scholarly journals Magnetic fields in elliptical galaxies: using the Laing–Garrington effect in radio galaxies and polarized emission from background radio sources

2021 ◽  
Vol 508 (1) ◽  
pp. 1371-1388
Author(s):  
Hilay Shah ◽  
Amit Seta
Keyword(s):  
Magnetic Fields ◽  
Galaxy Evolution ◽  
Large Scale ◽  
Elliptical Galaxies ◽  
Small Scale ◽  
Radio Sources ◽  
Scale Field ◽  
Order Of Magnitude ◽  
Non Gaussian ◽  
Polarization Fraction

ABSTRACT Magnetic fields in elliptical galaxies are poorly constrained because of a lack of significant synchrotron emission from them. In this paper, we explore the properties of magnetic fields in ellipticals using two methods. First, we exploit the Laing–Garrington effect (asymmetry in the observed polarization fraction between radio galaxy jets) for 57 galaxies with redshifts up to 0.5. We use the differences in polarization fraction and rotation measure between the jet and counterjet to estimate the small- and large-scale magnetic fields in and around ellipticals (including their circumgalactic medium). We find that the small-scale field (at scales smaller than the driving scale of turbulence, approximately 300 pc) lies in the range 0.1–2.75 $\mu{\rm G}$. The large-scale field (at scales of 100 kpc) is an order of magnitude smaller than the small-scale field. In the second method, we cross-match the Faraday rotation measures (RMs) of a few hundred (out of 3098) extragalactic radio sources with galaxy catalogues to explore the effect of the number and morphology of intervening galaxies on the observed RM distribution. We use both Gaussian and non-Gaussian functions to describe the RM distribution and to derive its statistical properties. Finally, using the difference in the observed polarization fraction between the intervening spirals and ellipticals, we estimate the small-scale magnetic fields at the centre of ellipticals to be ∼6 $\mu{\rm G}$. Both methods with different observations and analysis techniques give magnetic field strengths consistent with previous studies (≤10 $\mu{\rm G}$). The results can be used to constrain dynamo theories and galaxy evolution simulations.

scholarly journals Viscosity and Large-Scale Magnetic Fields from Accretion Disc Dynamos

1997 ◽  
Vol 163 ◽  
pp. 190-200
Author(s):  
Christopher A. Tout
Keyword(s):  
Magnetic Fields ◽  
Large Scale ◽  
Small Scale ◽  
Cascade Process ◽  
Jet Formation ◽  
Magnetohydrodynamic Turbulence ◽  
Discs Large ◽  
Scale Field ◽  
Large Scale Field ◽  
Nova Outbursts

AbstractWe review those processes associated with accretion discs that are probably influenced by magnetic fields, specifically, accretiondisc viscosity, energy dissipation and jet formation. We consider how magnetic instabilities in the disc can lead to a self-sustaining dynamical dynamo and how this is manifested as magnetohydrodynamic turbulence in numerical simulations. We show that currently these models do not fit with observational constraints imposed by dwarf-nova outbursts. We also show that the drop in ionisation fraction does not lead to the apparently necessary drop in viscosity in quiescent cataclysmic variable discs. Large-scale magnetic fields are required to launch and collimate jets form discs. We describe an inverse cascade process that can construct sufficient large-scale field from small-scale field generated by a dynamo.

scholarly journals Magnetic Fields in our Galaxy: How much do we Know? I. Disk fields within a few kpc of the Sun

2001 ◽  
Vol 182 ◽  
pp. 181-184
Author(s):  
J.L. Han
Keyword(s):  
Magnetic Fields ◽  
Field Strength ◽  
Random Fields ◽  
Pitch Angle ◽  
Large Scale ◽  
Radio Sources ◽  
The Sun ◽  
Total Field ◽  
Scale Field ◽  
Large Scale Field

AbstractThe large scale magnetic fields of our Galaxy have been mostly revealed by rotation measures (RMs) of pulsars and extragalactic radio sources. In the disk of our Galaxy, the average field strength over a few kpc scale is about 1.8 μG, while the total field, including the random fields on smaller scales, has a strength of about 5 μG. The local regular field, if it is part of the large scale field of a bisymmetric form, has a pitch angle of about −8°. There are at least three, and perhaps five, field reversals from the Norma arm to the outer skirt of our Galaxy.

Fluctuations in the Galactic Magnetic Field

1990 ◽  
Vol 140 ◽  
pp. 55-58
Author(s):  
James M. Cordes ◽  
Andrew Clegg ◽  
John Simonetti
Keyword(s):  
Magnetic Field ◽  
Large Scale ◽  
Measure Data ◽  
Small Scale ◽  
Radio Sources ◽  
Galactic Magnetic Field ◽  
Rotation Measure ◽  
Scale Field ◽  
Large Scale Field ◽  
The Magnetic Field

We discuss small scale structure in the Galactic magnetic field as inferred from Faraday rotation measurements of extragalactic radio sources. The rotation measure data suggest a continuum of length scales extending from parsec scales down to at least 0.01 pc and perhaps to as small as 109 cm. Such turbulence in the magnetic field comprises a reservoir of energy that is comparable to the energy in the large scale field.

scholarly journals Global magnetic fields: variation of solar minima

2011 ◽  
Vol 7 (S286) ◽  
pp. 113-122
Author(s):  
Andrey G. Tlatov ◽  
Vladimir N. Obridko
Keyword(s):  
Magnetic Field ◽  
Magnetic Fields ◽  
Large Scale ◽  
Sunspot Cycle ◽  
Magnetic Activity ◽  
Small Scale ◽  
The Sun ◽  
Large Scale Magnetic Field ◽  
Scale Field ◽  
Large Scale Field

AbstractThe topology of the large-scale magnetic field of the Sun and its role in the development of magnetic activity were investigated using Hα charts of the Sun in the period 1887-2011. We have considered the indices characterizing the minimum activity epoch, according to the data of large-scale magnetic fields. Such indices include: dipole-octopole index, area and average latitude of the field with dominant polarity in each hemisphere and others. We studied the correlation between these indices and the amplitude of the following sunspot cycle, and the relation between the duration of the cycle of large-scale magnetic fields and the duration of the sunspot cycle.The comparative analysis of the solar corona during the minimum epochs in activity cycles 12 to 24 shows that the large-scale magnetic field has been slow and steadily changing during the past 130 years. The reasons for the variations in the solar coronal structure and its relation with long-term variations in the geomagnetic indices, solar wind and Gleissberg cycle are discussed.We also discuss the origin of the large-scale magnetic field. Perhaps the large-scale field leads to the generation of small-scale bipolar ephemeral regions, which in turn support the large-scale field. The existence of two dynamos: a dynamo of sunspots and a surface dynamo can explain phenomena such as long periods of sunspot minima, permanent dynamo in stars and the geomagnetic field.

The macrodynamics of α-effect dynamos in rotating fluids

1975 ◽  
Vol 67 (3) ◽  
pp. 417-443 ◽  
Author(s):  
W. V. R. Maekus ◽  
M. R. E. Proctor
Keyword(s):  
Magnetic Fields ◽  
Large Scale ◽  
Magnetic Energy ◽  
Finite Amplitude ◽  
Small Scale ◽  
Past Study ◽  
Ohmic Loss ◽  
General Will ◽  
Rotating Systems ◽  
Stellar Magnetic Fields

Past study of the large-scale consequences of forced small-scale motions in electrically conducting fluids has led to the ‘α-effect’ dynamos. Various linear kinematic aspects of these dynamos have been explored, suggesting their value in the interpretation of observed planetary and stellar magnetic fields. However, large-scale magnetic fields with global boundary conditions can not be force free and in general will cause large-scale motions as they grow. I n this paper the finite amplitude behaviour of global magnetic fields and the large-scale flows induced by them in rotating systems is investigated. In general, viscous and ohmic dissipative mechanisms both play a role in determining the amplitude and structure of the flows and magnetic fields which evolve. In circumstances where ohmic loss is the principal dissipation, it is found that determination of a geo- strophic flow is an essential part of the solution of the basic stability problem. Nonlinear aspects of the theory include flow amplitudes which are independent of the rotation and a total magnetic energy which is directly proportional to the rotation. Constant a is the simplest example exhibiting the various dynamic balances of this stabilizing mechanism for planetary dynamos. A detailed analysis is made for this case to determine the initial equilibrium of fields and flows in a rotating sphere.

scholarly journals The magnetic structure of our Galaxy: a review of observations

2008 ◽  
Vol 4 (S259) ◽  
pp. 455-466 ◽  
Author(s):  
JinLin Han
Keyword(s):  
Magnetic Fields ◽  
Magnetic Structure ◽  
Large Scale ◽  
Galactic Halo ◽  
Galactic Center ◽  
Galactic Plane ◽  
Galactic Disk ◽  
Dust Emission ◽  
Measure Data ◽  
Radio Sources

AbstractThe magnetic structure in the Galactic disk, the Galactic center and the Galactic halo can be delineated more clearly than ever before. In the Galactic disk, the magnetic structure has been revealed by starlight polarization within 2 or 3 kpc of the Solar vicinity, by the distribution of the Zeeman splitting of OH masers in two or three nearby spiral arms, and by pulsar dispersion measures and rotation measures in nearly half of the disk. The polarized thermal dust emission of clouds at infrared, mm and submm wavelengths and the diffuse synchrotron emission are also related to the large-scale magnetic field in the disk. The rotation measures of extragalactic radio sources at low Galactic latitudes can be modeled by electron distributions and large-scale magnetic fields. The statistical properties of the magnetized interstellar medium at various scales have been studied using rotation measure data and polarization data. In the Galactic center, the non-thermal filaments indicate poloidal fields. There is no consensus on the field strength, maybe mG, maybe tens of μG. The polarized dust emission and much enhanced rotation measures of background radio sources are probably related to toroidal fields. In the Galactic halo, the antisymmetric RM sky reveals large-scale toroidal fields with reversed directions above and below the Galactic plane. Magnetic fields from all parts of our Galaxy are connected to form a global field structure. More observations are needed to explore the untouched regions and delineate how fields in different parts are connected.

scholarly journals A Field Efficacy Evaluation of In2Care Mosquito Traps in Comparison with Routine Integrated Vector Management at Reducing Aedes aegypti

2021 ◽  
Vol 37 (4) ◽  
pp. 242-249
Author(s):  
Eva A. Buckner ◽  
Katie F. Williams ◽  
Samantha Ramirez ◽  
Constance Darrisaw ◽  
Juliana M. Carrillo ◽  
...  
Keyword(s):  
Aedes Aegypti ◽  
Large Scale ◽  
Pathogen Transmission ◽  
Small Scale ◽  
Integrated Vector Management ◽  
Control Programs ◽  
Vector Management ◽  
Scale Field ◽  
Large Scale Field ◽  
Eggs And Larvae

ABSTRACT Aedes aegypti is the predominant vector of dengue, chikungunya, and Zika viruses. This mosquito is difficult to control with conventional methods due to its container-inhabiting behavior and resistance to insecticides. Autodissemination of pyriproxyfen (PPF), a potent larvicide, has shown promise as an additional tool to control Aedes species in small-scale field trials. However, few large-scale field evaluations have been conducted. We undertook a 6-month-long large-scale field study to compare the effectiveness and operational feasibility of using In2Care Mosquito Traps (In2Care Traps, commercially available Aedes traps with PPF and Beauveria bassiana) compared to an integrated vector management (IVM) strategy consisting of source reduction, larviciding, and adulticiding for controlling Ae. aegypti eggs, larvae, and adults. We found that while the difference between treatments was only statistically significant for eggs and larvae (P < 0.05 for eggs and larvae and P > 0.05 for adults), the use of In2Care Traps alone resulted in 60%, 57%, and 57% fewer eggs, larvae, and adults, respectively, collected from that site compared to the IVM site. However, In2Care Trap deployment and maintenance were more time consuming and labor intensive than the IVM strategy. Thus, using In2Care Traps alone as a control method for large areas (e.g., >20 ha) may be less practical for control programs with the capacity to conduct ground and aerial larviciding and adulticiding. Based on our study results, we conclude that In2Care Traps are effective at suppressing Ae. aegypti and have the most potential for use in areas without sophisticated control programs and within IVM programs to target hotspots with high population levels and/or risk of Aedes-borne pathogen transmission.

scholarly journals Magnetosheath-cusp interface

2004 ◽  
Vol 22 (1) ◽  
pp. 183-212 ◽  
Author(s):  
S. Savin ◽  
L. Zelenyi ◽  
S. Romanov ◽  
I. Sandahl ◽  
J. Pickett ◽  
...  
Keyword(s):  
Boundary Layer ◽  
Solar Wind ◽  
Magnetic Fields ◽  
Large Scale ◽  
Traditional Approach ◽  
Qualitative Difference ◽  
Small Scale ◽  
Nonlinear Phenomena ◽  
Solar Wind Interaction ◽  
Linear Superposition

Abstract. We advance the achievements of Interball-1 and other contemporary missions in exploration of the magnetosheath-cusp interface. Extensive discussion of published results is accompanied by presentation of new data from a case study and a comparison of those data within the broader context of three-year magnetopause (MP) crossings by Interball-1. Multi-spacecraft boundary layer studies reveal that in ∼80% of the cases the interaction of the magnetosheath (MSH) flow with the high latitude MP produces a layer containing strong nonlinear turbulence, called the turbulent boundary layer (TBL). The TBL contains wave trains with flows at approximately the Alfvén speed along field lines and "diamagnetic bubbles" with small magnetic fields inside. A comparison of the multi-point measurements obtained on 29 May 1996 with a global MHD model indicates that three types of populating processes should be operative: large-scale (∼few RE) anti-parallel merging at sites remote from the cusp; medium-scale (few thousandkm) local TBL-merging of fields that are anti-parallel on average; small-scale (few hundredkm) bursty reconnection of fluctuating magnetic fields, representing a continuous mechanism for MSH plasma inflow into the magnetosphere, which could dominate in quasi-steady cases. The lowest frequency (∼1–2mHz) TBL fluctuations are traced throughout the magnetosheath from the post-bow shock region up to the inner magnetopause border. The resonance of these fluctuations with dayside flux tubes might provide an effective correlative link for the entire dayside region of the solar wind interaction with the magnetopause and cusp ionosphere. The TBL disturbances are characterized by kinked, double-sloped wave power spectra and, most probably, three-wave cascading. Both elliptical polarization and nearly Alfvénic phase velocities with characteristic dispersion indicate the kinetic Alfvénic nature of the TBL waves. The three-wave phase coupling could effectively support the self-organization of the TBL plasma by means of coherent resonant-like structures. The estimated characteristic scale of the "resonator" is of the order of the TBL dimension over the cusps. Inverse cascades of kinetic Alfvén waves are proposed for forming the larger scale "organizing" structures, which in turn synchronize all nonlinear cascades within the TBL in a self-consistent manner. This infers a qualitative difference from the traditional approach, wherein the MSH/cusp interaction is regarded as a linear superposition of magnetospheric responses on the solar wind or MSH disturbances. Key words. Magnetospheric physics (magnetopause, cusp, and boundary layers) – Space plasma physics (turbulence; nonlinear phenomena)

scholarly journals Galactic Dynamos — a Challenge for Observers

1993 ◽  
Vol 157 ◽  
pp. 283-297
Author(s):  
Rainer Beck
Keyword(s):  
Large Scale ◽  
General Distribution ◽  
Future Research ◽  
Small Scale ◽  
Dynamo Theory ◽  
Galactic Winds ◽  
Scale Field ◽  
Field Lines ◽  
Galactic Dynamos ◽  
The Magnetic Field

Results of linear αΩ-dynamo models are confronted with radio polarization observations of spiral galaxies. The general distribution of polarized emission and the magnetic field pitch angle can be described with sufficient accuracy. The occurrance of systematic large-scale variations in Faraday rotation (RM) is the strongest argument in favour of dynamo theory. However, the predominance of axisymmetric SO modes could not be confirmed by observations; S1 modes are about equally frequent. The azimuthal variations of field pitch angles and, in two cases, the phases of the RM variations are inconsistent with a classical αΩ-dynamo. Locally deviating RM values indicate field lines bending out of the plane. There is increasing evidence that galactic fields cannot be described by simple dynamo modes. This calls for more realistic dynamo models, taking into account non-axisymmetric velocity fields and galactic winds.Interpretation of radio observations is difficult because Faraday depolarization can seriously affect the data. Observations of small-scale field structures are summarized which show the path for future research. Instrumental needs for such investigations are discussed.

scholarly journals Characterising the surface magnetic fields of T Tauri stars with high-resolution near-infrared spectroscopy

2019 ◽  
Vol 630 ◽  
pp. A99 ◽  
Author(s):  
A. Lavail ◽  
O. Kochukhov ◽  
G. A. J. Hussain
Keyword(s):  
Magnetic Field ◽  
High Resolution ◽  
Magnetic Fields ◽  
Large Scale ◽  
Near Infrared ◽  
Field Measurements ◽  
T Tauri Stars ◽  
Small Scale ◽  
Surface Magnetic Field ◽  
T Tauri

Aims. In this paper, we aim to characterise the surface magnetic fields of a sample of eight T Tauri stars from high-resolution near-infrared spectroscopy. Some stars in our sample are known to be magnetic from previous spectroscopic or spectropolarimetric studies. Our goals are firstly to apply Zeeman broadening modelling to T Tauri stars with high-resolution data, secondly to expand the sample of stars with measured surface magnetic field strengths, thirdly to investigate possible rotational or long-term magnetic variability by comparing spectral time series of given targets, and fourthly to compare the magnetic field modulus ⟨B⟩ tracing small-scale magnetic fields to those of large-scale magnetic fields derived by Stokes V Zeeman Doppler Imaging (ZDI) studies. Methods. We modelled the Zeeman broadening of magnetically sensitive spectral lines in the near-infrared K-band from high-resolution spectra by using magnetic spectrum synthesis based on realistic model atmospheres and by using different descriptions of the surface magnetic field. We developped a Bayesian framework that selects the complexity of the magnetic field prescription based on the information contained in the data. Results. We obtain individual magnetic field measurements for each star in our sample using four different models. We find that the Bayesian Model 4 performs best in the range of magnetic fields measured on the sample (from 1.5 kG to 4.4 kG). We do not detect a strong rotational variation of ⟨B⟩ with a mean peak-to-peak variation of 0.3 kG. Our confidence intervals are of the same order of magnitude, which suggests that the Zeeman broadening is produced by a small-scale magnetic field homogeneously distributed over stellar surfaces. A comparison of our results with mean large-scale magnetic field measurements from Stokes V ZDI show different fractions of mean field strength being recovered, from 25–42% for relatively simple poloidal axisymmetric field topologies to 2–11% for more complex fields.

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