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Why coronal mass ejections are necessary for the dynamo

Published online by Cambridge University Press:  01 August 2006

Axel Brandenburg
Axel Brandenburg*
Affiliation:
Nordita, Blegdamsvej 17, DK-2100 Copenhagen Ø, Denmark email: [email protected] AlbaNova University Center, SE-106 91 Stockholm, Sweden
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Abstract

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Large scale dynamo-generated fields are a combination of interlocked poloidal and toroidal fields. Such fields possess magnetic helicity that needs to be regenerated and destroyed during each cycle. A number of numerical experiments now suggests that stars may do this by shedding magnetic helicity. In addition to plain bulk motions, a favorite mechanism involves magnetic helicity flux along lines of constant rotation. We also know that the sun does shed the required amount of magnetic helicity mostly in the form of coronal mass ejections. Solar-like stars without cycles do not face such strong constraints imposed by magnetic helicity evolution and may not display coronal activity to that same extent. I discuss the evidence leading to this line of argument. In particular, I discuss simulations showing the generation of strong mean toroidal fields provided the outer boundary condition is left open so as to allow magnetic helicity to escape. Control experiments with closed boundaries do not produce strong mean fields.

Keywords

hydrodynamics (magnetohydrodynamics:) MHDturbulenceSun: coronal mass ejections (CMEs)Sun: magnetic fieldsstars: mass loss
Type
Contributed Papers
Information
Proceedings of the International Astronomical Union , Volume 2 , Highlights of Astronomy 14: Highlights of Astronomy , August 2006 , pp. 291 - 292
Copyright
Copyright © International Astronomical Union 2007

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