Change search
ReferencesLink to record
Permanent link

Direct link
Faraday signature of magnetic helicity from reduced depolarization
KTH, Centres, Nordic Institute for Theoretical Physics NORDITA.ORCID iD: 0000-0002-7304-021X
2014 (English)In: Astrophysical Journal, ISSN 0004-637X, E-ISSN 1538-4357, Vol. 786, no 2, 91- p.Article in journal (Refereed) Published
Abstract [en]

Using one-dimensional models, we show that a helical magnetic field with an appropriate sign of helicity can compensate the Faraday depolarization resulting from the superposition of Faraday-rotated polarization planes from a spatially extended source. For radio emission from a helical magnetic field, the polarization as a function of the square of the wavelength becomes asymmetric with respect to zero. Mathematically speaking, the resulting emission occurs then either at observable or at unobservable (imaginary) wavelengths. We demonstrate that rotation measure (RM) synthesis allows for the reconstruction of the underlying Faraday dispersion function in the former case, but not in the latter. The presence of positive magnetic helicity can thus be detected by observing positive RM in highly polarized regions in the sky and negative RM in weakly polarized regions. Conversely, negative magnetic helicity can be detected by observing negative RM in highly polarized regions and positive RM in weakly polarized regions. The simultaneous presence of two magnetic constituents with opposite signs of helicity is shown to possess signatures that can be quantified through polarization peaks at specific wavelengths and the gradient of the phase of the Faraday dispersion function. Similar polarization peaks can tentatively also be identified for the bi-helical magnetic fields that are generated self-consistently by a dynamo from helically forced turbulence, even though the magnetic energy spectrum is then continuous. Finally, we discuss the possibility of detecting magnetic fields with helical and non-helical properties in external galaxies using the Square Kilometre Array.

Place, publisher, year, edition, pages
2014. Vol. 786, no 2, 91- p.
Keyword [en]
galaxies: magnetic fields, methods: data analysis, polarization
National Category
Physical Sciences
URN: urn:nbn:se:kth:diva-146552DOI: 10.1088/0004-637X/786/2/91ISI: 000335884500015ScopusID: 2-s2.0-84899460053OAI: diva2:724237
EU, European Research Council, 227952Swedish Research Council, 621-2011-5076 2012-5797

QC 20140612

Available from: 2014-06-12 Created: 2014-06-12 Last updated: 2014-06-12Bibliographically approved

Open Access in DiVA

No full text

Other links

Publisher's full textScopus

Search in DiVA

By author/editor
Brandenburg, Axel
By organisation
Nordic Institute for Theoretical Physics NORDITA
In the same journal
Astrophysical Journal
Physical Sciences

Search outside of DiVA

GoogleGoogle Scholar
The number of downloads is the sum of all downloads of full texts. It may include eg previous versions that are now no longer available

Altmetric score

Total: 19 hits
ReferencesLink to record
Permanent link

Direct link