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Astron. Astrophys. 318, L55-L58 (1997)

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Letter to the Editor

The rôle of absolute instability in the solar dynamo

S.M. Tobias 1, 2, M.R.E. Proctor 1 and E. Knobloch 2

1 Department of Applied Mathematics and Theoretical Physics, University of Cambridge, Cambridge CB3 9EW, UK
2 JILA, University of Colorado, Boulder, CO 80309-0440, USA

Received 3 September 1996 / Accepted 19 December 1996


The presence of the equator and poles has a profound effect on the critical dynamo number [FORMULA] for the onset of dynamo waves and their subsequent nonlinear evolution. The instability threshold is associated with the onset of absolute instability, which is necessary for the existence of a globally unstable mode. With increasing D the waves become very rapidly nonlinear and the region of activity extends to higher and higher latitudes. The wave frequency is selected in the polar regions where the mode amplitude is small. Secondary absolute instabilities lead to the coexistence of waves with different wavenumbers, frequencies and amplitudes at different latitudes.

Key words: MHD – Sun: magnetic fields – dynamo


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© European Southern Observatory (ESO) 1997

Online publication: July 8, 1998