NOAA AR 8210: EVOLUTION AND FLARES FROM MULTIBAND DIAGNOSTICS

NOAA 8210 has been a region showing a remarkable level of activity well before solar maximum. Dominated by a large, rapidly rotating spot, it produced several intense flares during its disk passage at the end of April–beginning of May 1998. We examine the development of AR 8210 in H and white light (WL) and study the evolution of its complex magnetic topology. While the other principal flares are briefly reviewed, the great X1.1/3B flare of 2 May, which was observed at Kanzelhöhe Solar Observatory during a SOHO/UVCS ground support campaign, is studied in detail. This event has been documented in full-disk H and Na-D intensitygrams, Dopplergrams, and magnetograms, with a time cadence of one minute each. The flare was associated with a CME and produced significant geomagnetic effects. Furthermore, we point out the perspectives for our planned Flare Monitoring and Alerting System, since the two new instruments (Magneto-Optical Filter and Digital H camera), which made their first operational run with the campaign, are crucial components for this program.     

Thermoelastic and structural properties of forsterite as a function of P and T: a computer simulation study, by semi-classical potentials

A theoretical study of some thermoelastic and structural properties of forsterite, at non-ambient conditions, is presented in this work. A site-dependent potential (NP) has been developed, and successfully used to carry out theoretical investigations on the lattice parameters, specific heat and tensorial elastic properties of forsterite, at P and T conditions representative of the actual thermobaric environment of the upper mantle of the Earth. Calculations have been performed in the framework of lattice dynamics and of quasi-harmonic approximation. The results, from high temperature or high pressure simulations, have been compared with experimental data. Calculations at high pressure and high temperature have provided unprecedented data, which are not easy to achieve experimentally. Received: October 10 1997 / Revised, accepted: January 26 1998     

Volcanic ash detection by GPS signal

We investigate the ability of GPS to detect volcanic plumes at Mt. Etna, Italy. We use a robust statistical approach to highlight whether the presence of a volcanic plume in the atmosphere may really affect the GPS undifferenced post-fit phase residuals. The proposed method has been tested for the September 4–5, 2007 activity of Mt. Etna. This eruption produced powerful lava fountains forming a weak, a few kilometers high plume for several hours, representing typical activity at Etna over the last 5 years. We analyzed data from nineteen Etna permanent GPS stations located on the volcano flanks at different heights and applied a statistical test based on four main steps: (a) realization of a simplified model representing the volcanic plume in atmosphere; (b) evaluation of the GPS satellite and station couples intersecting the plume; (c) calculation of the volcanic plume region crossed by the GPS signal; (d) application of a robust statistical test in order to see whether the volcanic plume affected the GPS signals. Results show that during the September 4–5, 2007 explosive activity, the GPS residuals definitely include the contribution of the volcanic plume. Our analysis shows that values of the GPS residuals are ten times smaller than those found for the Miyakejima eruption (Japan), highlighting a likely relationship between residuals and eruption intensity. In the future, data derived from the GPS stations located on Etna’s flanks could be used to improve the alerting system of volcanic ash, already operating at the Istituto Nazionale di Geofisica e Vulcanologia, Osservatorio Etneo.     

A focal plane detector design for a wide-band Laue-lens telescope

The energy range above 60 keV is important for the study of many open problems in high energy astrophysics such as the role of Inverse Compton with respect to synchrotron or thermal processes in GRBs, non thermal mechanisms in SNR, the study of the high energy cut-offs in AGN spectra, and the detection of nuclear and annihilation lines. Recently the development of high energy Laue lenses with broad energy bandpasses from 60 to 600keV have been proposed for a Hard X ray focusing Telescope (HAXTEL) in order to study the X-ray continuum of celestial sources. The required focal plane detector should have high detection efficiency over the entire operative range, a spatial resolution of about 1mm, an energy resolution of a few keV at 500keV and a sensitivity to linear polarization. We describe a possible configuration of the focal plane detector based on several CdTe/CZT pixelated layers stacked together to achieve the required detection efficiency at high energy. Each layer can operate both as a separate position sensitive detector and polarimeter or work with other layers to increase the overall photopeak efficiency. Each layer has a hexagonal shape in order to minimize the detector surface required to cover the lens field of view. The pixels would have the same geometry so as to provide the best coupling with the lens point spread function and to increase the symmetry for polarimetric studies.     

Experimental Evidences of the Effect of Fibres in Reinforcing a Sandy Gravel

Studies regarding fibre-reinforcement are restricted to clays, silty and sandy soils. No information is available on gravels. It is worth checking the effect of randomly oriented discrete fibres also on such soils to see if they can be beneficial and to get a better insight into the grain to soil interaction mechanism. In this paper, the effect of a small amount of fibres having high aspect ratio on a sandy gravel was analysed by means of tests carried out in a large triaxial apparatus. Specimens of both natural and fibre-reinforced sandy gravel were prepared by wet tamping at different relative densities, and were tested along monotonic and cyclic stress paths. The results show that the addition of a small amount of fibres causes a slight increase in peak strength and a larger increase in ultimate strength at small confining stress, with an overall more ductile behaviour. The cyclic tests at small confining stress and intermediate strain levels show that, for the lowest applied strain (of the order of 10⁻²%), the stiffness was larger for the reinforced specimens, with a much sharper decrease at larger strains and final values similar for the reinforced and non-reinforced materials.     

Cosmological Parameters from Cosmic Microwave Background Anisotropies: Status and Prospects

A survey is made of the present constraints on cosmological parameters from Cosmic Microwave Background Anisotropies. This revised version was published online in July 2006 with corrections to the Cover Date.     

Ephemeral rollover points and clinothem evolution in the modern Po Delta based on repeated bathymetric surveys

Reconstructions of ancient delta systems rely typically on a two-dimensional (2D) view of prograding clinothems but may miss their three-dimensional (3D) stratigraphic complexity which can, instead, be best documented on modern delta systems by integrating high-resolution geophysical data, historical cartography, core data and geomorphological reconstructions offshore. We quantitatively compare three precisely positioned, high-resolution multi-beam bathymetry maps in the delta front and pro delta sectors (0.3 to 10 m water depth) of Po di Pila, the most active of the modern Po Delta five branches. By investigating the detailed morphology of the prograding modern Po Delta, we shed new light on the mechanisms that control the topset to foreset transition in clinothems and show the temporal and spatial complexity of a delta and its pro delta slope, under the impact of oceanographic processes. This study documents the ephemeral nature of the rollover point at the transition between sandy topset (fluvial, delta plain to mouth-bar) and muddy seaward-dipping foreset deposits advancing, in this case, in >20 m of water depth. Three multibeam surveys, acquired between 2013 and 2016, document the complexity in space and time of the topset and foreset regions and their related morphology, a diagnostic feature that could not be appreciated using solely 2D, even very high-resolution, seismic profiles. In addition, the comparison of bathymetric surveys gathered with one-year lapses shows the migration of subaqueous sand dunes on the clinothem topset, the formation of ephemeral cut-and-fill features at the rollover point (few m below mean sea level), the presence of collapse depressions derived by sagging of sediments and fluid expulsion (possibly induced by storm waves) on the foreset, and splays of sand likely reflecting gravity flows on the lower foreset. Though the modern Po Delta is anthropogenic in many respects, its subaqueous clinothem can be studied as a scale model for ancient clinothems that are less resolved geometrically and far less constrained chronologically.     

Benthic fluxes of mercury species in a lagoon environment (Grado Lagoon,Northern Adriatic Sea,Italy)

The role of the major biogeochemical processes in Hg cycling at the sediment–water interface was investigated in the Grado Lagoon (Northern Adriatic Sea). This wetland system has been extensively contaminated from the Idrija Hg Mine (Slovenia) through the Isonzo River suspended load carried by tidal fluxes. Three approaches were used to study the sediment–water exchange of total Hg (THg), methylmercury (MeHg), reactive Hg (RHg) and dissolved gaseous Hg (DGHg): (1) estimation of diffusive fluxes from porewater and overlying water concentrations, (2) measurements of benthic fluxes using a deployed light benthic chamber in situ and (3) measurements of benthic fluxes during oxic–anoxic transition with a laboratory incubation experiment. The THg solid phase, ranging between 9.5 and 14.4 μg g⁻¹, showed slight variability with depth and time. Conversely, MeHg contents were highest (up to 21.9 ng g⁻¹) at the surface; they tended to decrease to nearly zero concentration with depth, thus suggesting that MeHg production and accumulation occur predominantly just below the sediment–water interface. Porewater MeHg concentrations (0.9–7.9 ng L⁻¹, 0.15–15% of THg) varied seasonally; higher contents were observed in the warmer period. The MeHg diffusive fluxes (up to 17 ng m⁻² day⁻¹) were similar to those in the nearby Gulf of Trieste [Covelli, S., Horvat, M., Faganeli, J., Brambati, A., 1999. Porewater distribution and benthic flux of mercury and methylmercury in the Gulf of Trieste (Northern Adriatic Sea). Estuar. Coast. Shelf Sci. 48, 415–428], although the lagoon sediments contained four-fold higher THg concentrations. Conversely, the THg diffusive fluxes in the lagoon (up to 110 ng m⁻² day⁻¹) were one- to two-fold higher than those previously estimated for the Gulf of Trieste. The diurnal MeHg benthic fluxes were highest in summer at both sites (41,000 and 33,000 ng m⁻² day⁻¹ at the fishfarm and in the open lagoon, respectively), thus indicating the influence of temperature on microbial processes. The diurnal variations of dissolved THg and especially MeHg were positively correlated with O₂ and inversely with DIC, suggesting an important influence of benthic photosynthetic activities on lagoon benthic Hg cycling, possibly through the production of organic matter promptly available for methylation. The results from the dark chamber incubated in the laboratory showed that the regeneration of dissolved THg was slightly affected by the oxic–anoxic transition. Conversely, the benthic flux of MeHg was up to 15-fold higher in sediments overlain by O₂ depleted waters. In the anoxic phase, the MeHg fluxes proceeded in parallel with Fe fluxes and the methylated form reached approximately 100% of dissolved THg. The MeHg is mostly released into overlying water (mean recycling efficiency of 89%) until the occurrence of sulphide inhibition, due to scavenging of the available Hg substrate for methylation. The results suggest that sediments in the Grado Lagoon, especially during anoxic events, should be considered as a primary source of MeHg for the water column.