfellsjökull, respectively. The remaining three populations originate from Grímsvötn, Hekla and Katla; however, their attribution to individual eruptions remains ambiguous. These findings highlight the potential to extend the Late Holocene tephrochronological framework of northern Europe to the west Eurasian Arctic. The detection of at least three basaltic tephras in the core suggests that basaltic shards can be transported over larger distances than previously known and that peatlands are well suited to preserve such components. 相似文献
59.
60.
Nina N. Belcheva Maxim V. Zakhartsev Nadezhda V. Dovzhenko Avianna F. Zhukovskaya Victor Ya. Kavun Victor P. Chelomin 《Ocean Science Journal》2011,46(2):85-94
The digestive gland and gills of the mussel Crenomytilus grayanus extracted from three locations — (i) sampled from a clean and (ii) polluted site and (iii) transplanted from the nonpolluted
to polluted site - were analysed for antioxidant enzymes (superoxide dismutase, catalase, glutathione reductase), total oxyradical
scavenging capacity and levels of lipid peroxidation products (malondialdehyde, conjugated dienes and lipofuscin). Perturbation
of redox status was found in both digestive gland and gill tissues of mussels living in the polluted area. As the activities
of superoxide dismutase and catalase were 1.2–3 times higher, the total oxyradical scavenging capacity was lower by 20–35%
and the levels of lipid peroxidation products were 2–7 times higher compared to mussels from the reference site. In transplanted
mussels, the lipid peroxidation process in both tissues was significantly stimulated (the level of conjugated dienes was increased
1.7–2.5-fold; malondialdehyde and lipofuscin contents were increased 3.5–5-fold) and the total oxyradical scavenging capacity
fell by 50–70%. In addition, the transplantation generally resulted in transient and variable responses of antioxidant enzymes
for both tissues. Complex response-behaviour of the antioxidant enzymes strongly points to the necessity of employing a combined
approach that takes into account activities of antioxidant enzymes and the total oxyradical scavenging capacity, as well as
measurement of oxidative damage (e.g., lipid peroxidation) to evaluate the physiological health of molluscs. 相似文献