scholarly journals LOFAR observations of the XMM-LSS field

2019 ◽  
Vol 622 ◽  
pp. A4 ◽  
Author(s):  
C. L. Hale ◽  
W. Williams ◽  
M. J. Jarvis ◽  
M. J. Hardcastle ◽  
L. K. Morabito ◽  
...  
Keyword(s):  
Large Scale ◽  
Angular Resolution ◽  
Low Frequency ◽  
Primary Beam ◽  
Electromagnetic Spectrum ◽  
Radio Sources ◽  
Spectral Indices ◽  
The North ◽  
True Source ◽  
Scale Calibration

We present observations of the XMM Large-Scale Structure (XMM-LSS) field observed with the LOw Frequency ARray (LOFAR) at 120–168 MHz. Centred at a J2000 declination of −4.5°, this is a challenging field to observe with LOFAR because of its low elevation with respect to the array. The low elevation of this field reduces the effective collecting area of the telescope, thereby reducing sensitivity. This low elevation also causes the primary beam to be elongated in the north-south direction, which can introduce side lobes in the synthesised beam in this direction. However the XMM-LSS field is a key field to study because of the wealth of ancillary information, encompassing most of the electromagnetic spectrum. The field was observed for a total of 12 h from three four-hour LOFAR tracks using the Dutch array. The final image presented encompasses ∼27 deg2, which is the region of the observations with a >50% primary beam response. Once combined, the observations reach a central rms of 280μJy beam−1at 144 MHz and have an angular resolution of 7.5 × 8.5″. We present our catalogue of detected sources and investigate how our observations compare to previous radio observations. This includes investigating the flux scale calibration of these observations compared to previous measurements, the implied spectral indices of the sources, the observed source counts and corrections to obtain the true source counts, and finally the clustering of the observed radio sources.

scholarly journals The Low-Frequency Array (LOFAR): Opening a New Window on the Universe

2002 ◽  
Vol 199 ◽  
pp. 474-483
Author(s):  
Namir E. Kassim ◽  
T. Joseph W. Lazio ◽  
William C. Erickson ◽  
Patrick C. Crane ◽  
R. A. Perley ◽  
...  
Keyword(s):  
Angular Resolution ◽  
Broad Band ◽  
Low Frequency ◽  
Electromagnetic Spectrum ◽  
Interferometric Imaging ◽  
Very Large Array ◽  
Long Wavelength ◽  
Calibration Techniques ◽  
Long Baselines ◽  
The Universe

Decametric wavelength imaging has been largely neglected in the quest for higher angular resolution because ionospheric structure limited interferometric imaging to short (< 5 km) baselines. The long wavelength (LW, 2—20 m or 15—150 MHz) portion of the electromagnetic spectrum thus remains poorly explored. The NRL-NRAO 74 MHz Very Large Array has demonstrated that self-calibration techniques can remove ionospheric distortions over arbitrarily long baselines. This has inspired the Low Frequency Array (LOFAR)—-a fully electronic, broad-band (15—150 MHz)antenna array which will provide an improvement of 2—3 orders of magnitude in resolution and sensitivity over the state of the art.

scholarly journals Internal tide energy flux over a ridge measured by a co-located ocean glider and moored ADCP

2019 ◽  
Author(s):  
Rob Hall ◽  
Barbara Berx ◽  
Gillian Damerell
Keyword(s):  
Energy Flux ◽  
Large Scale ◽  
Low Frequency ◽  
Internal Tide ◽  
Small Scale ◽  
Potential Density ◽  
The North ◽  
Model Study ◽  
Continental Shelves ◽  
Alternative Approach

Abstract. Internal tide energy flux is an important diagnostic for the study of energy pathways in the ocean, from large-scale input by the surface tide, to small-scale dissipation by turbulent mixing. Accurate calculation of energy flux requires repeated full-depth measurements of both potential density (ρ) and horizontal current velocity (u) over at least a tidal cycle and over several weeks to resolve the internal spring-neap cycle. Typically, these observations are made using full-depth oceanographic moorings that are vulnerable to being fished-out by commercial trawlers when deployed on continental shelves and slopes. Here we test an alternative approach to minimise these risks, with u measured by a low-frequency ADCP moored near the seabed and ρ measured by an autonomous ocean glider holding station by the ADCP. The method is used to measure the M2 internal tide radiating from the Wyville Thompson Ridge in the North Atlantic. The observed energy flux (4.2 ± 0.2 kW m−1) compares favourably with historic observations and a previous numerical model study. Error in the energy flux calculation due to imperfect co-location of the glider and ADCP is estimated by sub-sampling potential density in an idealised internal tide field along pseudorandomly distributed glider paths. The error is considered acceptable (

Energetics of the Western Hemisphere Circulation Pattern

2019 ◽  
Vol 32 (22) ◽  
pp. 7857-7870 ◽  
Author(s):  
Xin Tan ◽  
Ming Bao ◽  
Xuejuan Ren
Keyword(s):  
North Atlantic ◽  
Large Scale ◽  
Low Frequency ◽  
Lower Troposphere ◽  
Circulation Pattern ◽  
Western Hemisphere ◽  
Self Organizing Maps ◽  
Atlantic Sector ◽  
The North ◽  
Energy Maintenance

Abstract The Western Hemisphere (WH) circulation pattern, identified by self-organizing maps cluster analysis, is a low-frequency atmospheric regime that influences the fluctuations of large-scale circulation over the North Pacific–North American–North Atlantic areas. The reanalysis datasets from ECMWF are used to estimate the energetics of the WH pattern in this study. The composite results based on monthly WH events reveal that the kinetic energy (KE) associated with the WH pattern is maintained through the barotropic conversion from the climatological-mean westerlies, mainly in the Atlantic jet exit regions. The KE could also be gained through the barotropic feedback forcing from transient eddies. The corresponding baroclinic conversion of available potential energy (APE) from the climatological-mean state, which contributes most efficiently to the energy maintenance of the WH pattern, is obvious in the middle and lower troposphere, owing to the thermal contrast of the colder continent and warmer ocean over the North America–North Atlantic sector. The baroclinic conversion associated with the heat flux on the climatological temperature gradient is consistent with the southwestward-tilting height anomalies from 850 to 500 hPa. The baroclinic feedback from transient eddies contributes negatively to the energy conversion and destroys the maintenance of the WH pattern.

scholarly journals Spectral Index - Redshift Relation for Radio Galaxies and Quasars

1996 ◽  
Vol 175 ◽  
pp. 569-570
Author(s):  
R.D. Dagkesamanskii
Keyword(s):  
Flux Density ◽  
Spectral Index ◽  
Low Frequency ◽  
Radio Galaxies ◽  
Cosmological Evolution ◽  
Radio Sources ◽  
Spectral Indices ◽  
Extragalactic Radio Sources

Cosmological evolution of synchrotron spectra of the powerful extragalactic radio sources was studied by many authors. Some indications of such an evolution had been found firstly by analysis of ‘spectral index - flux density’ (α – S) relation for the sample of relatively strong radio sources. Later Gopal-Krishna and Steppe extended the analysis to weaker sources and found that the slope of αmed(S) curve changes dramatically at intermediate flux densities. Gopal-Krishna and Steppe pointed out that the maxima of the αmed(S) curve and of differential source counts are at almost the same flux density ranges (see, Fig. 2). It has to be noticed that the all mentioned results were obtained using the low-frequency spectral indices and on the basis of low frequency samples.

scholarly journals The Murchison Widefield Array Commissioning Survey: A Low-Frequency Catalogue of 14 110 Compact Radio Sources over 6 100 Square Degrees

2014 ◽  
Vol 31 ◽  
Author(s):  
Natasha Hurley-Walker ◽  
John Morgan ◽  
Randall B. Wayth ◽  
Paul J. Hancock ◽  
Martin E. Bell ◽  
...  
Keyword(s):  
Flux Density ◽  
Noise Level ◽  
Angular Resolution ◽  
Low Frequency ◽  
Radio Sources ◽  
Reduction Strategy ◽  
Sky Survey ◽  
Field Boundaries ◽  
Murchison Widefield Array ◽  
Finding Method

AbstractWe present the results of an approximately 6 100 deg2 104–196 MHz radio sky survey performed with the Murchison Widefield Array during instrument commissioning between 2012 September and 2012 December: the MWACS. The data were taken as meridian drift scans with two different 32-antenna sub-arrays that were available during the commissioning period. The survey covers approximately 20.5 h < RA < 8.5 h, − 58° < Dec < −14°over three frequency bands centred on 119, 150 and 180 MHz, with image resolutions of 6–3 arcmin. The catalogue has 3 arcmin angular resolution and a typical noise level of 40 mJy beam− 1, with reduced sensitivity near the field boundaries and bright sources. We describe the data reduction strategy, based upon mosaicked snapshots, flux density calibration, and source-finding method. We present a catalogue of flux density and spectral index measurements for 14 110 sources, extracted from the mosaic, 1 247 of which are sub-components of complexes of sources.

scholarly journals Revisiting the humid Roman hypothesis: novel analyses depict oscillating patterns

2011 ◽  
Vol 7 (4) ◽  
pp. 2355-2389 ◽  
Author(s):  
B. J. Dermody ◽  
H. J. de Boer ◽  
M. F. P. Bierkens ◽  
S. L. Weber ◽  
M. J. Wassen ◽  
...  
Keyword(s):  
North Atlantic ◽  
Large Scale ◽  
Mediterranean Climate ◽  
Eastern Mediterranean ◽  
Low Frequency ◽  
Interdisciplinary Approach ◽  
Archaeological Site ◽  
Roman Period ◽  
Spatial And Temporal Patterns ◽  
The North

Abstract. Previous studies have proposed that potential vegetation in the Mediterranean maintained a wetter climate during the Roman Period until the initiation of large scale deforestation. The reduction in evapotranspirative fluxes associated with deforestation is suggested to have caused climatic aridification leading to the establishment of the present-day Mediterranean climate. There is also evidence to indicate that during the Roman Period Mediterranean climate was influenced by low frequency fluctuations in sea level pressure over the North Atlantic, termed here: the Centennial North Atlantic Oscillation (CNAO). In order to understand the importance of each of these mechanisms and disentangle their respective signals in the proxy record, we have employed an interdisciplinary approach that exploits a range of tools and data sources. An analysis of archaeological site distribution and historical texts demonstrate that climate did not increase in aridity since the Roman Period. Using an Earth system model of intermediate complexity prescribed with a reconstruction of ancient deforestation, we find that Mediterranean climate was insensitive to deforestation in the Late Holocene. A novel analysis of a composite of proxy indicators of climatic humidity depicts spatial and temporal patterns consistent with the CNAO. The link between the CNAO during the Roman Period and climatic humidity signals manifest in our composite analysis are demonstrated using a modelling approach. Finally, we present evidence indicating that fluctuations in the CNAO contributed to triggering a societal tipping point in the Eastern Mediterranean at the end of the Roman Period.

scholarly journals Diffuse radio sources in clusters of galaxies: models and long-wavelength radio observations

2006 ◽  
Vol 2 (14) ◽  
pp. 369-371
Author(s):  
Craig L. Sarazin
Keyword(s):  
Large Scale ◽  
Low Frequency ◽  
Radio Sources ◽  
Clusters Of Galaxies ◽  
Radio Observations ◽  
Hadronic Interactions ◽  
Long Wavelength ◽  
Cluster Environment ◽  
Cluster Mergers ◽  
Radio Halos

AbstractClusters of galaxies contain several types of diffuse radio sources with very steep radio spectra which are associated with the cluster environment, including central radio bubbles, cluster radio relics, and cluster radio halos. Radio halos and relics are found only in merging clusters. Cluster radio relics may be produced by particle acceleration in merger shocks, while radio halos, may result from electron re-acceleration by turbulence produced by mergers. Secondary production of electrons and positrons by hadronic interactions also plays a role. If cluster radio halos and relics are related to mergers, then deep low frequency radio surveys could detect 1000's of clusters. Long-wavelength radio observations have a great potential to help us understand clusters and large scale structure, and can provide a diagnostic of cluster mergers, which affect the use of clusters in cosmological and dark energy studies.

scholarly journals The North Atlantic Oscillation Response to Large-Scale Atmospheric Anomalies in the Northeastern Pacific

2013 ◽  
Vol 70 (9) ◽  
pp. 2854-2874 ◽  
Author(s):  
Marie Drouard ◽  
Gwendal Rivière ◽  
Philippe Arbogast
Keyword(s):  
North Atlantic Oscillation ◽  
North Atlantic ◽  
Wave Breaking ◽  
Large Scale ◽  
Wave Train ◽  
Low Frequency ◽  
The North ◽  
Northeastern Pacific ◽  
The North Atlantic ◽  
The North Atlantic Oscillation

Abstract Ingredients in the North Pacific flow influencing Rossby wave breakings in the North Atlantic and the intraseasonal variations of the North Atlantic Oscillation (NAO) are investigated using both reanalysis data and a three-level quasigeostrophic model on the sphere. First, a long-term run is shown to reproduce the observed relationship between the nature of the synoptic wave breaking and the phase of the NAO. Furthermore, a large-scale, low-frequency ridge anomaly is identified in the northeastern Pacific in the days prior to the maximum of the positive NAO phase both in the reanalysis and in the model. A large-scale northeastern Pacific trough anomaly is observed during the negative NAO phase but does not systematically precede it. Then, short-term linear and nonlinear simulations are performed to understand how the large-scale ridge anomaly can act as a precursor of the positive NAO phase. The numerical setup allows for analysis of the propagation of synoptic waves in the eastern Pacific in the presence of a large-scale ridge or trough anomaly and their downstream impact onto the Atlantic jet when they break. The ridge acts in two ways. First, it tends to prevent the downstream propagation of small waves compared to long waves. Second, it deflects the propagation of the wave trains in such a way that they mainly propagate equatorward in the Atlantic. The two modes of action favor the anticyclonic wave breaking and, therefore, the positive NAO phase. With the trough, the wave train propagation is more zonal, disturbances are more meridionally elongated, and cyclonic wave breaking is more frequent in the Atlantic than in the ridge case.

scholarly journals A low-frequency radio halo survey of the South Pole Telescope SZ-selected clusters with the GMRT

2020 ◽  
Vol 500 (2) ◽  
pp. 2236-2249
Author(s):  
Ramij Raja ◽  
Majidul Rahaman ◽  
Abhirup Datta ◽  
Reinout J van Weeren ◽  
Huib T Intema ◽  
...  
Keyword(s):  
Large Scale ◽  
Low Frequency ◽  
Radio Sources ◽  
Synchrotron Emission ◽  
Diffuse Emission ◽  
Radio Halo ◽  
South Pole Telescope ◽  
New Radio ◽  
Upper Limits ◽  
Correlation Line

ABSTRACT The presence of non-thermal electrons and large-scale magnetic fields in the intracluster medium is known through the detection of megaparsec (Mpc) scale diffuse radio synchrotron emission. Although a significant amount of progress in finding new diffuse radio sources has happened in the last decade, most of the investigation has been constrained towards massive low-redshift clusters. In this work, we explore clusters with redshift z &gt; 0.3 in search of diffuse radio emission, at 325 MHz with the Giant Metrewave Radio Telescope. This campaign has resulted in the discovery of two new radio haloes (SPT-CL J0013−4906 and SPT-CL J0304−4401) along with two other detections (SPT-CL J2031−4037 and SPT-CL J2248−4431), previously reported (at 325 MHz) in the literature. In addition, we detect a halo candidate in one cluster in our sample, and upper limits for haloes are placed in eight clusters where no diffuse emission is detected. In the P1.4–LX plane, the detected haloes follow the observed correlation, whereas the upper limits lie above the correlation line, indicating the possibility of future detection with sensitive observations.

scholarly journals Large-scale changes of the semidiurnal tide along North Atlantic coasts from 1846 to 2018

Ocean Science ◽  
2021 ◽  
Vol 17 (1) ◽  
pp. 17-34
Author(s):  
Lucia Pineau-Guillou ◽  
Pascal Lazure ◽  
Guy Wöppelmann
Keyword(s):  
Sea Level ◽  
North Atlantic ◽  
Large Scale ◽  
Low Frequency ◽  
Underlying Mechanism ◽  
East Atlantic ◽  
North West ◽  
North East ◽  
The North ◽  
The North Atlantic

Abstract. We investigated the long-term changes of the principal tidal component M2 along North Atlantic coasts, from 1846 to 2018. We analysed 18 tide gauges with time series starting no later than 1940. The longest is Brest with 165 years of observations. We carefully processed the data, particularly to remove the 18.6-year nodal modulation. We found that M2 variations are consistent at all the stations in the North-East Atlantic (Cuxhaven, Delfzijl, Hoek van Holland, Newlyn, Brest), whereas some discrepancies appear in the North-West Atlantic. The changes started long before the 20th century and are not linear. The secular trends in M2 amplitude vary from one station to another; most of them are positive, up to 2.5 mm/yr at Wilmington since 1910. Since 1990, the trends switch from positive to negative values in the North-East Atlantic. Concerning the possible causes of the observed changes, the similarity between the North Atlantic Oscillation and M2 variations in the North-East Atlantic suggests a possible influence of the large-scale atmospheric circulation on the tide. Our statistical analysis confirms large correlations at all the stations in the North-East Atlantic. We discuss a possible underlying mechanism. A different spatial distribution of mean sea level (corresponding to water depth) from one year to another, depending on the low-frequency sea-level pressure patterns, could impact the propagation of the tide in the North Atlantic basin. However, the hypothesis is at present unproven.

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