EDP Sciences
Free access
Volume 407, Number 1, August III 2003
Page(s) 7 - 12
Section Cosmology
DOI http://dx.doi.org/10.1051/0004-6361:20030783

A&A 407, 7-12 (2003)
DOI: 10.1051/0004-6361:20030783

Relaxation of writhe and twist of a bi-helical magnetic field

T. A. Yousef1 and A. Brandenburg2

1  Faculty of Engineering Science and Technology, Norwegian University of Science and Technology, Kolbjørn Hejes vei 2B, 7491 Trondheim, Norway
2  NORDITA, Blegdamsvej 17, 2100 Copenhagen Ø, Denmark

(Received 7 March 2003 / Accepted 22 May 2003)

In the past few years suggestions have emerged that the solar magnetic field might have a bi-helical contribution with oppositely polarized magnetic fields at large and small scales, and that the shedding of such fields may be crucial for the operation of the dynamo. It is shown that, if a bi-helical field is shed into the solar wind, positive and negative contributions of the magnetic helicity spectrum tend to mix and decay. Even in the absence of turbulence, mixing and decay can occur on a time scale faster than the resistive one provided the two signs of magnetic helicity originate from a single tube. In the presence of turbulence, positively and negatively polarized contributions mix rapidly in such a way that the ratio of magnetic helicity to magnetic energy is largest both at the largest scale and in the dissipation range. In absolute units the small scale excess of helical fields is however negligible.

Key words: magnetohydrodynamics (MHD) -- turbulence

Offprint request: T. A. Yousef, tarek.yousef@mtf.ntnu.no

© ESO 2003