Scaledependence of magnetic helicity in the solar wind 

Axel Brandenburg Submitted: 20120401 23:18
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 onedimensional spectra of the magnetic energy and magnetic helicity under the assumption that the Taylor frozeninflow hypothesis is valid. That is a wellsatisfied assumption for the data used in this study. The magnetic helicity derives from the skewsymmetric terms of the threedimensional 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 ≈ 2 AU1 (or frequency ν ≈ 2 μHz) at distances below 2.8 AU and at k ≈ 30 AU1 (or ν ≈ 25 μHz) 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 1045 Mx2 cycle1 at large scales and to a three times lower value at smaller scales.
Authors: Brandenburg, A., Subramanian, K., Balogh, A., & Goldstein, M. L.
Projects: None

Publication Status: Astrophys. J. 734, 9 (2011)
Last Modified: 20120404 08:13


