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          Institute: MPI für Radioastronomie     Collection: Publikationen des MPI für Radioastronomie     Display Documents



ID: 125337.0, MPI für Radioastronomie / Publikationen des MPI für Radioastronomie
Systematic bias in interstellar magnetic field estimates
Authors:Beck, R.; Shukurov, A.; Sokoloff, D.; Wielebinski, R.
Language:English
Date of Publication (YYYY-MM-DD):2003-11
Title of Journal:Astronomy & Astrophysics
Journal Abbrev.:Astron. Astrophys.
Volume:411
Issue / Number:2
Start Page:99
End Page:107
Review Status:Peer-review
Audience:Experts Only
Abstract / Description:Faraday rotation of the polarization plane in magnetized thermal plasma provides one of the most efficient methods to deduce regular magnetic fields from radio astronomical observations. Since the Faraday rotation measure RM is proportional to an integral, along the line of sight, of magnetic field weighted with thermal electron density, RM is believed to yield the regular magnetic field averaged over large volume. Here we show that this is not the case in a turbulent medium where fluctuations in magnetic field and electron density are not statistically independent, and so contribute to RM. For example, in the case of pressure equilibrium, magnetic field can be anticorrelated with plasma density to produce a negative contribution. As a result, the strength of the regular magnetic field obtained from RM can be underestimated if the fluctuations in electron density and magnetic field are neglected. The anticorrelation also reduces the standard deviation of RM. We further discuss the effect of the positive correlations where the standard treatment of RM leads to an overestimated magnetic field. Because of the anisotropy of the turbulent magnetic field, the regular magnetic fields strength, obtained from synchrotron emission using standard formulae, can be overestimated. A positive correlation between cosmic-ray number density and magnetic field leads to an overestimate of the strengths of the regular and total fields. These effects can explain the difference between the strengths of the regular Galactic magnetic field as indicated by RM and synchrotron emissivity data and reconcile the magnetic field strength in the Solar vicinity with typical strength of regular magnetic fields in external galaxies.
Free Keywords:magnetic fields, polarization, turbulence, ISM : magnetic fields, galaxies : ISM
Comment of the Author/Creator:Date: 2003, NOV
External Publication Status:published
Document Type:Article
Communicated by:N. N.
Affiliations:MPI für Radioastronomie
External Affiliations:Max Planck Inst Radioastron, D-53121 Bonn, Germany.; Univ Newcastle Upon Tyne, Sch Math & Stat, Newcastle Upon Tyne NE1 7RU, Tyne & Wear, England.; Moscow MV Lomonosov State Univ, Dept Phys, Moscow 119992, Russia.
Identifiers:ISI:000186536600015 [ID No:1]
ISSN:0004-6361 [ID No:2]
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