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Astron. Astrophys. 359, 364-372 (2000)

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Grand minima in a buoyancy-driven solar dynamo

M.A.J.H. Ossendrijver

Kiepenheuer-Institut für Sonnenphysik, Schöneckstrasse 6, 79108 Freiburg, Germany (mathieu@kis.uni-freiburg.de)

Received 22 December 1999 / Accepted 3 April 2000


Numerical simulations of a 2D mean-field model are presented which show that grand minima, typical for the long-term behaviour of solar magnetic activity, can be produced by a dynamo that features an alpha effect based on the buoyancy instability of magnetic fluxtubes. The buoyancy-driven alpha effect functions only if the magnetic field strength exceeds a minimal value necessary for instability. It opens the possibility of dynamo action within the solar overshoot layer, where a strong magnetic field, [FORMULA] G, is thought to be stored. The existence of a magnetic threshold for dynamo action can lead to interruptions of the magnetic cycle, similar to the grand minima of solar activity. Transitions across the instability threshold are triggered by magnetic-flux injections from the convection zone. This is modelled by allowing for a small-scale kinematic alpha effect in the convection zone, and convective downdrafts that penetrate the overshoot layer.

Key words: Sun: magnetic fields – Sun: activity – magnetic fields – Magnetohydrodynamics (MHD)

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© European Southern Observatory (ESO) 2000

Online publication: June 30, 2000