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Research Area Astro Sciences
Principal Investigator(s) Maarit Korpi
  • University of Helsinki, Observatory, Finland
  • AlbaNova University Center, Nordita, Sweden


The mechanism responsible for the observed magnetic activity phenomena of the Sun still remains controversial: the challenge is to understand how ordered magnetic fields can arise in the turbulent solar convection zone. One possible mechanism is a large-scale hydromagnetic dynamo, the basic ingredients of which include convective turbulence and large-scale shear due to solar differential rotation. Whilst simple theoretical arguments suggest that such a dynamo should be capable of efficient large-scale magnetic field generation, numerical simulations of turbulent shearing convection exhibiting large-scale dynamo action have been lacking. As a possible explanation of this it has been suggested that large-scale dynamos cannot co-exist with the small-scale dynamo, which inevitably arises in the high Reynolds number (Rm) regime, and which washes out any large-scale structures. We have, for the first time, produced a set of runs just reaching the desired Rm regime where the small-scale dynamo is operating, which also show clear large-scale dynamo action. Our results indicate that shear is the key ingredient, not only as a generator of magnetic fields via streching, but also as a mediator of magnetic helicity fluxes. The latter is vital for the dynamo for dumping its own garbage, the small-scale magnetic helicity, out of the system. We propose to investigate this issue further with higher resolution runs reaching higher Rm, more realistic shear profile and boundary conditions, to check whether or not the large-scale dynamo action persists also in this regime. The result (positive or negative) will have very important implications for dynamo theory.

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