Launching mechanisms of astrophysical jets

The combination of accretion disks and supersonic jets is used to model many active astrophysical objects, viz., young stars, relativistic stars, and active galactic nuclei. However, existing theories on the physical processes by which these structures transfer angular momentum and energy from disks to jets through viscous or magnetic torques are still relatively approximate. Global stationary solutions do not permit understanding the formation and stability of these structures; and global numerical simulations that include both the disk and jet physics are often limited to relatively short time scales and astrophysically out-of-range values of viscosity and resistivity parameters that are instead crucial to defining the coupling of the inflow/outflow dynamics. Along these lines we discuss self-consistent time-dependent simulations of the launching of supersonic jets by magnetized accretion disks, using high resolution numerical techniques. We shall concentrate on the effects of the disk physical parameters, and discuss under which conditions steady state solutions of the type proposed in the self-similar models of Blandford and Payne can be reached and maintained in a self-consistent nonlinear stationary state.     

Resistive instabilities in coronal conditions

Resistive instabilities in a context referring to the solar corona are rigorously investigated. Various equilibrium configurations are considered, differing, among other things, by their behaviour with respect to fast, ideal instabilities. The computations presented cover in a unified scheme all known regimes of resistive modes and allow one to determine the fastest timescale over which resistivity can play a role. Comparisons with previous work as well as possible extensions are presented.     

A tensor-tensor theory of gravitation

One approach to a tensor-tensor theory of gravitation is proposed as an attempt to represent Mach's principle in a consistent way. In this theory the geometrodynamic properties of General Relativity are still valid, but not necessarily its field equations. Einstein's equations may be obtained as a special case.Most of this work was developed during the author's stay in the Centro de Investigación y de Estudios Avanzados del Instituto Politécnico Nacional, México 14, D. F.     

Distribution of macrobenthic assemblages along a marine gradient in Mediterranean eutrophic coastal lagoons

Coastal lagoons can be characterised by strong environmental changes along the outer marine‐inner brackish gradient, which are reflected in the patterns of distribution of organisms and composition of assemblages. Close to the sea the assemblages are dominated by marine animals, while in the innermost confined areas, there would be estuarine taxa and organisms typical of organically‐enriched areas. During the last decades, human impact has been severe in these habitats and has often profoundly altered the distribution of animals and plants along the gradient. In this study we have analysed the distribution of macrofauna along the outer marine‐inner brackish gradient in five Italian lagoons, affected by severe human disturbance on the Adriatic, Tyrrhenian and Sardinian coasts. We aimed to measure changes in the composition of assemblages according to the distance from the sea, consistent among lagoons and time, after the occurrence of eutrophication, repeated distrophic crisis and other human interventions. We considered existing data from campaigns done during 1994, 1995 and 1999. We arranged different datasets according to the available data, in order to measure the spatial distribution of macrofauna genera with distance from the sea and through time. Analyses showed differences among assemblages related to the gradient. Nevertheless, the species characterizing the assemblages were all typical of organically‐enriched areas, which testified to an impoverishment of diversity and functioning of all these systems.     

Assessment of toxicological status of a SW Mediterranean segment population of striped dolphin (Stenella coeruleoalba) using skin biopsy

Various studies have revealed high concentrations of contaminants such as organochlorines (OCs) and heavy metals in Mediterranean cetaceans. A geographical trend of contamination (PCBs and DDTs) has been found for striped dolphin (Stenella coeruleoalba). In this study we used a non-lethal approach (skin biopsy) to investigate bioaccumulation of OCs, including polychlorobiphenyls (PCBs), DDTs, polychlorodibenzo-p-dioxins (PCDDs), polychlorodibenzofurans (PCDFs), trace elements (Hg, Cd, Pb) and CYP1A activity (BPMO) in nine striped dolphins sampled in the Aeolian area (Sicily - Italy) in summer 2002. The arithmetic mean value of BPMO activity in this group was 43.46 AUF/g tissue/h. This value is approximately 3 times and 5 times lower, respectively, than the value found in the Ionian and in the Ligurian groups. Skin biopsies of striped dolphins emerged as a suitable material for assessing the toxicological status of the various Mediterranean groups.     

<Superscript>40</Superscript>Ar/<Superscript>39</Superscript>Ar ages of the<Emphasis Type="Italic"> AD</Emphasis> 79 eruption of Vesuvius,Italy

The Italian volcano, Vesuvius, erupted explosively in AD 79. Sanidine from pumice collected at Casti Amanti in Pompeii and Villa Poppea in Oplontis yielded a weighted-mean ⁴⁰Ar/³⁹Ar age of 1925±66 years in 2004 (1σ uncertainty) from incremental-heating experiments of eight aliquants of sanidine. This is the calendar age of the eruption. Our results together with the work of Renne et al. (1997) and Renne and Min (1998) demonstrate the validity of the ⁴⁰Ar/³⁹Ar method to reconstruct the recent eruptive history of young, active volcanoes.     

Application of the topographic datuming operator to a data setfrom the Eastern Arabian Peninsula

We apply a redatuming methodology, designed to handle rugged topography and the presence of high‐velocity layers near the acquisition surface, to a 2D land seismic data set acquired in Saudi Arabia. This methodology is based on a recently developed prestack operator, which we call the topographic datuming operator (TDO). The TDO, unlike static corrections, allows for the movement of reflections laterally with respect to their true locations, corresponding to the new datum level. Thus, it mitigates mispositioning of events and velocity bias introduced by the assumption of surface consistency and the time‐invariant time shifts brought about by static corrections. Using the shallow velocities estimated from refracted events, the TDO provides a superior continuity of reflections and better focusing than that obtained from conventional static corrections in most parts of the processed 2D line. The computational cost of applying the TDO is only slightly higher than static corrections. The marginal additional computational cost and the possibility of estimating, after TDO redatuming, stacking velocities that are not affected by a spurious positive bias, as in the case of static corrections, are further advantages of the proposed methodology. The likelihood of strong heterogeneities in the most complex part of the line limits the applicability of any approach based upon geometrical optics; however, the TDO produces results that are slightly better than those obtained from static corrections because of its ability to partially collapse diffractions generated in the near surface.     

SPATIAL DISTRIBUTION AND MORPHOLOGICAL FEATURES OF GULLIES IN AN AGROPASTORAL AREA IN SARDINIA, ITALY

1 INTRODUCTION Many studies on soil erosion by water have been carried out in the Mediterranean area, since erosion is considered as one of the most important land degradation processes in these environments (UNEP, 1994). Those studies mainly focused on r…