Dual polarization measurements of MWA beampatterns at 137 MHz

ABSTRACT The wide adoption of low-frequency radio interferometers as a tool for deeper and higher resolution astronomical observations has revolutionized radio astronomy. Despite their construction from static, relatively simple dipoles, the sheer number of distinct elements introduces new, complicated instrumental effects. Their necessary remote locations exacerbate failure rates, while electronic interactions between the many adjacent receiving elements can lead to non-trivial instrumental effects. The Murchison Widefield Array (MWA) employs phased array antenna elements (tiles), which improve collecting area at the expense of complex beam shapes. Advanced electromagnetic simulations have produced the fully embedded element (FEE) simulated beam model which has been highly successful in describing the ideal beam response of MWA antennas. This work focuses on the relatively unexplored aspect of various in-situ, environmental perturbations to beam models and represents the first large-scale, in-situ, all-sky measurement of MWA beam shapes at multiple polarizations and pointings. Our satellite based beam measurement approach enables all-sky beam response measurements with a dynamic range of ∼50 dB, at 137 MHz.

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A calibration and imaging strategy at 300 MHz with the Murchison Widefield Array (MWA)

Publications of the Astronomical Society of Australia ◽

10.1017/pasa.2021.55 ◽

2021 ◽

Vol 38 ◽

Author(s):

Jaiden H. Cook ◽

Nicholas Seymour ◽

Marcin Sokolowski

Keyword(s):

Dynamic Range ◽

Low Frequency ◽

High Dynamic Range ◽

Primary Beam ◽

Main Lobe ◽

Beam Model ◽

Highly Sensitive ◽

Imaging Strategy ◽

Murchison Widefield Array ◽

Beam Patterns

Abstract At relatively high frequencies, highly sensitive grating sidelobes occur in the primary beam patterns of low frequency aperture arrays (LFAA) such as the Murchison Widefield Array (MWA). This occurs when the observing wavelength becomes comparable to the dipole separation for LFAA tiles, which for the MWA occurs at ${\sim}300$ MHz. The presence of these grating sidelobes has made calibration and image processing for 300 MHz MWA observations difficult. This work presents a new calibration and imaging strategy which employs existing techniques to process two example 300 MHz MWA observations. Observations are initially calibrated using a new 300 MHz sky-model which has been interpolated from low frequency and high frequency all-sky surveys. Using this 300 MHz model in conjunction with the accurate MWA tile primary beam model, we perform sky-model calibration for the two example observations. After initial calibration a self-calibration loop is performed by all-sky imaging each observation. We mask the main lobe of the all-sky image, and perform a sky-subtraction by estimating the masked image visibilities. We then image the main lobe of the sky-subtracted visibilities, which results in high dynamic range images of the two example observations. These images have been convolved with a Gaussian to a resolution of $2.4$ arcminutes, with a maximum sensitivity of ${{\sim}}31\,\textrm{mJy/beam}$ . The calibration and imaging strategy demonstrated in this work opens the door to performing science at 300 MHz with the MWA, which was previously an inaccessible domain. With this paper we release the code described below and the cross-matched catalogue along with the code to produce a sky-model in the range 70–1 400 MHz.

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GaLactic and Extragalactic All-sky Murchison Widefield Array (GLEAM) survey III: South Galactic Pole data release

Publications of the Astronomical Society of Australia ◽

10.1017/pasa.2021.5 ◽

2021 ◽

Vol 38 ◽

Author(s):

T. M. O. Franzen ◽

N. Hurley-Walker ◽

S. V. White ◽

P. J. Hancock ◽

N. Seymour ◽

...

Keyword(s):

Large Scale ◽

Angular Resolution ◽

Beam Model ◽

Spectral Variability ◽

Extragalactic Source ◽

Automated Algorithm ◽

Data Release ◽

Murchison Widefield Array ◽

Source Populations ◽

One Year

Abstract We present the South Galactic Pole (SGP) data release from the GaLactic and Extragalactic All-sky Murchison Widefield Array (GLEAM) survey. These data combine both years of GLEAM observations at 72–231 MHz conducted with the Murchison Widefield Array (MWA) and cover an area of 5 113 $\mathrm{deg}^{2}$ centred on the SGP at $20^{\mathrm{h}} 40^{\mathrm{m}} < \mathrm{RA} < 05^{\mathrm{h}} 04^{\mathrm{m}}$ and $-48^{\circ} < \mathrm{Dec} < -2^{\circ} $ . At 216 MHz, the typical rms noise is ${\approx}5$ mJy beam–1 and the angular resolution ${\approx}2$ arcmin. The source catalogue contains a total of 108 851 components above $5\sigma$ , of which 77% have measured spectral indices between 72 and 231 MHz. Improvements to the data reduction in this release include the use of the GLEAM Extragalactic catalogue as a sky model to calibrate the data, a more efficient and automated algorithm to deconvolve the snapshot images, and a more accurate primary beam model to correct the flux scale. This data release enables more sensitive large-scale studies of extragalactic source populations as well as spectral variability studies on a one-year timescale.

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A general Cluster data and global MHD simulation comparison

Annales Geophysicae ◽

10.5194/angeo-26-3411-2008 ◽

2008 ◽

Vol 26 (11) ◽

pp. 3411-3428 ◽

Cited By ~ 3

Author(s):

P. Daum ◽

M. H. Denton ◽

J. A. Wild ◽

M. G. G. T. Taylor ◽

J. Šafránková ◽

...

Keyword(s):

Large Scale ◽

Numerical Models ◽

Mhd Simulation ◽

Global Context ◽

Cluster Data ◽

Verification Methods ◽

Modelling Studies ◽

Global Mhd Simulation ◽

The Many

Abstract. Among the many challenges facing the space weather modelling community today, is the need for validation and verification methods of the numerical models available describing the complex nonlinear Sun-Earth system. Magnetohydrodynamic (MHD) models represent the latest numerical models of this environment and have the unique ability to span the enormous distances present in the magnetosphere, from several hundred kilometres to several thousand kilometres above the Earth's surface. This makes it especially difficult to develop verification and validation methods which posses the same range spans as the models. In this paper we present a first general large-scale comparison between four years (2001–2004) worth of in situ Cluster plasma observations and the corresponding simulated predictions from the coupled Block-Adaptive-Tree-Solarwind-Roe-Upwind-Scheme (BATS-R-US) MHD code. The comparison between the in situ measurements and the model predictions reveals that by systematically constraining the MHD model inflow boundary conditions a good correlation between the in situ observations and the modeled data can be found. These results have an implication for modelling studies addressing also smaller scale features of the magnetosphere. The global MHD simulation can therefore be used to place localised satellite and/or ground-based observations into a global context and fill the gaps left by measurements.

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High fidelity spectroscopic imaging at low radio frequencies to estimate plasma parameters of solar coronal mass ejections at higher coronal heights

10.5194/egusphere-egu21-11089 ◽

2021 ◽

Author(s):

Devojyoti Kansabanik ◽

Surajit Mondal ◽

Divya Oberoi ◽

Angelos Vourlidas

Keyword(s):

Radio Emission ◽

Coronal Mass Ejections ◽

Large Scale ◽

Dynamic Range ◽

High Dynamic Range ◽

Physical Parameters ◽

Plasma Parameters ◽

Radio Imaging ◽

Gyrosynchrotron Emission ◽

Murchison Widefield Array

<p>Coronal Mass Ejections (CMEs) are large-scale explosive eruptions of magnetised plasma from the Sun into the Heliosphere. Measuring the physical parameters of CMEs is crucial for understanding their physics and for assessing their geo-effectiveness. Radio observations offer the most direct means for estimating these plasma parameters when gyrosynchrotron (GS) emission is detected from the CME plasma. However, since the first detection by Bastian et al.2001, only a handful of studies have successfully detected GS emission from CME plasma. This is usually attributed to the challenges involved in obtaining the high dynamic range imaging required for observing this faint gyrosynchrotron emission in the vicinity of active solar emissions.</p><p>The newly developed imaging pipeline (Mondal et al., 2019) designed for the data from Murchison Widefield Array (MWA) marks a significant improvement in metrewave solar radio imaging. Our work suggests that we should now be able to routinely detect GS emission from CME plasma. We present an example where we have successfully detected radio emission from CME plasma and modelled it as GS emission, leading to reliable estimates of CME magnetic field as well as the distribution of energetic electrons (Mondal et al. 2020). In a different example we are able to detect the radio emission from the CME plasma out to as far as 8.3 solar radii. We find that the observed spectra are not always consistent with simple GS models. This highlights that more complicated physics might be at play and points to the need for building more detailed models for interpreting these emissions. We hope that with the availability of polarimetric imaging capability, which we are in the process of developing, this technique will provide a robust way to routinely measure CME magnetic fields along with its other physical parameters. We note that these are the weakest detections of GS emissions from CME plasma reported yet.</p>

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Calibration and Stokes Imaging with Full Embedded Element Primary Beam Model for the Murchison Widefield Array

Publications of the Astronomical Society of Australia ◽

10.1017/pasa.2017.54 ◽

2017 ◽

Vol 34 ◽

Cited By ~ 18

Author(s):

M. Sokolowski ◽

T. Colegate ◽

A. T. Sutinjo ◽

D. Ung ◽

R. Wayth ◽

...

Keyword(s):

Low Frequency ◽

Primary Beam ◽

Beam Model ◽

Efficient Calculation ◽

Wide Range ◽

Simulation Based ◽

Soil Effects ◽

Murchison Widefield Array ◽

The Individual ◽

Bow Tie

AbstractThe Murchison Widefield Array (MWA), located in Western Australia, is one of the low-frequency precursors of the international Square Kilometre Array (SKA) project. In addition to pursuing its own ambitious science programme, it is also a testbed for wide range of future SKA activities ranging from hardware, software to data analysis. The key science programmes for the MWA and SKA require very high dynamic ranges, which challenges calibration and imaging systems. Correct calibration of the instrument and accurate measurements of source flux densities and polarisations require precise characterisation of the telescope’s primary beam. Recent results from the MWA GaLactic Extragalactic All-sky Murchison Widefield Array (GLEAM) survey show that the previously implemented Average Embedded Element (AEE) model still leaves residual polarisations errors of up to 10–20% in Stokes Q. We present a new simulation-based Full Embedded Element (FEE) model which is the most rigorous realisation yet of the MWA’s primary beam model. It enables efficient calculation of the MWA beam response in arbitrary directions without necessity of spatial interpolation. In the new model, every dipole in the MWA tile (4 × 4 bow-tie dipoles) is simulated separately, taking into account all mutual coupling, ground screen, and soil effects, and therefore accounts for the different properties of the individual dipoles within a tile. We have applied the FEE beam model to GLEAM observations at 200–231 MHz and used false Stokes parameter leakage as a metric to compare the models. We have determined that the FEE model reduced the magnitude and declination-dependent behaviour of false polarisation in Stokes Q and V while retaining low levels of false polarisation in Stokes U.

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Infrasound as a Cue for Seabird Navigation

Frontiers in Ecology and Evolution ◽

10.3389/fevo.2021.740027 ◽

2021 ◽

Vol 9 ◽

Author(s):

Samantha C. Patrick ◽

Jelle D. Assink ◽

Mathieu Basille ◽

Susana Clusella-Trullas ◽

Thomas A. Clay ◽

...

Keyword(s):

Large Scale ◽

Animal Species ◽

Bird Species ◽

Low Frequency ◽

Temporal Consistency ◽

Physiological Mechanisms ◽

Breeding Sites ◽

Spatio Temporal ◽

Future Work

Seabirds are amongst the most mobile of all animal species and spend large amounts of their lives at sea. They cross vast areas of ocean that appear superficially featureless, and our understanding of the mechanisms that they use for navigation remains incomplete, especially in terms of available cues. In particular, several large-scale navigational tasks, such as homing across thousands of kilometers to breeding sites, are not fully explained by visual, olfactory or magnetic stimuli. Low-frequency inaudible sound, i.e., infrasound, is ubiquitous in the marine environment. The spatio-temporal consistency of some components of the infrasonic wavefield, and the sensitivity of certain bird species to infrasonic stimuli, suggests that infrasound may provide additional cues for seabirds to navigate, but this remains untested. Here, we propose a framework to explore the importance of infrasound for navigation. We present key concepts regarding the physics of infrasound and review the physiological mechanisms through which infrasound may be detected and used. Next, we propose three hypotheses detailing how seabirds could use information provided by different infrasound sources for navigation as an acoustic beacon, landmark, or gradient. Finally, we reflect on strengths and limitations of our proposed hypotheses, and discuss several directions for future work. In particular, we suggest that hypotheses may be best tested by combining conceptual models of navigation with empirical data on seabird movements and in-situ infrasound measurements.

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Large-scale chromosomal restructuring is induced by the transposable element tam3 at the nivea locus of antirrhinum majus.

Genetics ◽

10.1093/genetics/119.1.171 ◽

1988 ◽

Vol 119 (1) ◽

pp. 171-184

Author(s):

C Martin ◽

S Mackay ◽

R Carpenter

Keyword(s):

Transposable Element ◽

Large Scale ◽

Molecular Level ◽

Chromosomal Rearrangements ◽

Low Frequency ◽

Antirrhinum Majus ◽

Inverted Duplication ◽

Imprecise Excision ◽

Flanking Sequences

Abstract The transposable element, Tam3, gives rise to large-scale (greater than 1 kb) chromosomal rearrangements at a low frequency, when it is inserted at the nivea locus of Antirrhinum majus. Although some deletions may result from imprecise excision of Tam3, rearrangements involving deletion, dispersion and inverted duplication of flanking sequences, where Tam3 remains in situ, have also been identified. These rearrangements have been mapped at the molecular level, and the behavior of Tam3 following rearrangement has been observed. It is clear that Tam3 has enormous potential to restructure chromosomes through successive rounds of large-scale rearrangements. The mechanisms by which such rearrangements might arise are discussed.

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A Large-Scale, Low-Frequency Murchison Widefield Array Survey of Galactic H ii Regions between 260 < l < 340

Publications of the Astronomical Society of Australia ◽

10.1017/pasa.2016.19 ◽

2016 ◽

Vol 33 ◽

Cited By ~ 12

Author(s):

L. Hindson ◽

M. Johnston-Hollitt ◽

N. Hurley-Walker ◽

J. R. Callingham ◽

H. Su ◽

...

Keyword(s):

Large Scale ◽

Low Frequency ◽

Photon Flux ◽

H Ii Regions ◽

Spectral Indices ◽

Multiple Frequency ◽

The Galaxy ◽

Frequency Survey ◽

Murchison Widefield Array ◽

Bremsstrahlung Process

AbstractWe have compiled a catalogue of H ii regions detected with the Murchison Widefield Array between 72 and 231 MHz. The multiple frequency bands provided by the Murchison Widefield Array allow us identify the characteristic spectrum generated by the thermal Bremsstrahlung process in H ii regions. We detect 306 H ii regions between 260° < l < 340° and report on the positions, sizes, peak, integrated flux density, and spectral indices of these H ii regions. By identifying the point at which H ii regions transition from the optically thin to thick regime, we derive the physical properties including the electron density, ionised gas mass, and ionising photon flux, towards 61 H ii regions. This catalogue of H ii regions represents the most extensive and uniform low frequency survey of H ii regions in the Galaxy to date.

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