scholarly journals Using low-frequency pulsar observations to study the 3-D structure of the Galactic magnetic field

2017 ◽  
Vol 12 (S333) ◽  
pp. 151-156
Author(s):  
C. Sobey ◽  
Keyword(s):  
Magnetic Field ◽  
Faraday Rotation ◽  
Efficient Method ◽  
Galactic Halo ◽  
Three Dimensional ◽  
Low Frequency ◽  
Dimensional Structure ◽  
Galactic Magnetic Field ◽  
Fractional Bandwidth ◽  
Murchison Widefield Array

AbstractThe Galactic magnetic field (GMF) plays a role in many astrophysical processes and is a significant foreground to cosmological signals, such as the Epoch of Reionization (EoR), but is not yet well understood. Dispersion and Faraday rotation measurements (DMs and RMs, respectively) towards a large number of pulsars provide an efficient method to probe the three-dimensional structure of the GMF. Low-frequency polarisation observations with large fractional bandwidth can be used to measure precise DMs and RMs. This is demonstrated by a catalogue of RMs (corrected for ionospheric Faraday rotation) from the Low Frequency Array (LOFAR), with a growing complementary catalogue in the southern hemisphere from the Murchison Widefield Array (MWA). These data further our knowledge of the three-dimensional GMF, particularly towards the Galactic halo. Recently constructed or upgraded pathfinder and precursor telescopes, such as LOFAR and the MWA, have reinvigorated low-frequency science and represent progress towards the construction of the Square Kilometre Array (SKA), which will make significant advancements in studies of astrophysical magnetic fields in the future. A key science driver for the SKA-Low is to study the EoR, for which pulsar and polarisation data can provide valuable insights in terms of Galactic foreground conditions.

scholarly journals Studying Magnetic Fields using Low-frequency Pulsar Observations

2017 ◽  
Vol 13 (S337) ◽  
pp. 299-303
Author(s):  
C. Sobey ◽  
Keyword(s):  
Magnetic Field ◽  
Magnetic Fields ◽  
Large Scale ◽  
Galactic Halo ◽  
Three Dimensional ◽  
Low Frequency ◽  
Small Scale ◽  
Radio Telescopes ◽  
Astrophysical Plasmas ◽  
Murchison Widefield Array

AbstractLow-frequency polarisation observations of pulsars, facilitated by next-generation radio telescopes, provide powerful probes of astrophysical plasmas that span many orders of magnitude in magnetic field strength and scale: from pulsar magnetospheres to intervening magneto-ionic plasmas including the ISM and the ionosphere. Pulsar magnetospheres with teragauss field strengths can be explored through their numerous emission phenomena across multiple frequencies, the mechanism behind which remains elusive. Precise dispersion and Faraday rotation measurements towards a large number of pulsars probe the three-dimensional large-scale (and eventually small-scale) structure of the Galactic magnetic field, which plays a role in many astrophysical processes, but is not yet well understood, especially towards the Galactic halo. We describe some results and ongoing work from the Low Frequency Array (LOFAR) and the Murchison Widefield Array (MWA) radio telescopes in these areas. These and other pathfinder and precursor telescopes have reinvigorated low-frequency science and build towards the Square Kilometre Array (SKA), which will make significant advancements in studies of astrophysical magnetic fields in the next 50 years.

scholarly journals Low-frequency Faraday rotation measures towards pulsars using LOFAR: probing the 3D Galactic halo magnetic field

2019 ◽  
Vol 484 (3) ◽  
pp. 3646-3664 ◽  
Author(s):  
C Sobey ◽  
A V Bilous ◽  
J-M Grießmeier ◽  
J W T Hessels ◽  
A Karastergiou ◽  
...  
Keyword(s):  
Magnetic Field ◽  
Faraday Rotation ◽  
Galactic Halo ◽  
Low Frequency

scholarly journals The three-dimensional structure of swirl-switching in bent pipe flow

2017 ◽  
Vol 835 ◽  
pp. 86-101 ◽  
Author(s):  
Lorenz Hufnagel ◽  
Jacopo Canton ◽  
Ramis Örlü ◽  
Oana Marin ◽  
Elia Merzari ◽  
...  
Keyword(s):  
Reynolds Number ◽  
Three Dimensional ◽  
Low Frequency ◽  
Dimensional Structure ◽  
Pipe Bend ◽  
Dean Vortices ◽  
Dean Vortex ◽  
Piping Systems ◽  
Bent Pipe ◽  
Pipe Radius

Swirl-switching is a low-frequency oscillatory phenomenon which affects the Dean vortices in bent pipes and may cause fatigue in piping systems. Despite thirty years worth of research, the mechanism that causes these oscillations and the frequencies that characterise them remain unclear. Here we show that a three-dimensional wave-like structure is responsible for the low-frequency switching of the dominant Dean vortex. The present study, performed via direct numerical simulation, focuses on the turbulent flow through a $90^{\circ }$ pipe bend preceded and followed by straight pipe segments. A pipe with curvature 0.3 (defined as ratio between pipe radius and bend radius) is studied for a bulk Reynolds number $Re=11\,700$, corresponding to a friction Reynolds number $Re_{\unicode[STIX]{x1D70F}}\approx 360$. Synthetic turbulence is generated at the inflow section and used instead of the classical recycling method in order to avoid the interference between recycling and swirl-switching frequencies. The flow field is analysed by three-dimensional proper orthogonal decomposition (POD) which for the first time allows the identification of the source of swirl-switching: a wave-like structure that originates in the pipe bend. Contrary to some previous studies, the flow in the upstream pipe does not show any direct influence on the swirl-switching modes. Our analysis further shows that a three-dimensional characterisation of the modes is crucial to understand the mechanism, and that reconstructions based on two-dimensional POD modes are incomplete.

scholarly journals Modeling the spatial distribution of magnetic fields of low frequency multiple sources

2021 ◽  
Vol 5 (2) ◽  
pp. 34-37
Author(s):  
Larysa Levchenko
Keyword(s):  
Magnetic Field ◽  
Electromagnetic Compatibility ◽  
Three Dimensional ◽  
Low Frequency ◽  
Electrical Equipment ◽  
Technical Equipment ◽  
Multiple Sources ◽  
Matlab Package ◽  
Many Sources ◽  
The Magnetic Field

The work is show that in conditions of dense the location of electrical equipment in the premises, buildings and on territories to ensure the regulatory levels of electromagnetic compatibility of personnel and the population, it is advisable to carry out preliminary modeling of the propagation of electromagnetic fields it is advisable. Considering the insignificant shielding of the magnetic field by the equipment cases, it is advisable to carry out modeling based on the magnetic component of the electromagnetic field. The mathematical ratio of the propagation of the magnetic field of individual sources, taking into account their dipole model, has been determined. The modeling was carried out for sources of the dipole and dipole-quadrupole types. Three-dimensional models of sources with the propagation of dipole, quadrupole and dipole-quadrupole harmonics of the field are provided, using the Matlab package. Application software has been developed in the C environment, using the SQL server database, and modeling of the propagation of the magnetic field of many sources in a certain plane has been carried out. This result is show that even for electrical equipment that operates to determine the reduced magnetic field isolines experimentally very difficult. At the design stages of equipment placement, modeling is the only tool for predicting the electromagnetic environment, which determines the electromagnetic compatibility of technical equipment and the electromagnetic safety of personnel and the public.

scholarly journals A new look at the galactic magnetic field

1979 ◽  
Vol 84 ◽  
pp. 317-319
Author(s):  
P. P. Kronberg ◽  
M. Simard-Normandin
Keyword(s):  
Magnetic Field ◽  
Faraday Rotation ◽  
Linear Polarization ◽  
Radio Sources ◽  
Galactic Magnetic Field ◽  
Equal Area ◽  
Large Sample ◽  
Extragalactic Radio Sources ◽  
First Order ◽  
New Look

We have measured the linear polarization of a new large sample of extragalactic radio sources, and by combining these with polarization values already in the literature, we have been able to compute a large number of rotation measures, with improved quality. We have also investigated the depolarization properties of these sources and as a result have been able to identify most sources with a large internally generated Faraday rotation. Figure 1 shows the rotation measures of 475 extragalactic radio sources on an equal-area projection, after “cleaning out” the extragalactic effects to first order.

Galactic magnetic-field models derived from Faraday-rotation data

1967 ◽  
Vol 31 ◽  
pp. 391-392
Author(s):  
R. D. Davies
Keyword(s):  
Magnetic Field ◽  
Faraday Rotation ◽  
Component Model ◽  
The Sun ◽  
Galactic Magnetic Field ◽  
Two Component ◽  
Net Flux ◽  
The Magnetic Field ◽  
Disk Component ◽  
Spiral Arm

The distribution of rotation measures for 86 sources suggests a two-component model for the magnetic field: a disk component directed towardl= 95°, and a component in the local spiral arm, directed alongl= 70° and 250°, with opposite senses above and below the plane. The latter may be due to a looped field in a cloud surrounding the Sun; its net flux is 3·5 micro-gauss.

Low-frequency seismology and the three-dimensional structure of the Earth

1989 ◽  
Vol 328 (1599) ◽  
pp. 329-349 ◽  
Keyword(s):  
Large Scale ◽  
Inner Core ◽  
Three Dimensional ◽  
Low Frequency ◽  
Quantitative Agreement ◽  
Phase Behaviour ◽  
Dimensional Structure ◽  
Order Structure ◽  
Three Dimensional Structure ◽  
The Earth

We attempt to catalogue those features of the three-dimensional structure of the Earth that are well-constrained by low-frequency data (i.e. periods longer than about 125 seconds). The dominant signals in such data are the surface-wave equivalent modes whose phase characteristics are mainly affected by a large scale structure of harmonic degree two in the upper mantle. Available aspherical models predict this phase behaviour quite well, but do not give an accurate prediction of the observed waveforms and we must appeal to higher-order structure an d /o r coupling effects to give the observed complexity of the data. Strong splitting of modes which sample the cores of the Earth is also observed and, though we do not yet have a model of aspherical structure which gives quantitative agreement with these data, anisotropy or large-scale aspherical structure in the inner core appears to be required to model the observed signal.

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