VLF, magnetic bay and Pi2 substorm signatures at auroral and midlatitude ground stations

A superposed epoch analysis of 100–300 substorms is performed to determine the median size and shape of the substorm-associated VLF chorus, magnetic bay, and Pi2 pulsation burst observed at the near-auroral Halley research station, Antarctica, and at the midlatitude Faraday station at three different local times (2230, 2330, 0130 MLT). The spatial and temporal properties of the magnetic bay signatures are compared with the University of York implementation of the Kisabeth–Rostoker substorm current wedge (SCW) model and the Weimer pulse model, respectively. These constitute the best analytical models of the substorm to date. It is shown that the polarities and relative amplitudes of the observed magnetic bays in the H, D, and Z components at Halley at midnight MLT and at Faraday in the premidnight sector are consistent with the York model for a SCW 3 hours wide in MLT with its westward electrojet at 67°S magnetic latitude. In particular the little-discussed Z component of the bay agrees with the model and is shown to be the clearest substorm signature of the three components, especially at midlatitude. The midnight and postmidnight bays are similar to the premidnight case but progressively smaller and cannot be fully reconciled with the model. The shape of the H and Z bays at Halley and the D bays at Faraday fit a normalized Weimer pulse well, with Weimer's 2 h−1 recovery rate, but the other components do not. The D component at Halley and H at Faraday do fit the Weimer pulse shape but with a faster recovery rate of 4 h−1. It is proposed that this is due to the effect of a decaying current in the SCW combining with the geometrical effect of changing SCW configuration and position relative to the observing station. The Z component at Faraday recovers more slowly than the 2 h−1 Weimer prediction; we cannot explain this. Secondary bays at Halley and Faraday show a clear tendency to recur after 2 hours. Inflection points just prior to onset at Halley and Faraday are argued to be related to reduced convection associated with northward turning of the IMF. The median substorm signature at Halley in the Pi2 frequency band (7–25 mHz) is well correlated with the bay structure, showing that it is part of a broader band, possibly turbulent, spectrum in the substorm-dependent DP2 current. There is evidence of a minor additional narrow band component occurring at substorm onset. This is the dominant signal at Faraday which shows the classic midlatitude substorm signature, a short Pi2 pulsation burst at onset, that decreases progressively in intensity with increasing local time, implying a source region biased to the evening side or else preferred propagation to the east from a near-midnight source

Influence of mollusk species on marine DELTA R determinations

Radiocarbon ages were measured on replicate samples of burnt grain and 5 mollusk species collected from a single sealed layer at an archaeological site (Hornish Point) on the west coast of South Uist, Scotland. The aim was to examine the impact of using different mollusk species on ΔR determinations that are calculated using the paired terrestrial/marine sample approach. The mollusk species examined inhabit a range of environments and utilize a variety of food sources within the intertidal zone. Several authors have suggested that these factors may be responsible for observed variations in the 14C activity of mollusk shells that were contemporaneous in a single location. This study found no significant variation in the <sup>14</sup>C ages of the mollusk species, and consequently, no significant variation in calculated values of ΔR. The implication is that in an area where there are no carboniferous rocks or significant local inputs of freshwater to the surface ocean, any of a range of marine mollusk species can be used in combination with short-lived terrestrial material from the same secure archaeological context to accurately determine a ΔR value for a particular geographic location and period in time

Energetics, bonding mechanism and electronic structure of metal/ceramic interfaces

Progress are reported on: electronic structure of PdO, PtO, and AgO (band structure calculations); ab initio calculations of electronic structure of TiO{sub 2}(110) surface; and electronic structure of VO{sub 2} and TiO{sub 2} thin films and multilayers. (DLC

Energetics, bonding mechanism and electronic structure of metal/ceramic interfaces

Among the tremendous variety of oxide materials currently used for technological applications, those based on titanium and vanadium oxides are not only among the most important, but are also the most intriguing. Relatively small changes in crystallographic modifications and valencies of metal atoms, as the wide class of Magnelli phases and related compounds, may result in abrupt metal-insulator transitions and sharp changes of magnetic ordering with critical temperatures that may vary in the hundreds of degrees range. When doped by other transition, or s and p metals, these oxides exhibit a vast variety of complicated cooperative phenomena,starting from unusual ferroelectricity up to superconductivity at moderately high temperatures. Most of these unusual materials have been extensively studied and used in the bulk or polycrystalline thin film form. For the fabrication of novel devices of micron or submicron sizes, however, single crystal films and superlattices are preferred or required. Moreover, one may expect that such man-made'' artificial structures will reveal new and unusual physical properties. These are the reasons for the Renaissance in experimental and theoretical investigations of these oxides that is now occurring

Arbuscular mycorrhiza status of gold and uranium tailings and surrounding soils of South Africa's deep level gold mines. II. Infectivity

AbstractAn AMF infectivity study and spore viability assessment was performed on substrata obtained from gold and uranium mine tailings dumps (‘slimes dams’) in the North West and Free State provinces of South Africa. Three slimes dams in each region were categorized as recently vegetated (RV), old vegetated (OV) and never vegetated (NV), and dams then divided into five zones based on elevation above ground level, steepness and broad chemical differences. Rhizosphere samples were collected from two of three plant species common to all sites; Eragrostis curvula, Atriplex semibaccata and Cynodon dactylon, as well as from bare areas, in order to allow comparisons across all site categories because of the rarity of the grasses on the lower slope of NV dams. Infectivity was determined by the mean infection percentage method from a bioassay of the substrata using Eragrostis curvula cv Ermelo as a host plant. There was no difference in total infectivity between North West and Free State substrata, but within regions, there were differences in infectivity between rehabilitation ages, between zones, and between rhizosphere and bare areas. Toepaddock substrata and veld soil produced the highest total infection levels overall. On both dams and veld, total arbuscular levels differed between rhizosphere and bare substrata, and the percentage of arbuscules (max. 15.4%) and vesicles (max. 22.0%) as a proportion of total infection structures was low. A low correlation between infectivity and total spore numbers was also found. Spore numbers and the numbers of viable spores increased with zone down the slimes dams to the veld, and also differed within zones between rhizosphere and bare substrata with marked interactive effects. Substratum organic matter (SOM) levels differed between regions, and between zones within the North West region increasing with distance down the slopes to the veld, and were strongly correlated with total spore numbers. Substratum pH values and most AMF parameters were positively correlated in the order of RV>OV>NV dams, indicating that natural colonization of acidic NV sites by AMF is at very low rates, and that AMF colonizing RV slopes are not surviving the transition from RV to OV, with the associated increase in acidity, conductivity and decline in plant cover. Substratum conductivity differed between zones in both regions, with minor interaction between region and zone, and was negatively correlated with pH, AMF infectivity and total spore numbers. Our findings demonstrate that the ameliorant effects of liming and irrigation on substratum pH and conductivity are short-lived, but despite the physico-chemical constraints, a significant measurable AMF inoculum potential does exist on all substrata. Amelioration of tailings with organic matter and use of acid and salt-tolerant AMF would be expected to contribute to more persistent AMF communities and vegetation on gold and uranium slimes dams

The Approach to Ergodicity in Monte Carlo Simulations

The approach to the ergodic limit in Monte Carlo simulations is studied using both analytic and numerical methods. With the help of a stochastic model, a metric is defined that enables the examination of a simulation in both the ergodic and non-ergodic regimes. In the non-ergodic regime, the model implies how the simulation is expected to approach ergodic behavior analytically, and the analytically inferred decay law of the metric allows the monitoring of the onset of ergodic behavior. The metric is related to previously defined measures developed for molecular dynamics simulations, and the metric enables the comparison of the relative efficiencies of different Monte Carlo schemes. Applications to Lennard-Jones 13-particle clusters are shown to match the model for Metropolis, J-walking and parallel tempering based approaches. The relative efficiencies of these three Monte Carlo approaches are compared, and the decay law is shown to be useful in determining needed high temperature parameters in parallel tempering and J-walking studies of atomic clusters.Comment: 17 Pages, 7 Figure

Uniform algebras and approximation on manifolds

Let $\Omega \subset \mathbb{C}^n$ be a bounded domain and let $\mathcal{A} \subset \mathcal{C}(\bar{\Omega})$ be a uniform algebra generated by a set $F$ of holomorphic and pluriharmonic functions. Under natural assumptions on $\Omega$ and $F$ we show that the only obstruction to $\mathcal{A} = \mathcal{C}(\bar{\Omega})$ is that there is a holomorphic disk $D \subset \bar{\Omega}$ such that all functions in $F$ are holomorphic on $D$, i.e., the only obstruction is the obvious one. This generalizes work by A. Izzo. We also have a generalization of Wermer's maximality theorem to the (distinguished boundary of the) bidisk

Impact ionization in GaAs: A screened exchange density-functional approach

Results are presented of a fully ab initio calculation of impact ionization rates in GaAs within the density functional theory framework, using a screened-exchange formalism and the highly precise all-electron full-potential linearized augmented plane wave method. The calculated impact ionization rates show a marked orientation dependence in k space, indicating the strong restrictions imposed by the conservation of energy and momentum. This anisotropy diminishes as the impacting electron energy increases. A Keldysh type fit performed on the energy-dependent rate shows a rather soft edge and a threshold energy greater than the direct band gap. The consistency with available Monte Carlo and empirical pseudopotential calculations shows the reliability of our approach and paves the way to ab initio calculations of pair production rates in new and more complex materials