SCALE DEPENDENCE OF MAGNETIC HELICITY IN THE SOLAR WIND

Journal Article · Fri Jun 10 00:00:00 EDT 2011 · Astrophysical Journal

DOI:https://doi.org/10.1088/0004-637X/734/1/9· OSTI ID:21576700

Brandenburg, Axel ^[1]; Subramanian, Kandaswamy ^[2]; Balogh, Andre ^[3]

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1. NORDITA, AlbaNova University Center, Roslagstullsbacken 23, SE-10691 Stockholm (Sweden)
2. Inter-University Centre for Astronomy and Astrophysics, Post Bag 4, Ganeshkhind, Pune 411 007 (India)
3. International Space Science Institute, Hallerstrasse 6, Bern CH-3012 (Switzerland)

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 {approx} 2 AU{sup -1} (or frequency {nu} {approx} 2 {mu}Hz) at distances below 2.8 AU and at k {approx} 30 AU{sup -1} (or {nu} {approx} 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{sup 45} Mx{sup 2} cycle{sup -1} at large scales and to a three times lower value at smaller scales.

• OSTI ID: 21576700
• Journal Information: Astrophysical Journal, Vol. 734, Issue 1; Other Information: DOI: 10.1088/0004-637X/734/1/9; ISSN 0004-637X
• Country of Publication: United States
• Language: English

Related Subjects

79 ASTROPHYSICS
COSMOLOGY AND ASTRONOMY
CORRELATIONS
DIFFUSION
FOURIER TRANSFORMATION
HELICITY
MAGNETIC FIELDS
MAGNETOHYDRODYNAMICS
SOLAR WIND
TENSORS
THREE-DIMENSIONAL CALCULATIONS
TURBULENCE
FLUID MECHANICS
HYDRODYNAMICS
INTEGRAL TRANSFORMATIONS
MECHANICS
PARTICLE PROPERTIES
SOLAR ACTIVITY
STELLAR ACTIVITY
STELLAR WINDS
TRANSFORMATIONS