Galactic and Extragalactic Magnetic Fields

By Rainer Beck. 2000

339 citations recorded by the source corpus.

The current state of research of the Galactic magnetic field is reviewed critically. The average strength of the total field derived from radio synchrotron data, under the energy equipartition assumption, is 6±2 μG locally and about 10±3 μG at 3 kpc Galactic radius. These values agree well with the estimates using the locally measured cosmic-ray energy spectrum and the radial variation of protons derived from γ-rays. Optical and synchrotron polarization data yield a strength of the local regular field of 4±1 μG, but this value is an upper limit if the field strength fluctuates within the beam or if anisotropic fields are present. Pulsar rotation measures, on the other hand, give only 1.4±0.2 μG, a lower limit if fluctuations in regular field strength and thermal electron density are anticorrelated along the pathlength. The local regular field may be part of a `magnetic arm’ between the optical arms. However, the global structure of the regular Galactic field is not yet known. Several large-scale field reversals in the Galaxy were detected from rotation measure data, but a similar phenomenon was not observed in external galaxies. The Galactic field may be young in terms of dynamo action so that reversals from the chaotic seed field are preserved, or a mixture of dynamo modes causes the reversals, or the reversals are signatures of large-scale anisotropic field loops. The Galaxy is surrounded by a thick disk of radio continuum emission of similar extent as in edge-on spiral galaxies. While the local field in the thin disk is of even symmetry with respect to the plane (quadrupole), the global thick-disk field may be of dipole type. The Galactic center region hosts highly regular fields of up to milligauss strength which are oriented perpendicular to the plane. A major extension of the data base of pulsar rotation measures and Zeeman splitting measurements is required to determine the structure of the Galactic field. Further polarization surveys of the Galactic plane at wavelengths of 6 cm or shorter may directly reveal the fine structure of the local magnetic field.

Subfields: Astronomy and Astrophysics, Nuclear and High Energy Physics

View Paper: https://www.semanticscholar.org/paper/8175b64a5ab15170bb3c85dfa7d657b7e9ee8397

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Citations in the corpus

DERIVING THE GLOBAL STRUCTURE OF THE GALACTIC MAGNETIC FIELD FROM FARADAY ROTATION MEASURES OF EXTRAGALACTIC SOURCES (2011, background): “The magnetic field (MF) of our Galaxy is thought to contain both regular and turbulent components (see, e.g., Beck (2001, 2008)).”
Galactic winds driven by cosmic ray streaming (2012, background): “This is expected given the near equipartition of energy in the Milky Way disc between magnetic fields, CRs and the thermal pressure (Zweibel & Heiles 1997; Beck 2001; Cox 2005).”
Cosmology with inhomogeneous magnetic fields (2006, background): “However, the galactic dynamo cannot work without an initial seed field [18]-[20] and these seeds may require B-fields of primeval origin [21]-[44].”
Simulating the Formation of Molecular Clouds. II. Rapid Formation from Turbulent Initial Conditions (2006, background): “Magnetic Field Strength Since there is now considerable observational evidence for the presence of dynamically significant magnetic fields in interstellar gas (Beck 2001; Heiles & Crutcher 2005), we included a magnetic field in the majority of our simulations.”
Astrophysical magnetic fields and nonlinear dynamo theory (2004, background): “Coherence of RM on a large scale is indeed seen in a number of galaxies (for example, M31 [95], NGC 6946 [74], NGC 2997 [97]).”
THE PHYSICS OF THE FAR-INFRARED–RADIO CORRELATION. I. CALORIMETRY, CONSPIRACY, AND IMPLICATIONS (2009, methodology): “…parametrize the global average magnetic field of galaxies as B = 6 ( Σg 0.0025 g cm−2 )a µG, (12) where a is determined from comparing with the FRC, and where the normalization has been chosen to match fiducial numbers at the Solar Circle (as in Boulares & Cox 1990; Strong et al. 2000; Beck 2001).”
Science with ASKAP (2008, background): “External galaxies often show magnetic field structures inclined or even perpendicular to the plane [17].”
The physics of galactic winds driven by active galactic nuclei (2012, background): “In ordinary spiral galaxies, B ∼ 1− 10 µG (Beck 2001) and synchrotron cooling is clearly subdominant.”
Separation of anomalous and synchrotron emissions using WMAP polarization data (2008, background): “On the other hand, and similarly to Sun et al. (2007) who did not consider the presence of anomalous emission at 23 GHz, model 1 requires Bturb/B0 ∼ 1.5 to fit the data, which is significantly stronger than the value obtained by Phillipps et al. (1981); Beck (2001).”
LATE-TIME COSMOLOGY WITH 21 cm INTENSITY MAPPING EXPERIMENTS (2014, neutral): “The galactic magnetic field is a non-linear superposition of an overall coherent mode, tied to the spiral structure of the Galaxy, with a turbulent stochastic mode on small scales (Beck 2001).”
Radio observational constraints on Galactic 3D-emission models (2007, background): “1 in Beck 2001).”
Magnetism in the spiral galaxy NGC 6946: magnetic arms, depolarization rings, dynamo modes, and helical fields (2007, background): “In the local Milky Way, starlight polarization and radio synchrotron data yield an average value of q 0.6 (see Beck 2001 for a review).”
The 511 keV emission from positron annihilation in the Galaxy (2010, background): “Besides, synchrotron polarimetry indicates that the local ratio of ordered (regular + anisotropic random) to total fields is ≃ 0.6 (Beck, 2001), implying an ordered field ≃ 3 µG near the Sun.”