ORCID Profile
0000-0002-4376-5455
Current Organisation
University of Tasmania
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Publisher: American Geophysical Union (AGU)
Date: 02-2020
DOI: 10.1029/2019JF005354
Abstract: The Southern Ocean (in the region 60–180° E) south of the Indian Ocean, Australia, and the West Pacific is noted for the frequent occurrence and severity of its storms. These storms give rise to high‐ litude secondary microseisms from sources, including the deep ocean regions, and primary microseisms where the swells impinge on submarine topographic features. A better understanding of the varying microseism wavefield enables improvements to seismic imaging and development of proxy observables to complement sparse in situ wave observations and hindcast models of the global ocean wave climate. We analyze 12–26 years of seismic data from 11 seismic stations either on the East Antarctic coast or sited in the Indian Ocean, Australia, and New Zealand. The power spectral density of the seismic wavefield is calculated to explore how the time‐changing microseism intensity varies with (i) sea ice coverage surrounding Antarctica and (ii) the Southern Annular Mode (SAM) climate index. Variations in sea ice extent are found to be the dominant control on the microseism intensity at Antarctic stations, which exhibit a seasonal pattern phase‐shifted by 4–5 months compared to stations in other continents. Peaks in extremal intensity at East Antarctic stations occur in March–April, with the highest peaks for secondary microseisms occurring during negative SAM events. This relationship between microseism intensity and the SAM index is opposite to that observed on the Antarctic Peninsula. This work informs the complexity of microseism litudes in the Southern Hemisphere and assists ongoing interdisciplinary investigations of interannual variability and long‐term trends.
Publisher: Oxford University Press (OUP)
Date: 20-02-2018
DOI: 10.1093/MNRAS/STY433
Publisher: Copernicus GmbH
Date: 25-07-2023
Publisher: Cambridge University Press (CUP)
Date: 2020
DOI: 10.1017/PASA.2019.49
Abstract: We present a detailed analysis of the radio galaxy PKS $2250{-}351$ , a giant of 1.2 Mpc projected size, its host galaxy, and its environment. We use radio data from the Murchison Widefield Array, the upgraded Giant Metre-wavelength Radio Telescope, the Australian Square Kilometre Array Pathfinder, and the Australia Telescope Compact Array to model the jet power and age. Optical and IR data come from the Galaxy And Mass Assembly (GAMA) survey and provide information on the host galaxy and environment. GAMA spectroscopy confirms that PKS $2250{-}351$ lies at $z=0.2115$ in the irregular, and likely unrelaxed, cluster Abell 3936. We find its host is a massive, ‘red and dead’ elliptical galaxy with negligible star formation but with a highly obscured active galactic nucleus dominating the mid-IR emission. Assuming it lies on the local M – $\\sigma$ relation, it has an Eddington accretion rate of $\\lambda_{\\rm EDD}\\sim 0.014$ . We find that the lobe-derived jet power (a time-averaged measure) is an order of magnitude greater than the hotspot-derived jet power (an instantaneous measure). We propose that over the lifetime of the observed radio emission ( ${\\sim} 300\\,$ Myr), the accretion has switched from an inefficient advection-dominated mode to a thin disc efficient mode, consistent with the decrease in jet power. We also suggest that the asymmetric radio morphology is due to its environment, with the host of PKS $2250{-}351$ lying to the west of the densest concentration of galaxies in Abell 3936.
Publisher: American Astronomical Society
Date: 05-06-2015
Publisher: Oxford University Press (OUP)
Date: 12-02-2022
Abstract: We present a method for applying spatially resolved adiabatic and radiative loss processes to synthetic radio emission from hydrodynamic simulations of radio sources from active galactic nuclei (AGNs). Lagrangian tracer particles, each representing an ensemble of electrons, are injected into simulations and the position, grid pressure, and time since the last strong shock are recorded. These quantities are used to track the losses of the electron packet through the radio source in a manner similar to the Radio AGN in Semi-analytic Environments formalism, which uses global source properties to calculate the emissivity of each particle ex situ. Freedom in the choice of observing parameters, including redshift, is provided through the post-processing nature of this approach. We apply this framework to simulations of jets in different environments, including asymmetric ones. We find a strong dependence of radio source properties on frequency and redshift, in good agreement with observations and previous modelling work. There is a strong evolution of radio spectra with redshift due to the more prominent inverse-Compton losses at high redshift. Radio sources in denser environments have flatter spectral indices, suggesting that spectral index asymmetry may be a useful environment tracer. We simulate intermediate Mach number jets that disrupt before reaching the tip of the lobe, and find that these retain an edge-brightened Fanaroff–Riley Type II morphology, with the most prominent emission remaining near the tip of the lobes for all environments and redshifts we study.
Publisher: Springer Science and Business Media LLC
Date: 30-10-2023
Publisher: Oxford University Press (OUP)
Date: 05-10-2016
Publisher: Oxford University Press (OUP)
Date: 18-11-2022
Abstract: We present an analytical model for the evolution of extended active galactic nuclei (AGNs) throughout their full lifecycle, including the initial jet expansion, lobe formation, and eventual remnant phases. A particular focus of our contribution is on the early jet expansion phase, which is traditionally not well captured in analytical models. We implement this model within the Radio AGN in Semi-Analytic Environments (RAiSE) framework, and find that the predicted radio source dynamics are in good agreement with hydrodynamic simulations of both low-powered Fanaroff-Riley Type-I and high-powered Type-II radio lobes. We construct synthetic synchrotron surface brightness images by complementing the original RAiSE model with the magnetic field and shock-acceleration histories of a set of Lagrangian tracer particles taken from an existing hydrodynamic simulation. We show that a single set of particles is sufficient for an accurate description of the dynamics and observable features of Fanaroff-Riley Type-II radio lobes with very different jet parameters and ambient density profile normalizations. Our new model predicts that the lobes of young (≲10 Myr) sources will be both longer and brighter than expected at the same age from existing analytical models, which lack a jet-dominated expansion phase this finding has important implications for interpretation of radio galaxy observations. The RAiSE code, written in python, is publicly available on github and pypi.
Publisher: Oxford University Press (OUP)
Date: 18-05-2022
Abstract: Quantifying the energetics and lifetimes of remnant radio-loud active galactic nuclei (AGNs) is much more challenging than for active sources due to the added complexity of accurately determining the time since the central black hole switched off. Independent spectral modelling of remnant lobes enables the derivation of the remnant ratio, Rrem (i.e. ‘off-time/source age’) however, the requirement of high-frequency (≳5 GHz) coverage makes the application of this technique over large-area radio surveys difficult. In this work, we propose a new method, which relies on the observed brightness of backflow of Fanaroff–Riley type II lobes, combined with the Radio AGN in Semi-Analytic Environments (RAiSE) code, to measure the duration of the remnant phase. Sensitive radio observations of the remnant radio galaxy J2253-34 are obtained to provide a robust comparison of this technique with the canonical spectral analysis and modelling methods. We find that the remnant lifetimes modelled by each method are consistent spectral modelling yields Rrem = 0.23 ± 0.02, compared to Rrem = 0.26 ± 0.02 from our new method. We examine the viability of applying our proposed technique to low-frequency radio surveys using mock radio source populations, and examine whether the technique is sensitive to any intrinsic properties of radio AGNs. Our results show that the technique can be used to robustly classify active and remnant populations, with the most confident predictions for the remnant ratio, and thus off-time, in the longest lived radio sources (& Myr) and those at higher redshifts (z & 0.1).
Publisher: Cambridge University Press (CUP)
Date: 2022
DOI: 10.1017/PASA.2022.4
Abstract: We present Hubble Space Telescope Wide Field Camera 3 photometric and grism observations of the candidate ultra-high-redshift ( $z $ ) radio galaxy, GLEAM J0917–0012. This radio source was selected due to the curvature in its 70–230 MHz, low-frequency Murchison Widefield Array radio spectrum and its faintness in K -band. Follow-up spectroscopic observations of this source with the Jansky Very Large Array and Atacama Large Millimetre Array were inconclusive as to its redshift. Our F105W and F0986M imaging observations detect the host of GLEAM J0917–0012 and a companion galaxy, $\\sim$ one arcsec away. The G102 grism observations reveal a single weak line in each of the spectra of the host and the companion. To help identify these lines we utilised several photometric redshift techniques including template fitting to the grism spectra, fitting the ultraviolet (UV)-to-radio photometry with galaxy templates plus a synchrotron model, fitting of the UV-to-near-infrared photometry with EAZY , and fitting the radio data alone with RAiSERed . For the host of GLEAM J0917–0012 we find a line at $1.12\\,\\mu$ m and the UV-to-radio spectral energy distribution (SED) fitting favours solutions at $z\\sim 2$ or $z\\sim 8$ . While this fitting shows a weak preference for the lower redshift solution, the models from the higher redshift solution are more consistent with the strength of the spectral line. The redshift constraint by RAiSERed of $ .5$ also supports the interpretation that this line could be Lyman $-\\alpha$ at $z=8.21$ however EAZY favours the $z\\sim 2$ solution. We discuss the implications of both solutions. For the companion galaxy we find a line at $0.98\\,\\mu$ m and the SED fitting favours solutions at $z $ implying that the line could be the [OII] $\\lambda3727$ doublet at $z=1.63$ (although the EAZY solution is $z\\sim 2.6\\pm 0.5$ ). Further observations are still required to unambiguously determine the redshift of this intriguing candidate ultra-high-redshift radio galaxy.
Publisher: Oxford University Press (OUP)
Date: 30-04-2020
Abstract: Feedback from radio jets associated with active galactic nuclei (AGNs) plays a profound role in the evolution of galaxies. Kinetic power of these radio jets appears to show temporal variation, but the mechanism(s) responsible for this process are not yet clear. Recently, the LOw Frequency ARray (LOFAR) has uncovered large populations of active, remnant, and restarted radio jet populations. By focusing on LOFAR data in the Lockman Hole, in this work we use the Radio AGNs in Semi-Analytic Environments (RAiSE) dynamical model to present the first self-consistent modelling analysis of active, remnant, and restarted radio source populations. Consistent with other recent work, our models predict that remnant radio lobes fade quickly. Any high (& per cent) observed fraction of remnant and restarted sources therefore requires a dominant population of short-lived jets. We speculate that this could plausibly be provided by feedback-regulated accretion.
Publisher: Oxford University Press (OUP)
Date: 30-05-2020
DOI: 10.1093/GJI/GGAA265
Abstract: Accurate measurement of the local component of geodetic motion at GPS stations presents a challenge due to the need to separate this signal from the tectonic plate rotation. A pressing ex le is the observation of glacial isostatic adjustment (GIA) which constrains the Earth’s response to ice unloading, and hence, contributions of ice-covered regions such as Antarctica to global sea level rise following ice mass loss. While both vertical and horizontal motions are of interest in general, we focus on horizontal GPS velocities which typically contain a large component of plate rotation and a smaller local component primarily relating to GIA. Incomplete separation of these components introduces significant bias into estimates of GIA motion vectors. We present the results of a series of tests based on the motions of GPS stations from East Antarctica: (1) signal separation for sets of synthetic data that replicate the geometric character of non-separable, and separable, GIA-like horizontal velocities and (2) signal separation for real GPS station data with an appraisal of uncertainties. For both synthetic and real motions, we compare results where the stations are unweighted, and where each station is areal-weighted using a metric representing the inverse of the spatial density of neighbouring stations. From the synthetic tests, we show that a GIA-like signal is recoverable from the plate rotation signal providing it has geometric variability across East Antarctica. We also show that areal-weighting has a very significant effect on the ability to recover a GIA-like signal with geometric variability, and hence on separating the plate rotation and local components. For the real data, assuming a rigid Antarctic plate, fitted plate rotation parameters compare well with other studies in the literature. We find that 25 out of 36 GPS stations examined in East Antarctica have non-zero local horizontal velocities, at the 2σ level, after signal separation. We make the code for weighted signal separation available to assist in the consistent appraisal of separated signals, and the comparison of likely uncertainty bounds, for future studies.
Publisher: Cambridge University Press (CUP)
Date: 2022
DOI: 10.1017/PASA.2022.42
Abstract: While unobscured and radio-quiet active galactic nuclei are regularly being found at redshifts $z 6$ , their obscured and radio-loud counterparts remain elusive. We build upon our successful pilot study, presenting a new s le of low-frequency-selected candidate high-redshift radio galaxies (HzRGs) over a sky area 20 times larger. We have refined our selection technique, in which we select sources with curved radio spectra between 72–231 MHz from the GaLactic and Extragalactic All-sky Murchison Widefield Array (GLEAM) survey. In combination with the requirements that our GLEAM-selected HzRG candidates have compact radio morphologies and be undetected in near-infrared $K_{\\rm s}$ -band imaging from the Visible and Infrared Survey Telescope for Astronomy Kilo-degree Infrared Galaxy (VIKING) survey, we find 51 new candidate HzRGs over a sky area of approximately $1200\\ \\mathrm{deg}^2$ . Our s le also includes two sources from the pilot study: the second-most distant radio galaxy currently known, at $z=5.55$ , with another source potentially at $z \\sim 8$ . We present our refined selection technique and analyse the properties of the s le. We model the broadband radio spectra between 74 MHz and 9 GHz by supplementing the GLEAM data with both publicly available data and new observations from the Australia Telescope Compact Array at 5.5 and 9 GHz. In addition, deep $K_{\\rm s}$ -band imaging from the High-Acuity Widefield K -band Imager (HAWK-I) on the Very Large Telescope and from the Southern Herschel Astrophysical Terahertz Large Area Survey Regions $K_{\\rm s}$ -band Survey (SHARKS) is presented for five sources. We discuss the prospects of finding very distant radio galaxies in our s le, potentially within the epoch of reionisation at $z \\gtrsim 6.5$ .
Publisher: Oxford University Press (OUP)
Date: 30-03-2019
DOI: 10.1093/MNRAS/STZ922
Publisher: Copernicus GmbH
Date: 15-05-2023
DOI: 10.5194/EGUSPHERE-EGU23-1524
Abstract: Uncertainty, as applied to geophysical and multivariate initiatives to constrain aspects of Earth-ice interactions for East Antarctica, provides a number of approaches to appraise and interrogate research results.& We discuss a number of use cases: 1) making use of multiple uncertainty metrics 2) making comparisons between spatially variable maps of inferred properties such as geothermal heat flow 3) extrapolating crustal structure given the likelihood of tectonic boundaries and 4) providing research results for interdisciplinary studies in forms that facilitate ensemble approaches.& It proves extremely useful to assess a research finding, such as a mapped geophysical property, through multiple uncertainty metrics (e.g., standard deviation, information entropy, data count).& However, a thoughtful appraisal of multiple metrics could be misleading, i.e., potentially not useful in isolation, in a case where there are significant unquantified uncertainties.& Uncertainties supplied with the mapped geophysical properties can potentially be extended to capture this broader range, but that range in turn could become less helpful as the fine detail in the quantified uncertainty will be lost.& In the case of a property such as geothermal heat flow, indirectly determined for East Antarctica, insights can be drawn by subtracting a forward model map from an empirically determined result (e.g. Aq1) to yield the non-steady state components excluded in the forward model.& In such investigations, including the maximum and minimum possible difference between maps is essential to understand which non-steady state anomalies are real, and which could be artifacts attributable to (quantified) uncertainty.& In further use cases, we show how the few available seismic measurements that constrain the crust and upper mantle structure of East Antarctica can be placed in context, given the likelihood of major tectonic boundaries beneath the ice, and link this to published constraints on the seismic structure (and hence, rheology) of the deeper lithosphere.& In terms of how the solid Earth interacts with the ice sheet above, the impact of fine scale-length variations in spatial uncertainty may be investigated in relation to, for ex le, ice sheet modelling. For a large region and relatively unexplored region such as East Antarctica, uncertainty yields many and varied insights.&
Publisher: Oxford University Press (OUP)
Date: 15-11-2017
Publisher: Copernicus GmbH
Date: 15-05-2023
DOI: 10.5194/EGUSPHERE-EGU23-898
Abstract: This work aims to contribute to progress in the detection of hidden or transient hydrological events. Passive seismic methods offer high temporal resolution and the ability to monitor seismic sources hidden from direct view, making it an ideal candidate to complement other in-situ and satellite methods in these cases. The dynamics of a glacier can be greatly affected by its hydrological system, whether this be through water mediated ice fracturing, or the influence the water has on friction at the ice-bed interface. Effective detection of moving meltwater is therefore of great interest for anticipating future glacier changes and sensitivities.To effectively infer any hidden process from the observed seismic waveforms, we require a physically rigorous modelling framework. Our work therefore combines hydrodynamic models depicting meltwater flow with seismic wave propagation methods to produce synthetic seismograms. The hydrodynamic model of choice is smoothed particle hydrodynamics (SPH). This is a full, three-dimensional computational fluid dynamics method, meaning we can make minimal assumptions on the exact seismogenic mechanism prior to simulation. SPH has the capacity to capture a broad range of signal-generating processes that may prove to be of interest for modelling meltwater flow, such as fluid-solid impact events, free-surface behaviour (e.g., wave breaks), and some forms of turbulence. Beyond the modelling of complex flow, SPH also allows a simple implementation of arbitrarily shaped solid boundaries and the computation of force of the water on these boundaries a necessary output for waveform simulation.We propose a correspondence between different types of meltwater flow and the attributes of the waveforms they produce, as a step towards better detection and characterisation of hidden and short-lived events. Across a erse set of model geometries and flow types, we anticipate the collection of synthetically generated signals will be useful for categorising archived and real-time signals according to a mechanistic process using unsupervised machine learning methods in ongoing work.
Publisher: Elsevier BV
Date: 12-2022
Publisher: EDP Sciences
Date: 07-2022
DOI: 10.1051/0004-6361/202142733
Abstract: The recent detection of the quasi-stellar object (QSO) VIKING J231818.3−311346 (hereafter VIK J2318−3113) at redshift z = 6.44 in the Rapid ASKAP Continuum Survey (RACS) uncovered its radio-loud nature, making it one of the most distant known to date in this class. By using data from several radio surveys of the Galaxy And Mass Assembly 23 h field and from a dedicated follow-up, we were able to constrain the radio spectrum of VIK J2318−3113 in the observed range ∼0.1–10 GHz. At high frequencies (0.888–5.5 GHz in the observed frame) the QSO presents a steep spectrum ( α r = 1.24, with S ν ∝ ν − α r ), while at lower frequencies (0.4–0.888 GHz in the observed frame) it is nearly flat. The overall spectrum can be modelled by either a curved function with a rest-frame turnover around 5 GHz, or with a smoothly varying double power law that is flat below a rest-frame break frequency of about 20 GHz and above which it significantly steepens. Based on the model adopted, we estimated that the radio jets of VIK J2318−3113 must be a few hundred years old in the case of a turnover, or less than a few × 10 4 years in the case of a break in the spectrum. Having multiple observations at two frequencies (888 MHz and 5.5 GHz), we further investigated the radio variability previously reported for this source. We found that the marginally significant flux density variations are consistent with the expectations from refractive interstellar scintillation, even though relativistic effects related to the orientation of the source may still play a non-negligible role. Further radio and X-ray observations are required to conclusively discern the nature of this variation.
Publisher: Cambridge University Press (CUP)
Date: 2021
DOI: 10.1017/PASA.2020.49
Abstract: The remnant phase of a radio galaxy begins when the jets launched from an active galactic nucleus are switched off. To study the fraction of radio galaxies in a remnant phase, we take advantage of a $8.31$ deg $^2$ subregion of the GAMA 23 field which comprises of surveys covering the frequency range 0.1–9 GHz. We present a s le of 104 radio galaxies compiled from observations conducted by the Murchison Widefield Array (216 MHz), the Australia Square Kilometer Array Pathfinder (887 MHz), and the Australia Telescope Compact Array (5.5 GHz). We adopt an ‘absent radio core’ criterion to identify 10 radio galaxies showing no evidence for an active nucleus. We classify these as new candidate remnant radio galaxies. Seven of these objects still display compact emitting regions within the lobes at 5.5 GHz at this frequency the emission is short-lived, implying a recent jet switch off. On the other hand, only three show evidence of aged lobe plasma by the presence of an ultra-steep-spectrum ( $\\alpha -1.2$ ) and a diffuse, low surface brightness radio morphology. The predominant fraction of young remnants is consistent with a rapid fading during the remnant phase. Within our s le of radio galaxies, our observations constrain the remnant fraction to $4\\%\\lesssim f_{\\mathrm{rem}} \\lesssim 10\\%$ the lower limit comes from the limiting case in which all remnant candidates with hotspots are simply active radio galaxies with faint, undetected radio cores. Finally, we model the synchrotron spectrum arising from a hotspot to show they can persist for 5–10 Myr at 5.5 GHz after the jets switch of—radio emission arising from such hotspots can therefore be expected in an appreciable fraction of genuine remnants.
Publisher: Copernicus GmbH
Date: 15-05-2023
DOI: 10.5194/EGUSPHERE-EGU23-913
Abstract: We present an analytic framework to model seismic body waves due to supraglacial, englacial or subglacial flows in solid ice based on a smoothed particle hydrodynamic (SPH) simulation. Consisting of two parts, i) hydrodynamic modelling and ii) seismic wave propagation, the flexible framework allows for a pre-existing fluid simulation to be supplied to generate synthetic seismic signals. The field of glacier-related seismology has seen rapid development in recent years, with an expanded availability of passive seismic datasets that contain records of seismic disturbances generated by glacier processes. Some of these processes, such as basal slip and crevasse propagation, have mechanisms with plate tectonic deformation counterparts, however, many glacier signals are generated by moving melt water. This contribution aims to inform the interpretation of such signals.Our approach tracks the trajectories of fluid particles near the water-ice interface, as recorded in standard simulation outputs, to create a catalogue describing the energetics of each collision. We illustrate the capability of this framework using four end-member cases of water flow along surface channels with different geometries. Seismic signals are simulated at a variety of locations around the channel based on the impulse of the database of simulated collisions. We consider the change in character of the seismic waveforms by modelling frequency-dependent attenuation and weak dispersion in the glacial ice, in addition to the standard geometric spreading. The acceleration time series produced in this work are invariant to the temporal and spatial resolution of the hydrodynamic simulation, provided more than some minimum resolution is used. These time series may be converted to velocity or displacement for comparison with observed seismic signals.Investigating the seismic waves generated for our four channel geometries, we find distinct waveform envelope shapes with different first and later litude peaks matching initial and subsequent collisions of the melt water surge with the supraglacial channel walls. The change in waveform character with distance is also captured such that the character attributes due to the process and the those due to the propagation effects may be understood. The flexibility inherent in the model framework will allow for the generation of the seismic signals from simulations of a variety of different water flow geometries including simple 3D channels into and through a glacier. We make the code available as an open source resource for the polar geophysics community with the aim of adding to the toolbox of available approaches to inform the potential future seismic monitoring of melt water movement and related glacier processes.
Publisher: Oxford University Press (OUP)
Date: 06-10-2020
Abstract: Next-generation radio surveys are expected to detect tens of millions of active galactic nuclei (AGNs) with a median redshift of $z$ ≥ 1. Beyond targeted surveys, the vast majority of these objects will not have spectroscopic redshifts, while photometric redshifts for high-redshift AGNs are of limited quality, and even then require optical and infrared photometry. We propose a new approach to measure the redshifts of lobed radio galaxies based exclusively on radio-frequency imaging and broad-band radio photometry. Specifically, our algorithm uses the lobe flux density, angular size and width, and spectral shape to derive probability density functions for the most likely source redshift based on the Radio AGN in Semi-analytic Environments dynamical model. The full physically based model explains 70 per cent of the variation in the spectroscopic redshifts of a high-redshift (2 & $z$ & 4) s le of radio AGNs, compared to at most 27 per cent for any one of the observed attributes in isolation. We find that upper bounds on the angular size, as expected for unresolved sources, are sufficient to yield accurate redshift measurements at $z$ ≥ 2. The error in the model upon calibration using at least nine sources with known spectroscopic redshifts is & per cent in redshift (as 1 + $z$) across all redshifts. We provide the python code for the calculation and calibration of our radio continuum redshifts in an online library.
Publisher: American Astronomical Society
Date: 10-07-2012
Publisher: Copernicus GmbH
Date: 20-07-2023
Publisher: Australian Mathematical Publishing Association, Inc.
Date: 19-03-2023
DOI: 10.21914/ANZIAMJ.V64.17438
Abstract: Recent higher-order explicit Runge–Kutta methods are compared with the classic fourth-order (RK4) method in long-term integration of both energy-conserving and lossy systems. By comparing quantity of function evaluations against accuracy for systems with and without known solutions, optimal methods are proposed. For a conservative system, we consider positional accuracy for Newtonian systems of two or three bodies and total angular momentum for a simplified Solar System model, over moderate astronomical timescales (tens of millions of years). For a nonconservative system, we investigate a relativistic two-body problem with gravitational wave emission. We find that methods of tenth and twelfth order consistently outperform lower-order methods for the systems considered here. doi: 10.1017/S1446181122000141
Publisher: Frontiers Media SA
Date: 17-08-2022
DOI: 10.3389/FEART.2022.963525
Abstract: Geothermal heat flow is inferred from the gradient of temperature values in boreholes or short-penetration probe measurements. Such measurements are expensive and logistically challenging in remote locations and, therefore, often targeted to regions of economic interest. As a result, measurements are not distributed evenly. Some tectonic, geologic and even topographic settings are overrepresented in global heat flow compilations other settings are underrepresented or completely missing. These limitations in representation have implications for empirical heat flow models that use catalogue data to assign heat flow by the similarity of observables. In this contribution, we analyse the s ling bias in the Global Heat Flow database of the International Heat Flow Commission the most recent and extensive heat flow catalogue, and discuss the implications for accurate prediction and global appraisals. We also suggest correction weights to reduce the bias when the catalogue is used for empirical modelling. From comparison with auxiliary variables, we find that each of the following settings is highly overrepresented for heat flow measurements continental crust, sedimentary rocks, volcanic rocks, and Phanerozoic regions with hydrocarbon exploration. Oceanic crust, cratons, and metamorphic rocks are underrepresented. The findings also suggest a general tendency to measure heat flow in areas where the values are elevated however, this conclusion depends on which auxiliary variable is under consideration to determine the settings. We anticipate that using our correction weights to balance disproportional representation will improve empirical heat flow models for remote regions and assist in the ongoing assessment of the Global Heat Flow database.
Publisher: Oxford University Press (OUP)
Date: 09-10-2017
Publisher: Oxford University Press (OUP)
Date: 13-03-2020
Abstract: We model the X-ray surface brightness distribution of emission associated with Fanaroff & Riley type-II radio galaxies. Our approach builds on the RAiSE dynamical model which describes broad-band radio frequency synchrotron evolution of jet-inflated lobes in a wide range of environments. The X-ray version of the model presented here includes: (1) inverse-Compton upscattering of cosmic microwave background radiation (2) the dynamics of the shocked gas shell and associated bremsstrahlung radiation and (3) emission from the surrounding ambient medium. We construct X-ray surface brightness maps for a mock catalogue of extended FR-IIs based on the technical characteristics of the eRosita telescope. The integrated X-ray luminosity function at low redshifts (z ≤ 1) is found to strongly correlate with the density of the ambient medium in all but the most energetic sources, whilst at high-redshift (z & 1) the majority of objects are dominated by inverse-Compton lobe emission due to the stronger cosmic microwave background radiation. By inspecting our mock spatial brightness distributions, we conclude that any extended X-ray detection can be attributed to AGN activity at redshifts z ≥ 1. We compare the expected detection rates of active and remnant high-redshift radio AGNs for eRosita and LOFAR, and future more sensitive surveys. We find that a factor of ten more remnants can be detected using X-ray wavelengths over radio frequencies at z & 2.2, increasing to a factor of 100 for redshifts z & 3.1.
Publisher: Copernicus GmbH
Date: 25-07-2023
DOI: 10.5194/EGUSPHERE-2023-1341
Abstract: Abstract. Given the high number and ersity of events in a typical cryoseismic dataset, in particular those recorded on ice sheet margins, it is desirable to use a semi-automated method of grouping similar events for reconnaissance and ongoing analysis. We present a workflow for employing semi-unsupervised cluster analysis to inform investigations of the processes occurring in glaciers and ice sheets. In this demonstration study, we make use of a seismic event catalogue previously compiled for the Whillans Ice Stream, for the 2010–2011 austral summer (outlined in companion paper, Latto et al., 2023). We address the challenges of seismic event analysis for a complex wavefield by clustering similar seismic events into groups using characteristic temporal, spectral, and polarization attributes of seismic time series with the k-means++ algorithm. This provides the basis for a reconnaissance analysis of a seismic wavefield that contains local events (from the ice stream) set in an ambient wavefield that itself contains a ersity of signals (mostly from the Ross Ice Shelf). As one result, we find that two clusters include stick-slip events that erge in terms of length and initiation locality (i.e. Central Sticky Spot and/or the grounding line). We also identify a swarm of high frequency signals on January 16–17, 2011 that are potentially associated with a surface melt event from the Ross Ice Shelf. Used together with the event detection presented in the companion paper, the semi-automated workflow could readily generalize to other locations, and as a possible benchmark procedure, could enable the monitoring of remote glaciers over time and comparisons between locations.
Publisher: Copernicus GmbH
Date: 20-07-2023
DOI: 10.5194/EGUSPHERE-2023-1340
Abstract: Abstract. Cryoseismology is a powerful toolset for progressing the understanding of the structure and dynamics of glaciers and ice sheets. It can enable the detection of hidden processes such as brittle fracture, basal sliding, transient hydrological processes, and calving. Due to the ersity and often low signal-to-noise levels of glacier processes, the automated detection of seismic events caused by such processes can pose a challenge. We present a novel approach for the automated detection of events in glacier environments, the multi-STA/LTA algorithm, with a focus on capturing the many signal types recorded on ice sheet margins. This develops the use of approaches that use the ratio between short and long time averages (sta,lta) of signal litude as the means of event detection. Implemented in the open source and widely used ObsPy python package, the algorithm constructs a hybrid characteristic function from a set of sta, lta pairs. We apply the multi-STA/LTA algorithm to data from a seismic array deployed on the Whillans Ice Stream (WIS) in West Antarctica (austral summer 2010–2011), to form an event catalogue. The new algorithm compares favorably with standard approaches, yielding a ersity of seismic events, including all previously identified stick-slip events (Pratt et al., 2014), teleseisms, and other noise-type signals. We investigate a partial association of seismicity with the tidal cycle, and a slight association with ice temperature changes of the Antarctic summer. The new algorithm and workflow has the potential to yield systematic catalogues for further cryoseismology studies: conventional glacier seismology, and those tailored to pattern recognition by machine learning.
Publisher: MDPI AG
Date: 19-07-2023
Abstract: Analytical models describing the dynamics of lobed radio sources are essential for interpretation of the tens of millions of radio sources that will be observed by the Square Kilometre Array and pathfinder instruments. We propose that historical models can be grouped into two classes in which the forward expansion of the radio source is driven by either the jet momentum flux or lobe internal pressure. The most recent generation of analytical models combines these limiting cases for a more comprehensive description. We extend the mathematical formalism of historical models to describe source expansion in non-uniform environments, and directly compare different model classes with each other and with hydrodynamic numerical simulations. We quantify differences in predicted observable characteristics for lobed radio sources due to the different model assumptions for their dynamics. We have made our code for the historical models analysed in this review openly available to the community.
Publisher: Springer Science and Business Media LLC
Date: 24-03-2022
DOI: 10.1007/S10665-022-10215-W
Abstract: Boussinesq theory can model quite accurately viscous flows that involve multiple fluids with interfaces between them, so long as there is not much difference between the densities of the various fluids. However, the Boussinesq approximation is generally poor when the density ratio between the fluids is large. Here, we propose an Extended Boussinesq approximate equation, that allows for large density ratios, while still remaining straightforward to implement. Ex les are given for planar Rayleigh–Taylor instability, where the Boussinesq and the novel Extended Boussinesq models are compared with the predictions of an SPH fluid dynamics code, to confirm this approach.
Publisher: Ubiquity Press, Ltd.
Date: 19-10-2021
DOI: 10.5334/JORS.365
Publisher: Copernicus GmbH
Date: 15-05-2023
DOI: 10.5194/EGUSPHERE-EGU23-4074
Abstract: Antarctic subglacial properties impact geothermal heat, subglacial sedimentation, and glacial isostatic adjustment critical parameters for predicting the ice sheet's response to warming oceans. However, the tectonic architecture of the Antarctic interior is unresolved, with results dependent on datasets or extrapolation used. Most existing deterministic suggestions are derived from qualitative observations and often presented as robust results however, they hide possible alternative interpretations.& Using information entropy as a measure of certainty, we present a robust tectonic segmentation model generated from similarity analysis of multiple geophysical and geological datasets. The use of information entropy provides us with an unbiased and transparent metric to communicate the ambiguities from the uncertainties of qualitative classifications. Information theory also allows us to test and optimise the methods and data to evaluate how choices impact the distribution of alternative output maps. We further discuss how this metric can quantify the predictive power of parameters as a function of regions with different tectonic settings.
Publisher: Cambridge University Press (CUP)
Date: 07-2022
DOI: 10.1017/S1446181122000141
Abstract: Recent higher-order explicit Runge–Kutta methods are compared with the classic fourth-order (RK4) method in long-term integration of both energy-conserving and lossy systems. By comparing quantity of function evaluations against accuracy for systems with and without known solutions, optimal methods are proposed. For a conservative system, we consider positional accuracy for Newtonian systems of two or three bodies and total angular momentum for a simplified Solar System model, over moderate astronomical timescales (tens of millions of years). For a nonconservative system, we investigate a relativistic two-body problem with gravitational wave emission. We find that methods of tenth and twelfth order consistently outperform lower-order methods for the systems considered here.
No related grants have been discovered for Ross Turner.