The rôle of absolute instability in the solar dynamo

The presence of the equator and poles has a profound effect on the critical dynamonumberDf 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 unstablemode. With increasingD thewaves 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.