Inversion of Lloyd Mirror Field for Determining a Source's Track

This paper describes a simple approach for inferring the depth and track of a sound source at short ranges by inversion of acoustic field data at a set of sea bottom hydrophones. At short ranges, the acoustic field consists of a dominant Lloyd mirror (LM) signal from the direct and surface-reflected ray paths and a series of bottom-reflected paths that modulate the LM signal. A computationally efficient propagation model based on the method of images is developed to calculate replica fields for the inversion. The matched field inversion method for inferring the source depth and track is demonstrated using data from an experiment carried out in shallow water off the east coast of Canada. The estimated values were in very good agreement with independent measurements taken during the experiment.     

Modification and testing of a one‐dimensional energy and mass balance model for supraglacial snowpacks

A one‐dimensional energy and mass balance snow model (SNTHERM) has been modified for use with supraglacial snowpacks and applied to a point on Haut Glacier d'Arolla, Switzerland. It has been adapted to incorporate the underlying glacier ice and a site‐specific, empirically derived albedo routine. Model performance was tested against continuous measurements of snow depth and meltwater outflow from the base of the snowpack, and intermittent measurements of surface albedo and snowpack density profiles collected during the 1993 and 2000 melt seasons. Snow and ice ablation was simulated accurately. The timing of the daily pattern of meltwater outflow was well reproduced, although magnitudes were generally underestimated, possibly indicating preferential flow into the snowpack lysimeter. The model was used to assess the quantity of meltwater stored temporally within the unsaturated snowpack and meltwater percolation rates, which were found to be in agreement with dye tracer experiments undertaken on this glacier. As with other energy balance studies on alpine valley glaciers, the energy available for melt was dominated by net radiation (64%), with a sizable contribution from sensible heat flux (36%) and with a negligible latent heat flux overall, although there was more complex temporal variation on diurnal timescales. A basic sensitivity analysis indicated that melt rates were most sensitive to radiation, air temperature and snowpack density, indicating the need to accurately extrapolate/interpolate these variables when developing a spatially distributed framework for this model. Copyright © 2008 John Wiley & Sons, Ltd.     

Time-series Paschen-β spectroscopy of SU Aurigae

We present time-series echelle spectra of the Paβ line of the T Tauri star SU Aur, observed over three consecutive nights. The line shows strong variability (∼10 per cent) over the velocity range 100–420 km s⁻¹ in the red broad absorption component, and weaker variability (∼2 per cent) over the velocity range  −200–0 km s⁻¹  in the blue wing. The variability in the velocity range  −200–0 km s⁻¹  is correlated with that in  200–400 km s⁻¹  , and the variability in these velocity ranges anticorrelates with that in  0–100 km s⁻¹  . The mean spectrum from the second night shows the suggestion of a blueshifted absorption component at about  −150 km s⁻¹  , similar to that found in the Hα and Hβ lines. We find the position of the subpeak in the red absorption component changes steadily with time, and its motion modulates at half the rotational period. We also find that the modulation of the line equivalent width is possibly associated with a half and a third of the rotational period, which is consistent with the surface Doppler images of SU Aur. Radiative transfer models of a rotationally modulated Paβ line, produced in the shock-heated magnetospheric accretion flow, are also presented. Models with a magnetic dipole offset reproduce the overall characteristics of the observed line variability, including the line equivalent width and the motion of the subpeak in the red absorption trough.     

Analyzing Animal Movement Characteristics From Location Data

When individuals of a species utilize an environment, they generate movement patterns at a variety of spatial and temporal scales. Field observations coupled with location technologies (e.g. GPS tags) enable the capture of detailed spatio‐temporal data regarding these movement patterns. These patterns contain information about species‐specific preferences regarding individual decision‐making, locational choices and the characteristics of the habitat in which the animal resides. Spatial Data Mining approaches can be used to extract repeated spatio‐temporal patterns and additional habitat preferences hidden within large spatially explicit movement datasets. We describe a method to determine the periodicity and directionality in movement exhibited by a migratory bird species. Results using a High Arctic‐nesting Svalbard Barnacle Goose movement data yielded undetected patterns that were secondarily corroborated with expert field knowledge. Individual revisits by the geese to specific locations in the breeding and wintering grounds of Svalbard, Norway and Solway, Scotland, occurred with a periodicity of 334 days . Further, the orientation of this movement was detected to be mostly north‐south. During long‐range migration the geese use the north‐south oriented Norwegian islands as “stepping stones”, Short‐range movement between mudbank roosts to feeding fields in Solway also retained a north‐south orientation.     

Kinetic isotopic fractionation during diffusion of ionic species in water

Experiments specifically designed to measure the ratio of the diffusivities of ions dissolved in water were used to determine . The measured ratio of the diffusion coefficients for Li and K in water (D_Li/D_K = 0.6) is in good agreement with published data, providing evidence that the experimental design being used resolves the relative mobility of ions with adequate precision to also be used for determining the fractionation of isotopes by diffusion in water. In the case of Li, we found measurable isotopic fractionation associated with the diffusion of dissolved LiCl (D_⁷Li/D_⁶Li=0.99772±0.00026). This difference in the diffusion coefficient of ⁷Li compared to ⁶Li is significantly less than that reported in an earlier study, a difference we attribute to the fact that in the earlier study Li diffused through a membrane separating the water reservoirs. Our experiments involving Mg diffusing in water found no measurable isotopic fractionation (D_²⁵Mg/D_²⁴Mg=1.00003±0.00006). Cl isotopes were fractionated during diffusion in water (D_³⁷Cl/D_³⁵Cl=0.99857±0.00080) whether or not the co-diffuser (Li or Mg) was isotopically fractionated. The isotopic fractionation associated with the diffusion of ions in water is much smaller than values we found previously for the isotopic fractionation of Li and Ca isotopes by diffusion in molten silicate liquids. A major distinction between water and silicate liquids is that water surrounds dissolved ions with hydration shells, which very likely play an important but still poorly understood role in limiting the isotopic fractionation associated with diffusion.     

Titanium in subduction zone fluids: First insights from abinitio molecular metadynamics simulations

The preferential incorporation of High-Field-Strength Elements (HFSE) in rutile (TiO₂), combined with its supposed stability in subduction zone settings, make it an ideal candidate to explain the low HFSE concentrations in subduction-derived magmas. The solubility behaviour of rutile is key to these arguments, but at present experimental and field-based evidence are contradictory.We have used abinitio molecular (meta)dynamics to investigate the coordination environment of Ti(IV) in pure water at 300 and 1000 K and densities ranging from 900-1260 kg m⁻³ (approximate pressures 0.9-3.6 GPa). In all high temperature simulations, the long-range structure of the solvent indicates a breakdown of the hydrogen bonding network as expected for supercritical water. The five-fold coordination of titanium to water is energetically most favourable in aqueous fluids at room temperature and pressure, separated from four and six-fold configurations by ∼175 and ∼200 kJ mol⁻¹, respectively. The average first shell Ti-O distance is 2.00 Å, in excellent agreement with bond lengths obtained from experiments. At similar densities and 1000 K, titanium is on average six-fold coordinate with water, and shows some degree of water dissociation in the first hydration shell. This coordination environment is remarkably persistent with increasing density from 1021 to 1260 kg m⁻³ at constant temperature (1000 K). At lower densities, however, (900 kg m⁻³ at 1000 K), the coordination with first shell water molecules is less than five. The observed coordination changes could promote association of titanium with peralkaline or peraluminous domains in the aqueous fluid, and thereby explain field-and laboratory based evidence of enhanced HFSE concentrations.This study demonstrates that abinitio molecular dynamics has considerable potential to access details of element behaviour in aqueous fluids at geologically relevant conditions that are impossible to examine otherwise. Changes in the solvent structure due to density variations lead to differences in solvent behaviour allowing access to new domains for fluid-solid interaction. Moreover, changes in the solvent structure are strongly linked to the effectiveness of element solvation.