Calculation of NMR parameters for bridging oxygens in H3T—O—T′H3 linkages (T,T′=Al,Si, P), for oxygen in SiH3O−, SiH3OH and SiH3OMg+ and for bridging fluorine in H3SiFSiH 3 +

Variations in the ¹⁷O nuclear quadrupole coupling constant, NQCC, and the ¹⁷O NMR shielding constant, σ^O, are evaluated for bridging oxygens in H₃T-O-T′H₃ linkages (with T, T′=Al, Si, P), and for nonbridging O in SiH₃O^?, SiH₃OH and SiH₃OMg⁺ and the ¹⁹F NMR shielding constant, σ^F, is evaluated for bridging F in H₃SiFSiH ₃ ^? using Hartree-Fock methods with large, flexible Gaussian basis sets. Trends in ¹⁷O NQCC as a function of T and T′ identity agree with experiment but the value for the Si-O-Al case is underestimated, indicative of neglected contributions from charge compensating cations. For H₃SiOSiH₃ the decrease in NQCC over the range from 180° to 140° is substantial but somewhat slower than the variation of -cosSi and σ^O decrease as Si of the nonbridging oxygen of SiH₃O^? compared to the bridging oxygen of H₃SiOSiH₃ but the calculated σ^Si in H₃SiOAlH ₃ ^? is too large and σ^O too small, indicative of important contributions from counter ions. By contrast, σ^O for PH₃OAlH₃ compared to SiH₃OSiH₃ is consistent with experiment. In H₃SiFSiH ₃ ⁺ (a model for bridging F in amorphous Si:H:F) the value of σ^Si is smaller and the NQCC at F is considerably larger than for H₃SiF, suggesting distinctive ²⁹Si and ¹⁹F NMR spectra for this species.     

Earth tides and polar motion

We establish a general theory that describes the rotational motion of a layered, oblate, elastic Earth under the influence of tidal forces when account is taken of the liquid outer core. We obtain a linearized version of the Navier-Stokes equation; within it not only have we retained the Coriolis and centrifugal acceleration terms, but also have included the nutational terms. We also make use of the Euler equation for angular momentum to analytically relate the nutational motion of the rotational axis with the oscillations of the liquid core and obtain a constraint for the nutational amplitude. Consideration of the Poisson equation for density variation completes our analytical model.We primarily discuss the equations of motion for the liquid core and present the solution as the sum of two terms: one being a component of the spheroidal displacement field, the other of the toroidal field. We also formulate the equations valid for the solid mantle when rotational effects are included, and establish the boundary conditions that must hold at the various interfaces in order that a complete integration of the differential system of equations be accomplished.We assume that the outer core consists of an inviscid fluid and ignore the existence of any boundary layer. We do not impose, however, any restriction on the stratification of the fluid. The dynamical coupling between liquid core and solid mantle is represented by a torque which is generated by the forced oscillations within the liquid core; these oscillations are in turn triggered by the diurnal tides.The expected influence of the liquid core/solid mantle boundary on the nutational motion is discussed in view of Poincare's results concerning a liquid core surrounded by a rigid shell. Comparison is finally made of our model with Molodenskii's 1961 theory for a neutral core and the 1976 Shen-Mansinha nutational theory for an unrestricted core.     

Albedo perturbation models: General formalism and applications to LAGEOS

The force due to radiation pressure on a satellite of arbitrary shape is written in a general form within a formalism similar to that used in the theory of radiative transfer in atmospheres. Then the corresponding integrals are evaluated for the simple case of a spherically symmetric satellite, and applied to model the perturbation due to the Earth-reflected radiation flux on LAGEOS. For this purpose, the optical behaviour of the Earth's surface and atmosphere is described as a combination of Lambertian diffusion (continents), partial specular reflection consistent with Fresnel law (oceans) and anisotropic diffusion according to Chandrasekhar's radiative transfer theory (clouds). The in-plane Gauss componentsT andS vs. mean anomaly are computed for a simple orbital geometry and for different models of the Earth's optical properties. A sensitive dependence is found on the assumed cloud distribution, with significant perturbations possibly arising from oceanic specular reflection when the satellite is close to the Earth's shadow boundaries.On leave from Astronomical Institute, Charles University, védská 8, 15000 Prague 5, Czechoslovakia     

Meteorites from the asteroid 6 Hebe

We have numerically integrated the orbits of 18 fictitious fragments ejected from the asteroid 6 Hebe, an S-type object about 200km across which is located very close to theg=g ₆ (orv ₆) secular resonance at a semimajor axis of 2.425AU and a (proper) inclination of 15° .0. A realistic ejection velocity distribution, with most fragments escaping at relative speeds of a few hundredsm/s, has been assumed. In four cases we have found that the resonance pumps up the orbital eccentricity of the fragments to values >0.6, which result into Earth-crossing, within a time span of 1Myr; subsequent close encounters with the Earth cause strongly chaotic orbital evolution. The closest Earth and Mars encounters recorded in our integration occur at miss distances of a few thousandths ofAU, implying collision lifetimes <10⁹ yr. Some other fragments affected by the secular resonance become Mars-crossers but not Earth-crossers over the integration time span. Two bodies are injected into the 3 : 1 mean motion resonance with Jupiter, and also display macroscopically chaotic behaviour leading to Earth-crossing. 6 Hebe is the first asteroid for which a realistic collisional/dynamical evolutionroute to generate meteorites has been fully demonstrated. It may be the parent body of one of the ordinary chondrite classes.     

Shallow landslides in pyroclastic soils: a distributed modelling approach for hazard assessment

An effective assessment of shallow landslide hazard requires spatially distributed modelling of triggering processes. This is possible by using physically based models that allow us to simulate the transient hydrological and geotechnical processes responsible for slope instability. Some simplifications are needed to address the lack of data and the difficulty of calibration over complex terrain at the catchment's scale. We applied two simple hydrological models, coupled with the infinite slope stability analysis, to the May 1998 landslide event in Sarno, Southern Italy. A quasi-dynamic model (Barling et al., 1994) was used to model the contribution to instability of lateral flow by simulating the time-dependent formation of a groundwater table in response to rainfall. A diffusion model [Water Resour. Res. 36 (2000) 1897] was used to model the role of vertical flux by simulating groundwater pressures that develop in response to heavy rainstorms. The quasi-dynamic model overestimated the slope instability over the whole area (more than 16%) but was able to predict correctly slope instability within zero order basins where landslides occurred and developed into large debris flows. The diffusion model simulated correctly the triggering time of more than 70% of landslides within an unstable area amounting to 7.3% of the study area. These results support the hypothesis that both vertical and lateral fluxes were responsible for landslide triggering during the Sarno event, and confirm the utility of such models as tools for hazard planning and land management.     

On theJ-integrals associated with close binary systems: New recursion formulae

The integralsJ _, ^m were introduced by Kopal for the numerical evaluation of the light changes exhibited by eclipsing binaries when both the tidal and rotational distrotions are taken into account.This paper is a sequel to a previous one to appear in this journal and aims at ascertaining some recursion formulae for these integrals to alleviate the computational complexity of the problem.Using a relationship existing between theJ-integrals and the Appell hypergeometric series of the first kind, we have been able to obtain recursion formulae affecting all three parametersm , of these integrals. The present stage of development has also allowed for a complete enumeration of all independent recursion formulae applicable to the case of partial eclipses.Various recursion formulae, given here for the first time, generalize previous results by Kopal which were valid form=0 ory=0.     

A procedure for landslide susceptibility zonation by the conditional analysis method

Numerous methods have been proposed for landslide probability zonation of the landscape by means of a Geographic Information System (GIS). Among the multivariate methods, i.e. those methods which simultaneously take into account all the factors contributing to instability, the Conditional Analysis method applied to a subdivision of the territory into Unique Condition Units is particularly straightforward from a conceptual point of view and particularly suited to the use of a GIS. In fact, working on the principle that future landslides are more likely to occur under those conditions which led to past instability, landslide susceptibility is defined by computing the landslide density in correspondence with different combinations of instability factors. The conceptual simplicity of this method, however, does not necessarily imply that it is simple to implement, especially as it requires rather complex operations and a high number of GIS commands. Moreover, there is the possibility that, in order to achieve satisfactory results, the procedure has to be repeated a few times changing the factors or modifying the class subdivision. To solve this problem, we created a shell program which, by combining the shell commands, the GIS Geographical Research Analysis Support System (GRASS) commands and the gawk language commands, carries out the whole procedure automatically. This makes the construction of a Landslide Susceptibility Map easy and fast for large areas too, and even when a high spatial resolution is adopted, as shown by application of the procedure to the Parma River basin, in the Italian Northern Apennines.     

Hierarchical spatial interaction among the Italian regions: a nonlinear relative dynamics approach

Regional interactions and spillover effects should be considered as important factors in growth analysis of regional economies. Using modified versions of the Dendrinos–Sonis model, this paper analyses the spatial hierarchical system of Italy. The interaction among Italian regions is considered at three different levels of spatial aggregation, the NUTS-1, NUTS-2 and NUTS-3 levels. Compared to more popular spatial econometric approaches, the Dendrinos–Sonis model and its extensions provide greater flexibility in the way interaction between regions is handled but the results strongly depend on the choice of a reference region.

Calibration of the DAMPE Plastic Scintillator Detector and its on-orbit performance

The DArk Matter Particle Explorer(DAMPE) is a space-borne apparatus for detecting the highenergy cosmic-ray-like electrons, γ-rays, protons and heavy ions. The Plastic Scintillator Detector(PSD)is the top-most sub-detector of the DAMPE. The PSD is designed to measure the charge of incident highenergy particles and it also serves as a veto detector for discriminating γ-rays from charged particles. In this paper, a PSD on-orbit calibration procedure is described, which includes the five steps of pedestal, dynode correlation, response to minimum-ionizing particles, light attenuation function and energy reconstruction.A method for reconstructing the charge of incident high energy cosmic-ray particles is introduced. The detection efficiency of each PSD strip is verified to be above 99.5%; the total efficiency of the PSD for charged particles is above 99.99%.