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The phenomenon of MHD wave refraction is useful in interpreting the properties of the magnetic fluctuations in certain parcels
of solar wind. In the physics of MHD wave refraction, variations in the Alfvén speed VAlf play a dominant role. Here, we compile statistics of the 1-min averages of VAlf at the location of the ACE spacecraft during its first 5 years of operation. We find that monthly distributions of VAlf are close to log-normal, with standard deviations σV as small as 0.11 in the logarithm. Variations in the monthly mean VAlf are correlated significantly with sunspot number. We also compile monthly distributions of the plasma β parameter. The distributions
of both VAlf and β are significantly narrower than they would be if the various solar wind parameters were statistically independent.
In the Tp–VAlf plane, we find a zone of avoidance at low VAlf: for VAlf ≤10 – 15 km/s, there are no samples in the 1-min data that are cooler than Tp = 10 000 – 15 000 K. This feature can be understood in the context of MHD wave refraction, although other explanations are
also possible. 相似文献
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R. Vargas E.A. Yépez J.L. Andrade G. Ángeles T. Arredondo A.E. Castellanos J. Delgado-Balbuena J. Garatuza-Payán E. González Del Castillo W. Oechel J.C. Rodríguez A. Sánchez-Azofeifa E. Velasco E.R. Vivoni C. Watts 《Atmósfera》2013,26(3):325-336
Understanding ecosystem processes from a functional point of view is essential to study relationships among climate variability, biogeochemical cycles, and surface-atmosphere interactions. Increasingly during the last decades, the eddy covariance (EC) method has been applied in terrestrial, marine and urban ecosystems to quantify fluxes of greenhouse gases (e.g., CO2, H2O) and energy (e.g., sensible and latent heat). Networks of EC systems have been established in different regions and have provided scientific information that has been used for designing environmental and adaptation policies. In this context, this article outlines the conceptual and technical framework for the establishment of an EC regional network (i.e., MexFlux) to measure the surface-atmosphere exchange of heat and greenhouse gases in Mexico. The goal of the network is to improve our understanding of how climate variability and environmental change influence the dynamics of Mexican ecosystems. First, we discuss the relevance of CO2 and water vapor exchange between terrestrial ecosystems and the atmosphere. Second, we briefly describe the EC basis and present examples of measurements in terrestrial and urban ecosystems of Mexico. Finally, we describe the conceptual and operational goals at short-, medium-, and long-term scales for continuity of the MexFlux network. 相似文献
106.
Joanna K. York George B. McManus Wim J. Kimmerer Anne M. Slaughter Toni R. Ignoffo 《Estuaries and Coasts》2014,37(3):576-588
We investigated trophic relationships involving microzooplankton in the low salinity zone of the San Francisco Estuary (SFE) as part of a larger effort aimed at understanding the dynamics of the food web supporting the endangered delta smelt, Hypomesus transpacificus. We performed 14 cascade experiments in which we manipulated the biomass of a copepod (Limnoithona tetraspina, Pseudodiaptomus forbesi, or Acartiella sinensis) and quantified responses of lower trophic levels including bacterioplankton, phytoplankton, and microzooplankton. Microzooplankton comprised a major food source for copepods; 9 out of 14 experiments showed removal of at least one group of microzooplankton by copepods. In contrast, the impact of copepods on phytoplankton was indirect; increased copepod biomass led to greater growth of phytoplankton in 3 of 14 experiments. Estimated clearance rates on microzooplankton were 4 mL day?1 for L. tetraspina and 2–6 mL day?1 for P. forbesi, whereas A. sinensis consumed mainly copepod nauplii. Complex trophic interactions, including omnivory, among copepods, microzooplankton, and different components of the phytoplankton likely obscured clear trends. The food web of the SFE is probably less efficient than previously thought, providing poor support to higher trophic levels; this inefficient food web is almost certainly implicated in the continuing low abundance of fishes, including the delta smelt that use the low salinity zone of the San Francisco Estuary. 相似文献
107.
Dense Tacheng rockfill material (TRM) exhibits strain softening and dilation during drained triaxial tests, and therefore, an adapted Rowe’s stress–dilatancy equation was proposed for TRM. This equation incorporates an internal state index related to the density and pressure, as well as the coefficient of particle breakage and rotation. The adapted Rowe’s stress–dilatancy equation indicates that the relationship between stress and dilatancy is not constant, but varies with density and pressure. This result is in agreement with TRM test data. A state-dependent model was established for TRM using generalized plasticity theory combined with the adapted Rowe’s stress–dilatancy equation. The model includes twelve constants calibrated using TRM test data from Group A, and this model was used to predict the strain softening and dilatancy behaviors of dense TRM. Furthermore, the model predictions were validated using test data from Group B. In summary, the model accurately represented the stress–strain and dilatancy behaviors of TRM over a wide range of densities and pressures. 相似文献
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Edo Berger 《New Astronomy Reviews》2011,55(1-2):1-22
The study of short-duration gamma-ray bursts (GRBs) experienced a complete revolution in recent years thanks to the discovery of the first afterglows and host galaxies starting in May 2005. These observations demonstrated that short GRBs are cosmological in origin, reside in both star forming and elliptical galaxies, are not associated with supernovae, and span a wide isotropic-equivalent energy range of ~1048–1052 erg. However, a fundamental question remains unanswered: What are the progenitors of short GRBs? The most popular theoretical model invokes the coalescence of compact object binaries with neutron star and/or black hole constituents. However, additional possibilities exist, including magnetars formed through prompt channels (massive star core-collapse) and delayed channels (binary white dwarf mergers, white dwarf accretion-induced collapse), or accretion-induced collapse of neutron stars. In this review I summarize our current knowledge of the galactic and sub-galactic environments of short GRBs, and use these observations to draw inferences about the progenitor population. The most crucial results are: (i) some short GRBs explode in dead elliptical galaxies; (ii) the majority of short GRBs occur in star forming galaxies; (iii) the star forming hosts of short GRBs are distinct from those of long GRBs, and instead appear to be drawn from the general field galaxy population; (iv) the physical offsets of short GRBs relative to their host galaxy centers are significantly larger than for long GRBs; (v) there is tentative evidence for large offsets from short GRBs with optical afterglows and no coincident hosts; (vi) the observed offset distribution is in good agreement with predictions for NS–NS binary mergers; and (vii) short GRBs trace under-luminous locations within their hosts, but appear to be more closely correlated with the rest-frame optical light (old stars) than the UV light (young massive stars). Taken together, these observations suggest that short GRB progenitors belong to an old stellar population with a wide age distribution, and generally track stellar mass. These results are fully consistent with NS–NS binary mergers and rule out a dominant population of prompt magnetars. However, a partial contribution from delayed magnetar formation or accretion-induced collapse is also consistent with the data. 相似文献
110.
《Astroparticle Physics》2012,35(10):615-624
The IceCube neutrino observatory in operation at the South Pole, Antarctica, comprises three distinct components: a large buried array for ultrahigh energy neutrino detection, a surface air shower array, and a new buried component called DeepCore. DeepCore was designed to lower the IceCube neutrino energy threshold by over an order of magnitude, to energies as low as about 10 GeV. DeepCore is situated primarily 2100 m below the surface of the icecap at the South Pole, at the bottom center of the existing IceCube array, and began taking physics data in May 2010. Its location takes advantage of the exceptionally clear ice at those depths and allows it to use the surrounding IceCube detector as a highly efficient active veto against the principal background of downward-going muons produced in cosmic-ray air showers. DeepCore has a module density roughly five times higher than that of the standard IceCube array, and uses photomultiplier tubes with a new photocathode featuring a quantum efficiency about 35% higher than standard IceCube PMTs. Taken together, these features of DeepCore will increase IceCube’s sensitivity to neutrinos from WIMP dark matter annihilations, atmospheric neutrino oscillations, galactic supernova neutrinos, and point sources of neutrinos in the northern and southern skies. In this paper we describe the design and initial performance of DeepCore. 相似文献