NASA Logo

NTRS

NTRS - NASA Technical Reports Server

Back to Results
scale dependence of magnetic helicity in the solar windWe 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 approximately equal to 2AU(sup -1) (or frequency nu approximately equal to 2 microHz) at distances below 2.8AU and at k approximately equal to 30AU(sup -1) (or nu approximately equal to 25 microHz) 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.
Document ID
20110015361
Document Type
Reprint (Version printed in journal)
Authors
Brandenburg, Axel
(AlbaNova Univ. Center)
Subramanian, Kandaswamy
(Inter-University Centre for Astronomy and Astrophysics Pune, India)
Balogh, Andre
(International Space Science Inst. Bern, Switzerland)
Goldstein, Melvyn L.
(NASA Goddard Space Flight Center Greenbelt, MD, United States)
Date Acquired
August 25, 2013
Publication Date
June 10, 2011
Publication Information
Publication: The Astrophysical Journal
Volume: 739
Issue: 9
Subject Category
Astrophysics
Report/Patent Number
GSFC.JA.4743.2011
Funding Number(s)
CONTRACT_GRANT: SRC 621-2007-4064
Distribution Limits
Public
Copyright
Other