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Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Nordic Institute for Theoretical Physics (Nordita).
2011 (English)In: Astrophysical Journal, ISSN 0004-637X, E-ISSN 1538-4357, Vol. 734, no 1, 9- p.Article in journal (Refereed) Published
Abstract [en]

We determine the magnetic helicity, along with the magnetic energy, at high latitudes using data from the Ulysses mission. The data set spans the time period from 1993 to 1996. The basic assumption of the analysis is that the solar wind is homogeneous. Because the solar wind speed is high, we follow the approach first pioneered by Matthaeus et al. by which, under the assumption of spatial homogeneity, one can use Fourier transforms of the magnetic field time series to construct one-dimensional spectra of the magnetic energy and magnetic helicity under the assumption that the Taylor frozen-in-flow hypothesis is valid. That is a well-satisfied assumption for the data used in this study. The magnetic helicity derives from the skew-symmetric terms of the three-dimensional magnetic correlation tensor, while the symmetric terms of the tensor are used to determine the magnetic energy spectrum. Our results show a sign change of magnetic helicity at wavenumber k approximate to 2 AU(-1) (or frequency nu approximate to 2 mu Hz) at distances below 2.8 AU and at k approximate to 30 AU(-1) (or nu approximate to 25 mu Hz) at larger distances. At small scales the magnetic helicity is positive at northern heliographic latitudes and negative at southern latitudes. The positive magnetic helicity at small scales is argued to be the result of turbulent diffusion reversing the sign relative to what is seen at small scales at the solar surface. Furthermore, the magnetic helicity declines toward solar minimum in 1996. The magnetic helicity flux integrated separately over one hemisphere amounts to about 10(45) Mx(2) cycle(-1) at large scales and to a three times lower value at smaller scales.

Place, publisher, year, edition, pages
2011. Vol. 734, no 1, 9- p.
Keyword [en]
magnetohydrodynamics (MHD), turbulence
National Category
Astronomy, Astrophysics and Cosmology
URN: urn:nbn:se:su:diva-67565DOI: 10.1088/0004-637X/734/1/9ISI: 000291026900009OAI: diva2:470609
authorCount :4Available from: 2011-12-29 Created: 2011-12-29 Last updated: 2011-12-29Bibliographically approved

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Brandenburg, Axel
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