The Australia telescope 20 GHz survey: hardware, observing strategy, and scanning survey catalog

The Australia Telescope 20 GHz (AT20G) survey is a large area (2?? sr), sensitive (40 mJy), high frequency (20 GHz) survey of the southern sky. The survey was conducted in two parts: an initial fast scanning survey, and a series of more accurate follow-up observations. The follow-up survey catalog has been presented by Murphy et al. (MNRAS 402:2403, 2010). In this paper we discuss the hardware setup and scanning survey strategy as well as the production of the scanning survey catalog.     

Simulation of screw dislocations in wadsleyite

The structure and energies of the cores of [100] and [001] screw dislocations in wadsleyite (β-Mg₂SiO₄) are calculated using a cluster-based combined elastic-atomistic method and a new parameterized interatomic potential model. For a core radius of 10 Å, core energies are found to be 2.5 and 4.4 eV/Å for the [100] and [001] dislocations, respectively. Both dislocations are associated with significant non-elastic displacement fields extending beyond the core with a radial component toward the dislocation line. The core of the [100] dislocation contains tetrahedrally coordinated magnesium, has a simple 2D structure and is spread parallel to (011) in a manner that suggests high mobility. In contrast, the core of the [001] dislocation has an extended and complex 3D structure involving the formation a large Si₆O₁₉ unit twisted around the dislocation line. This implies that movement of the [001] dislocation will be inhibited by the need to cleave Si–O bonds. These observations, combined with the anomalously low core energy of the [100] dislocation, explain the regular occurrence of [100] dislocations and very rare observation of [001] dislocations in experimentally deformed wadsleyite samples.     

Designing science web sites

From a scientist’s viewpoint a web site is one tool used to conduct research. From an artist’s viewpoint web sites are a form of visual composition. From a developer’s point of view a web site is a type of application. While web sites are a relatively new medium with a particular set of constraints, they do adhere to the same basic design principles that apply to other art forms. These design principles are the basic assumptions that affect the arrangement of elements within a composition. A successful design uses the principles and elements to achieve a visual goal in the composition. A web site designed for scientists has unique properties which are not shared by many other types of web sites. These properties influence the overall visual design of the web sites. Recently at the Institute of Geophysics and Planetary Physics at UCLA undertook a re-design of a number of its websites. In the effort, the use of visual design principles combined with the properties of a science web site were put to the test. In all, six different web sites were designed each with a difference science focus. We describe the process used to design the web sites which involve forming teams of designers, scientists and developers. We present example pages from each design and conclude with a discussion of what was learned during the process.     

Nontornadic convective wind fatalities in the United States

A database was compiled for the period 1977–2007 to assess the threat to life in the conterminous United States from nontornadic convective wind events. This study reveals the number of fatalities from these wind storms, their causes, and their unique spatial distributions. Nontornadic convective wind fatalities occur most frequently outdoors, in vehicles including aircraft, or while boating. Fatalities are most common in the Great Lakes and Northeast, with fewer fatalities observed in the central United States despite the climatological peak in severe thunderstorms in this region. Differences in fatality locations between tornadoes and nontornadic convective wind events highlight the unique combination of physical and social vulnerabilities involved in these deaths. Understanding these vulnerabilities is important to future reduction of nontornadic convective wind fatalities.     

The chemical-temporal evolution of lithospheric mantle underlying the North China Craton

Previous studies of samples of subcontinental lithospheric mantle (SCLM) that underlay the North China Craton (NCC) during the Paleozoic have documented the presence of thick Archean SCLM at this time. In contrast, samples of SCLM underlying the NCC during the Cenozoic are characterized by evidence for melt depletion during the Proterozoic, and relatively recent juvenile additions to the lithosphere. These observations, coupled with geophysical evidence for relatively thin lithosphere at present, have led to the conclusion that the SCLM underlying the NCC was thinned and modified subsequent to the late Paleozoic. In order to extend the view into both the Paleozoic and modern SCLM underlying the NCC, we examine mantle xenoliths and xenocrystic chromites extracted from three Paleozoic kimberlites (Tieling, Fuxian and Mengyin), and mantle xenoliths extracted from one Cenozoic basaltic center (Kuandian). Geochemical data suggest that most of the Kuandian xenoliths are residues of small degrees of partial melting from chemically primitive mantle. Sr-Nd-Hf isotopic analyses indicate that the samples were removed from long-term depleted SCLM that had later been variably enriched in incompatible elements. Osmium isotopic compositions of the two most refractory xenoliths are depleted relative to the modern convecting upper mantle and have model melt depletion ages that indicate melt depletion during Paleoproterozoic. Other relatively depleted xenoliths have Os isotopic compositions consistent with the modern convecting upper mantle. This observation is generally consistent with earlier data for xenoliths from other Cenozoic volcanic systems in the NCC and surrounding cratons. Thus, the present SCLM underlying the NCC has a complex age structure, but does not appear to retain materials with Archean melt depletion ages. Results for what are presumed to be early Paleozoic xenoliths from Teiling are generally highly depleted in melt components, e.g. have low Al₂O₃, but have also been metasomatically altered. Enrichment in light rare earth elements, low ε_Nd values (∼−10), and relatively high ⁸⁷Sr/⁸⁶Sr (0.707-0.710) are consistent with a past episode of metasomatism. Despite the metasomatic event, ¹⁸⁷Os/¹⁸⁸Os ratios are low and consistent with a late Archean melt depletion event. Thus, like results for xenoliths from other early Paleozoic volcanic centers within the NCC, these rocks sample dominantly Archean SCLM. The mechanism for lithospheric thinning is still uncertain. The complex age structure currently underlying the NCC requires either variable melt depletion over the entire history of this SCLM, or the present lithospheric material was partly or wholly extruded under the NCC from elsewhere by the plate collisions (collision with the Yangtze Craton and/or NNW subduction of the Pacific plate) that may have caused the thinning to take place.     

Influence of aerosol-radiative forcings on the diurnal and seasonal cycles of rainfall over West Africa and Eastern Atlantic Ocean using GCM simulations

Effects of aerosol radiative forcing on the diurnal and seasonal cycles of precipitation over West Africa and eastern Atlantic Ocean are investigated for the boreal summer season: June–July–August. An eight year (2000–2007) average of GCM simulated rainfall data is compared with the corresponding TRMM rainfall data. The comparison shows that the amplitude of the diurnal cycles of rainfall over land and ocean are reasonably well simulated. Over land, the phase of the simulated diurnal cycle of precipitation peaks several hours earlier than that of the TRMM data. Corresponding differences over the ocean(s) are relatively smaller. Some of the key features of the aerosol induced model simulated field anomalies are: (a) aerosol direct radiative forcing which increases the atmospheric stability and reduces the daytime moist convection and convective precipitation; (b) the aerosol induced changes in the diurnal cycle of precipitation are out of phase with those of the TRMM data over land, but are in-phase over the ocean; (c) aerosols reduce the amplitude of the diurnal cycle of precipitation over land and enhance it over ocean. However, the phase of the diurnal cycle is not affected much by the aerosol radiative forcing both over land and ocean. During the boreal summer, aerosol radiative forcing and induced circulation and precipitation cool the Sahel and the southern part of Sahara desert more than the adjacent areas to the north and south, thereby shifting the peak meridional temperature gradient northward. Consequently, an anomalous easterly jet is found north of its climatological location. This anomalous jet is associated with increased cyclonic circulation to the south of its axis, resulting in an anomalous monsoon rain belt in the Sahel.     

Chemical and genetic characterization of the ungrouped pallasite Lieksa

The meteorite Lieksa was found in 2017 in Löpönvaara, Finland, and later donated to the Finnish Museum of Natural History. Here, we report siderophile element concentrations, genetic isotopic data, and a metal–silicate segregation age for the meteorite. The ~280 g Lieksa is ~80% metal and ~20% silicate and oxide inclusions by volume, with the inclusions consisting primarily of Fe-rich olivine. Due to Lieksa's silicate content, coupled with a texture characterized by metal enclosing the silicates, it has been classified as a pallasite. Lieksa's olivine and bulk chemical characteristics are distinct from those of the known pallasite and iron meteorite groups, consistent with its classification as ungrouped. The meteorite exhibits a flat, chondrite-normalized highly siderophile element pattern, consistent with an origin as an early crystallization product from a metallic melt with chondritic relative abundances. Molybdenum, Ru, and ¹⁸³W isotopic data indicate that Lieksa formed in the non-carbonaceous (NC) domain of the solar nebula. Radiogenic ¹⁸²W abundances for Lieksa yield a model metal–silicate segregation age of 1.5 ± 0.8 Myr after calcium-aluminum-rich inclusion formation, which is within the range established for other NC-type pallasite and iron meteorite parent bodies.