Rocking vibrations of a rigid embedded foundation in a poroelastic half‐space

This paper presents an analysis of the rocking vibrations of a rigid cylindrical foundation embedded in poroelastic soil. The foundation is subjected to time‐harmonic rocking excitation and is perfectly bonded to the surrounding soil. The soil underlying the foundation base is represented by a homogeneous poroelastic half‐space, whereas the soil along the side of the foundation is modeled as an independent poroelastic stratum composed of a series of infinitesimally thin layers. The behavior of the soil is governed by Biot's poroelastodynamic theory. The contact surface between the foundation base and the poroelastic soil is assumed to be smooth and either fully permeable or impermeable. The dynamic interaction problem is solved by employing a simplified analytical method. Some numerical results for the nondimensional rocking dynamic impedance and nondimensional angular displacement amplitude of the foundation are presented to show the effect of nondimensional frequency of excitation, poroelastic material parameters, hydraulic boundary condition, depth ratio and mass ratio of the foundation. Copyright © 2009 John Wiley & Sons, Ltd.     

Catastrophe of Coronal Magnetic Flux Ropes Caused by Photospheric Motions

Using a 2.5-D, time-dependent ideal MHD model in Cartesian coordinates, we carried out numerical simulations to investigate the equilibrium and evolution properties of a magnetic configuration that consists of a coronal magnetic flux rope and a partly open photospheric background field, which is equivalent to that produced by a two-patch magnetic source on the photospheric surface. The axial and annular magnetic fluxes of the flux rope are given and fixed. The global magnetic configuration evolves in response to three types of changes of the background field: decreasing of the distance between the two sources, shrinking of the size of each source, and increasing of the shear in the closed component of the background field. As a result, the geometrical parameters of the flux rope, i.e. the height of the rope axis, the half-width of the rope and the length of the vertical current sheet below the rope, change due to the variation of the background field. It is shown that for a given coronal magnetic flux rope in a partly open background field, the variation of the geometrical parameters of the flux rope displays a catastrophic behavior, namely, there exists a critical point for each case, at which an infinitesimal change of the background field leads to a loss of equilibrium, and thus a jump of the flux rope. The implication of such a catastrophe in solar active phenomena is briefly discussed.     

Consolidation of clays subjected to three-dimensional embankment loadings

A Fourier transform technique is presented which may be used to analyse three-dimensional consolidation problems for soils which are loaded by embankment shaped loadings that are rectangular in plan (or of arbitrary shape which can be divided into rectangles in plan). The introduction of a Fourier transform effectively reduces the three-dimensional problem to one involving only two spatial dimensions and this reduces computer storage and saves computing and data preparation time. The material properties of the soil may vary arbitrarily in the plane which is perpendicular to the axis associated with the variable eliminated during the Fourier transform. Solutions obtained using the method are compared with existing solutions in order to verify the theory and assess accuracy, and illustrative examples are given for problems involving the consolidation of soils under embankment loadings.     

Under the shadow of the Magellanic Bridge: A measurement of the extragalactic background at ∼ 0.7 keV

We measured the extragalactic 0.7 keV X-ray background by observing the X-ray shadow of a neutral gas cloud in the Magellanic Bridge region. Two ROSAT PSPC observations of total 104 ks were complemented by a detailed H I mapping of the cloud with both the Parkes 64 m telescope and the Australia Telescope Compact Array. From the detected anti-correlation between the observed background intensity and the H I column density of the cloud, we derived the unabsorbed extragalactic background intensity as ∼ 28 keV s⁻¹ cm⁻² keV⁻¹ sr⁻¹ at ∼ 0.7 keV. The 95% confidence lower limit 18 keV s⁻¹ cm⁻² keV⁻¹ sr⁻¹ is greater than the expected point-like source contribution ? 14 keV s⁻¹ cm⁻² keV⁻¹ sr⁻¹, constrained by the mean source spectrum together with the total background intensity in the 1-2 keV band. A significant fraction of the 0.7 keV background likely arises in a diffuse hot intergalactic medium of a few million degrees, as has been predicted in hydrodynamic simulations of cosmological structure formation.Richard McCray     

A Two-Dimensional Alfvén-Wave-Driven Solar Wind Model

This paper presents a two-dimensional, Alfvén-wave-driven solar wind model, in which the wave energy is assumed to cascade from the low-frequency Alfvén waves to high-frequency ion cyclotron waves and to be transferred to the solar wind protons by cyclotron resonance at the Kolmogorov rate. A typical structure in the meridional plane consisting of a coronal streamer near the Sun, a fast wind in high latitudes, and a slow wind across the heliospheric current sheet, is found. The fast wind obtained in the polar region is essentially similar to that derived by previous one-dimensional flow-tube models, and its density profile in the vicinity of the Sun roughly matches relevant observations. The proton conditions at 1 AU are also consistent with observations for both the fast and slow winds. The Alfvén waves appear in the fast- and slow-wind regions simultaneously and have comparable amplitudes, which agrees with Helios observations. The acceleration and heating of the solar wind by the Alfvén waves are found to occur mainly in the near-Sun region. It is demonstrated in terms of one-dimensional calculations that the distinct properties of the fast and slow winds are mainly attributed to different geometries of the flow tubes associated with the two sorts of winds. In addition, the 2-D and 1-D simulations give essentially the same results for both the fast and the slow winds.     

G359.95-0.04: an energetic pulsar candidate near Sgr A*

We report the discovery of a prominent non-thermal X-ray feature located near the Galactic centre that we identify as an energetic pulsar wind nebula. This feature, G359.95-0.04, lies 1-lyr north of Sgr A* (in projection), is comet like in shape, and has a power-law spectrum that steepens with increasing distance from the putative pulsar. The distinct spectral and spatial X-ray characteristics of the feature are similar to those belonging to the rare class of ram-pressure confined pulsar wind nebulae. The luminosity of the nebula at the distance of Sgr A*, consistent with the inferred X-ray absorptions, is   L_x ∼ 1 × 10³⁴ erg s⁻¹  in the 2–10 keV energy band. The cometary tail extends back to a region centred at the massive stellar complex IRS 13 and surrounded by an enhanced diffuse X-ray emission, which may represent an associated supernova remnant. Furthermore, the inverse Compton scattering of the strong ambient radiation by the nebula consistently explains the observed TeV emission from the Galactic centre. We also briefly discuss plausible connections of G359.95-0.04 to other high-energy sources in the region, such as the young stellar complexes IRS 13 and SNR Sgr A East.     

A simple method using climatic variables to estimate canopy temperature,sensible and latent heat fluxes in a winter wheat field on the North China Plain

Estimation of evapotranspiration from a crop field is of great importance for detecting crop water status and proper irrigation scheduling. The Penman–Monteith equation is widely viewed as the best method to estimate evapotranspiration but it requires canopy resistance, which is very difficult to determine in practice. This paper presents a simple method simplified from the Penman–Monteith equation for estimating canopy temperature (T_c). The proposed method is a biophysically‐sound extended version of that proposed by Todorovic. The estimated canopy temperature is used to calculate sensible heat flux, and then latent heat flux is calculated as the residual of the surface energy balance. An eddy covariance (EC) system and an infrared thermometer (IRT) were installed in an irrigated winter wheat field on the North China Plain in 2004 and 2005, to measure T_c, and sensible and latent heat fluxes were used to test the modified Todorovic model (MTD). The results indicate that the original Todorovic model (TD) severely underestimates T_c and sensible heat flux, and hence severely overestimates the latent heat flux. However, the MTD model has good capability for estimating T_c, and gives acceptable results for latent heat flux at both half‐hourly and daily scales. The MTD model results also agreed well with the evapotranspiration calculated from the measured T_c. Copyright © 2008 John Wiley & Sons, Ltd.     

Wet and Dry Basalt Magma Evolution at Torishima Volcano, Izu Bonin Arc, Japan: the Possible Role of Phengite in the Downgoing Slab

The arc-front volcanoes of Sumisu (31·5°N, 140°E)and Torishima (30·5°N, 140·3°E) in thecentral Izu–Bonin arc are similar in size and rise asrelatively isolated edifices from the seafloor. Together theyprovide valuable along-arc information about magma generationprocesses. The volcanoes have erupted low-K basalts originatingfrom both wet and dry parental basaltic magmas (low-Zr basaltsand high-Zr basalts, respectively). Based on models involvingfluid-immobile incompatible element ratios (La/Sm), the parentalbasalts appear to result from different degrees of partial meltingof the same source mantle (20% and 10% for wet and dry basaltmagmas, respectively). Assuming that the wet basalts containgreater abundances of slab-derived components than their drycounterparts, geochemical comparison of these two basalt typespermits the identification of the specific elements involvedin fluid transport from the subducting slab. Using an extensiveset of new geochemical data from Torishima, where the top ofthe downgoing slab is about 100 km deep, we find that Cs, Pb,and Sr are variably enriched in the low-Zr basalts, which cannotbe accounted for by fractional crystallization or by differencesin the degree of mantle melting. These elements are interpretedto be selectively concentrated in slab-derived metasomatic fluids.Variations in K, high field strength element and rare earthelement concentrations are readily explained by variations inthe degree of melting between the low- and high-Zr basalts;these elements are not contained in the slab-derived fluids.Rb and Ba exhibit variable behaviour in the low-Zr basalts,ranging from immobile, similar to K, to mildly enriched in somelow-Zr basalts. We suggest that the K-rich mica, phengite, playsan important role in determining the composition of fluids releasedfrom the downgoing slab. In arc-front settings, where slab depthis 100 km, phengite is stable, and the fluids released fromthe slab contain little K. In back-arc settings, however, wherethe slab is at 100–140 km depth, phengite is unstable,and K-rich fluids are released. We conclude that cross-arc variationsin the K content of arc basalts are probably related to differingcompositions of released fluids or melts rather than the widelyheld view that such variations are controlled by the degreeof partial melting. KEY WORDS: arc volcano; degrees of melting; mantle wedge; water; wet and dry basalts