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An estimation of the Ocean Wave Heights using terrestrially seismic data
Full text linkTraditionally, there are different approaches to monitoring the ocean wave field consisting of 1) measurements using insitu buoys, 2) numerical ocean wave modelling using wind forecast, and 3) satellite altimetry. Each of these ocean wave monitoring techniques have their own advantages and disadvantages associated with their spatial and temporal resolution. For example, buoys are physical point measurements with excellent temporal resolution (e.g., sub-hourly), but their spatial resolution is very poor (e.g., single point in space). Buoys are also expensive to maintain; ‘Real-time’ wave height estimations from numerical wave
modelling is based on forecast wind, hence the model accuracy is dependent on wind prediction accuracy. . Compare to buoys, the temporal resolution of numerical models is poor (e.g., every 3 hours), but the spatial resolution is much better (various resolutions depending on the grid size); Satellite altimetry looks over a large region so the spatial coverage is very good but the temporal resolution is very poor (e.g., once every four days). In this work we consider terrestrial seismic (microseism) data as a proxy for wave heights. Under certain conditions, it has the potential for combined good spatial and temporal resolution, in quasireal
time. This technique is based on the relationship between secondary microseism amplitudes recorded on land and the ocean wave-wave interactions which excite the sea floor, generating these secondary microseisms. Here we take a data driven approach, implementing an Artificial Neural Network (ANN) to quantify the complex underlying relationship between ocean wave height and microseism amplitude. Thus far we trained the ANN using the available seismic and numerical simulated data and then used the trained ANN to estimate significant Ocean Wave Height (SWH) at particular location(s) in the Northeast Atlantic using amplitudes from seismic station distributed across Ireland. Our preliminary results look very promising and show relatively small residuals for measured wave height using the ANN compare to the real data for both small and moderate wave heights. However, currently larger residuals are seen for the largest wave heights. We expect this to improve as ever more data becomes available
Waveform Tomography of the Antarctic Plate
Full text linkThe Antarctic continent is a complex assemblage of geological units, ranging from Archean cratons in the east to a Cenozoic assembly of Mesozoic terranes in the west. Present are also the failed Lambert rift system, the inactive West Antarctic rift system and intraplate volcanism in Marie Byrd Land. Covered almost entirely by ice sheets, Antarctica's highly heterogeneous lithospheric structure and its upper mantle are among the least well-studied regions of the Earth’s interior.
The past two decades have seen a significant rise in the number of seasonal and temporary deployments as well as new permanent stations, supplementing and improving the still sparse station coverage in Antarctica. This provided a considerable improvement in both the quantity and quality of seismic data available for the Antarctic continent and its surrounding regions. We assemble a very large dataset of 0.8 million waveform fits, comprising all publicly accessible broadband data in the Southern Hemisphere, with sparser coverage elsewhere, for the best possible sampling of the Antarctic Plate’s crust and the upper mantle.
The new S-wave velocity tomographic model of the crust and upper mantle of Antarctica is computed using the Automated Multimode Inversion (AMI) scheme. AMI first extracts structural information from the surface, S- and multiple S-waves as sets of linearly independent equations. These equations are then combined into a single large linear system that is solved to obtain a tomographic model of the Antarctic crust and upper mantle. We observe the clear delineation of East and West Antarctica by a strong velocity gradient that bisects the continent extending from Coats Land to Victoria Land, following the Transantarctic Mountains. West Antarctica is observed to be underlain by low S-wave velocity anomalies connecting the Antarctic Peninsula, the Amundsen Sea Coast and Marie Byrd Land. The highest S-wave velocity anomalies are observed in central-eastern Antarctica, most of which is underlain by thick, cold cratonic lithosphere. Our tomography maps the boundaries of Antarctica’s cratonic lithosphere and, also, substantial intra-cratonic heterogeneity. It also reveals the patterns of the lithosphere-asthenosphere interactions beneath the cratons and the neighbouring Cenozoic terranes and offers new evidence on the origins of the Transantarctic Mountains and the intraplate volcanism in West Antarctica
Study of emission from the bow shocks of runaway massive stars at radio frequencies
Full text linkBow shocks from runaway massive stars can accelerate particles up to relativistic energies, making them excellent laboratories for studying different types of emission and their interaction with the interstellar medium those stars are passing through. Not many studies have been done in the radio frequencies regarding the thermal and non-thermal emission of bow shocks from runaway massive stars. The pioneering study made by Benaglia et al. [1], where BD+43 3654 was successfully observed with the Very Large Array at 1.42 and 4.86 GHz, reignated the need for further investigation of bow shocks in the radio frequencies, as traces of both thermal and non-thermal emission were found. Following their example, we repeated the experiment in higher radio frequencies of 4-12 GHz to determine the nature of the emission of BD+43 3654 even further [2]. In addition, we observed, at the same frequencies, a sample of 23 more sources that were found to be bow shock candidates [e.g. 3], in order to compare our findings with the BD+43 3654 and establish whether or not non-thermal emission is common in such sources and in what way. What is more, we developed 3D simulations of the stellar wind of massive runaway stars using Magnetohydrodynamics and the PhotoIonization of Nebula (PION) code. We compare our results with previous 2D Hydrodynamic simulations [4] as well as X-ray observations and their synthetic counterpart [5]. In this work we present in detail our findings
A Simultaneous Dual-site Technosignature Search Using International LOFAR Stations
Full text linkThe Search for Extraterrestrial Intelligence aims to find evidence of technosignatures, which can point toward the
possible existence of technologically advanced extraterrestrial life. Radio signals similar to those engineered on
Earth may be transmitted by other civilizations, motivating technosignature searches across the entire radio
spectrum. In this endeavor, the low-frequency radio band has remained largely unexplored; with prior radio
searches primarily above 1 GHz. In this survey at 110–190 MHz, observations of 1,631,198 targets from TESS and
Gaia are reported. Observations took place simultaneously with two international stations (noninterferometric) of
the Low Frequency Array in Ireland and Sweden. We can reject the presence of any Doppler drifting narrowband
transmissions in the barycentric frame of reference, with equivalent isotropic radiated power of 10 17 W, for
0.4 million (or 1.3 million) stellar systems at 110 (or 190) MHz. This work demonstrates the effectiveness of using
multisite simultaneous observations for rejecting anthropogenic signals in the search for technosignatures
InsituMarine Laboratory for Geosystems Research
Full text linkiMARL: The “Insitu Marine Laboratory for Geosystems Research” is a pool of various types of ocean sensors, hosted by the Dublin Institute for Advanced Studies
(DIAS).
• It comprises broadband Ocean Bottom Seismographs(OBS), acoustic sensors, and sensors for measuring absolute pressure and temperature in the water column.
• The sensor pool is mobile and can, in principle, be deployed around the world. However, the current focus is in the NE Atlantic, offshore Ireland.
• iMARL will allow for the detection of offshore earthquakes and offshore storms, as well as noise in the ocean and biologically generated acoustic signals (e.g.
from cetaceans).
• Impacts from this equipment will include: natural resources quantification, natural hazard estimation, environmental and baseline climate related “insitu” ocean
monitoring and the monitoring of marine noise pollution.
• Through an award to the Dublin Institute for Advanced Studies (DIAS) the iMARL infrastructure is funded by Science Foundation Ireland (SFI) with support from
the Geological Survey Irelan
Ambient noise autocorrelation scheme for imaging the P-wave reflectivity of the lithosphere
Full text linkImaging-spectroscopy of a band-split type II solar radio burst with the Murchison Widefield Array
Full text linkType II solar radio bursts are caused by magnetohydrodynamic (MHD) shocks driven by solar eruptive events such as coronal mass ejections (CMEs). Often, both fundamental and harmonic bands of type II bursts are split into sub-bands, which are generally believed to be coming from upstream and downstream regions of the shock; however, this explanation remains unconfirmed. Here, we present combined results from imaging analyses of type II radio burst band splitting and other fine structures observed by the Murchison Widefield Array (MWA) and extreme ultraviolet observations from Solar Dynamics Observatory (SDO)/Atmospheric Imaging Assembly (AIA) on 28 September 2014. The MWA provides imaging-spectroscopy in the range 80−300 MHz with a time resolution of 0.5 s and frequency resolution of 40 kHz. Our analysis shows that the burst was caused by a piston-driven shock with a driver speed of ∼112 km s−1 and shock speed of ∼580 km s−1. We provide rare evidence that band splitting is caused by emission from multiple parts of the shock (as opposed to the upstream–downstream hypothesis). We also examine the small-scale motion of type II fine structure radio sources in MWA images, and suggest that this motion may arise because of radio propagation effects from coronal turbulence, and is not due to the physical motion of the shock location. We present a novel technique that uses imaging spectroscopy to directly determine the effective length scale of turbulent density perturbations, which is found to be 1−2 Mm. The study of the systematic and small-scale motion of fine structures may therefore provide a measure of turbulence in different regions of the shock and corona
Inverse-Compton cooling of thermal plasma in colliding-wind binaries
No full textThe inverse-Compton effect (IC) is a widely recognized cooling mechanism for both relativistic and thermal electrons in various astrophysical environments, including the intergalactic medium and X-ray emitting plasmas. Its effect on thermal electrons is, however, frequently overlooked in theoretical and numerical models of colliding-wind binaries (CWB). In this article, we provide a comprehensive investigation of the impact of IC cooling in CWBs, presenting general results for when the photon fields of the stars dominate the cooling of the thermal plasma and when shocks at the stagnation point are expected to be radiative. Our analysis shows that IC cooling is the primary cooling process for the shocked-wind layer over a significant portion of the relevant parameter space, particularly in eccentric systems with large wind-momentum ratios, e.g. those containing a Wolf–Rayet and O-type star. Using the binary system WR 140 as a case study, we demonstrate that IC cooling leads to a strongly radiative shocked wind near periastron, which may otherwise remain adiabatic if only collisional cooling was considered. Our results are further supported by 2D and 3D simulations of wind–wind collisions. Specifically, 3D magnetohydrodynamic simulations of WR 140 show a significant decrease in hard-X-ray emission around periastron, in agreement with observations but in contrast to equivalent simulations that omit IC cooling. A novel method is proposed for constraining mass-loss rates of both stars in eccentric binaries where the wind-collision zone switches from adiabatic to radiative approaching periastron. IC scattering is an important cooling process in the thermal plasma of CWBs
