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An observational test for correlations between cosmic rays and magnetic fields

Lookup NU author(s): Dr Rodin Stepanov, Professor Anvar ShukurovORCiD


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We derive the magnitude of fluctuations in total synchrotron intensity in the Milky Way and M33, from both observations and theory under various assumptions about the relation between cosmic rays and interstellar magnetic fields. Given the relative magnitude of the fluctuations in the Galactic magnetic field (the ratio of the rms fluctuations to the mean magnetic field strength) suggested by Faraday rotation and synchrotron polarization, the observations are inconsistent with local energy equipartition between cosmic rays and magnetic fields. Our analysis of relative synchrotron intensity fluctuations indicates that the distribution of cosmic rays is nearly uniform at the scales of the order of and exceeding 100 pc, in contrast to strong fluctuations in the interstellar magnetic field at those scales. A conservative upper limit on the ratio of the fluctuation magnitude in the cosmic ray number density to its mean value is 0.2-0.4 at scales of the order of 100 pc. Our results are consistent with a mild anticorrelation between cosmic ray and magnetic energy densities at these scales, in both the Milky Way and M33. Energy equipartition between cosmic rays and magnetic fields may still hold, but at scales exceeding 1 kpc. Therefore, we suggest that equipartition estimates be applied to the observed synchrotron intensity smoothed to a linear scale of kiloparsec order (in spiral galaxies) to obtain the cosmic ray distribution and a large-scale magnetic field. Then the resulting cosmic ray distribution can be used to derive the fluctuating magnetic field strength from the data at the original resolution. The resulting random magnetic field is likely to be significantly stronger than existing estimates.

Publication metadata

Author(s): Stepanov R, Shukurov A, Fletcher A, Beck R, La Porta L, Tabatabaei F

Publication type: Article

Publication status: Published

Journal: Monthly Notices of the Royal Astronomical Society.

Year: 2014

Volume: 437

Issue: 3

Pages: 2201-2216

Print publication date: 20/11/2013

ISSN (print): 0035-8711

ISSN (electronic): 1365-2966

Publisher: Oxford University Press


DOI: 10.1093/mnras/stt2044


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Funder referenceFunder name
Newcastle University
Royal Society
NSF PHY05-51164National Science Foundation
YD-520.2013.2Council of the President of the Russian Federation
RPG-097Leverhulme Trust