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Issue A&A Volume 411, Number 2, November IV 2003 Page(s) 99 - 107 Section Formation, structure and evolution of stars DOI 10.1051/0004-6361:20031101

A&A 411, 99-107 (2003)
DOI: 10.1051/0004-6361:20031101

Systematic bias in interstellar magnetic field estimates

R. Beck¹, A. Shukurov^{1, 2}, D. Sokoloff ³ and R. Wielebinski<sup>1

1</sup>  Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
²  School of Mathematics and Statistics, University of Newcastle, Newcastle upon Tyne, NE1 7RU, UK
³  Department of Physics, Moscow State University, 119992 Moscow, Russia

(Received 6 March 2003 / Accepted 30 June 2003)

Abstract
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.

Key words: magnetic fields -- polarization -- turbulence -- ISM : magnetic fields -- galaxies: ISM

Offprint request: R. Beck, rbeck@mpifr-bonn.mpg.de

© ESO 2003

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