Three-dimensional electrical conductivity structure beneath Australia from inversion of geomagnetic observatory data: evidence for lateral variations in transition-zone temperature, water content and melt

In this paper, we report the 3-D electrical conductivity distribution beneath the Australian continent in the depth range 410-1600 km, which we have imaged by inverting C-response estimates from a regional network of geomagnetic observatories. The inversion scheme is based on a quasi-Newton optimization method while the forward algorithm relies on an integral-equation approach. To properly account for the ocean effect in responses at coastal observatories we included a high-resolution (1°×1°) fixed thin laterally varying surface conductance layer. As starting model in the inversion we considered a laboratory-based 3-D conductivity model of the region obtained from seismic surface wave data and thermodynamic modelling. This model provides a good fit to observed C-response estimates supporting its choice as initial model. The most striking feature of the obtained 3-D model is a high-conductivity anomaly in the lower part of the mantle transition zone (MTZ; 520-660 km depth) beneath southeastern Australia implying considerable lateral as radial heterogeneity in the conductivity structure. The high-conductivity region appears to be 0.5-1 log units more conductive than previous global and other regionalized 1-D models. Further analysis using laboratory-based conductivity models combined with thermochemical phase equilibrium computations shows that the strong conductivity anomaly implies water contents of around 0.1 wt per cent in the upper part and >0.4 wt per cent in the lower part of the MTZ. This implies a large MTZ water reservoir that likely totals one to three times that which currently resides in the oceans. The amount of water in the lower MTZ appears to exceed the experimentally determined water storage capacity of the main lower MTZ mineral ringwoodite, which, as a result, undergoes dehydration-induced partial melting. Including contributions to conductivity from a thin melt layer (20 km thick) located in the mid-MTZ increases conductivity locally in the melt layer to ∼1 S m−1, that is, about 0.5 log units more conductive than the average surrounding mantle. This provides an adequate explanation for the strong conductivity anomalies observed beneath part of the continent and points to lateral variations in melt in the MT

Discovery of Enhanced Radiative Recombination Continua of He-like Iron and Calcium from IC 443 and Its Implications

We present deep observations of the Galactic supernova remnant IC 443 with the {\it Suzaku X-ray satellite}. We find prominent K-shell lines from iron and nickel, together with a triangle residual at 8--10~keV, which corresponds to the energy of the radiative recombination continuum (RRC) of He-like iron. In addition, the wavy residuals have been seen at $\sim$5.1 and $\sim$5.5~keV. We confirm that the residuals show the first enhanced RRCs of He- and H-like calcium found in supernova remnants. These facts provide robust evidence for the recombining plasma. We reproduce the plasma in the 3.7--10~keV band using a recombining plasma model at the electron temperature 0.65~keV. The recombination parameter $n_{\rm e}t$ ($n_{\rm e}$ is electron density and $t$ is elapsed time after formation of a recombining plasma) and abundances of iron and nickel are strongly correlated, and hence the errors are large. On the other hand, the ratio of nickel to iron relative to the solar abundances is well constrained to 11$^{+4}_{-3}$ (1$\sigma$). A possibility is that the large abundance ratio is a result of an asymmetric explosion of the progenitor star.Comment: 4 pages, 5 figures, published in Ap

X-Ray Spectrum of a Peculiar Supernova Remnant G359.1-0.5

We present the Suzaku results of a supernova remnant (SNR), G359.1-0.5 in the direction of the Galactic center region. From the SNR, we find prominent K-shell lines of highly ionized Si and S ions, together with unusual structures at 2.5-3.0 and 3.1-3.6 keV. No canonical SNR plasma model, in either ionization equilibrium or under-ionization, can explain the structures. The energies and shapes of the structures are similar to those of the radiative transitions of free electrons to the K-shell of He-like Si and S ions (radiative recombination continuum: RRC). The presence of the strong RRC structures indicates that the plasma is in over-ionization. In fact, the observed spectrum is well fitted with an over-ionized plasma model. The best-fit electron temperature of 0.29 keV is far smaller than the ionization temperature of 0.77 keV, which means that G359.1-0.5 is in extreme condition of over-ionization. We report some cautions on the physical parameters, and comment possible origins for the over-ionized plasma.Comment: 7 pages, 5 figures, accepted for publication in PAS

Recombining Plasma and Hard X-ray Filament in the Mixed-Morphology Supernova Remnant W44

We report new features of the typical mixed-morphology (MM) supernova remnant (SNR) W44. In the X-ray spectra obtained with Suzaku, radiative recombination continua (RRCs) of highly ionized atoms are detected for the first time. The spectra are well reproduced by a thermal plasma in a recombining phase. The best-fit parameters suggest that the electron temperature of the shock-heated matters cooled down rapidly from $\sim1$,keV to $\sim 0.5$,keV, possibly due to adiabatic expansion (rarefaction) occurred $\sim20,000$ years ago. We also discover hard X-ray emission which shows an arc-like structure spatially-correlated with a radio continuum filament. The surface brightness distribution shows a clear anti-correlation with $^{12}$CO (J=2-1) emission from a molecular cloud observed with NANTEN2. While the hard X-ray is most likely due to a synchrotron enhancement in the vicinity of the cloud, no current model can quantitatively predict the observed flux.Comment: 10 pages, 5 figures, accepted for publication in PAS

Global 3-D electromagnetic forward modelling: a benchmark study

Global electromagnetic (EM) induction studies have been the focus of increasing attention during the past few years. A primary stimulus for this interest has been increased quality, coverage and variety of the newly available data sets especially from recent low-Earth-orbiting satellite missions. The combination of traditional ground-based data with satellite-borne measurements presents intriguing opportunity to attack the most challenging problem of deep EM studies: the recovery of 3-D variations of electrical conductivity in the Earth's mantle. But the reliable inference of deep-Earth electrical properties depends on the accuracy and efficiency of the underlying forward modelling solutions used to model 3-D electromagnetic induction in a heterogeneous sphere. Several 3-D forward solvers have been proposed over the last decade, which are based on staggered-grid finite difference, integral equation, finite element and spherical harmonic-finite element approaches. However, there has been no systematic intercomparison amongst the solvers. The goal of this paper is to conduct such a study in order to explore the relative merits of the different approaches when confronted with a set of synthetic models designed to probe the numerical accuracy of each. The results of the intercomparison are presented along with performance metrics to help assess the computational costs associated with each solutio