Modulated cycles in an illustrative solar dynamo model with competing α-effects

• Published in: Astronomy and astrophysics (Berlin) Vol. 563; pp. A116 - np
• Main Authors: Cole, L. C., Bushby, P. J.
• Format: Journal Article
• Language: English
• Published: EDP Sciences 01.03.2014
• Subjects: Bifurcations; Chaos theory; Differential equations; dynamo; magnetohydrodynamics (MHD); Mathematical analysis; Mathematical models; Modulation; Rotating generators; Solar cycles; Sun: activity; Sun: interior; Sun: magnetic fields
• ISSN: 0004-6361; 1432-0746
• DOI: 10.1051/0004-6361/201323285

Context. The large-scale magnetic field in the Sun varies with a period of approximately 22 years, although the amplitude of the cycle is subject to long-term modulation with recurrent phases of significantly reduced magnetic activity. It is believed that a hydromagnetic dynamo is responsible for producing this large-scale field, although this dynamo process is not well understood. Aims. Within the framework of mean-field dynamo theory, our aim is to investigate how competing mechanisms for poloidal field regeneration (namely a time-delayed Babcock-Leighton surface α-effect and an interface-type α-effect), can lead to the modulation of magnetic activity in a deep-seated solar dynamo model. Methods. We solve the standard αΩ dynamo equations in one spatial dimension, including source terms corresponding to both of the competing α-effects in the evolution equation for the poloidal field. This system is solved using two different methods. In addition to solving the one-dimensional partial differential equations directly, using numerical techniques, we also use a local approximation to reduce the governing equations to a set of coupled ordinary differential equations (ODEs), which are studied using a combination of analytical and numerical methods. Results. In the ODE model, it is straightforward to find parameters such that a series of bifurcations can be identified as the time delay is increased, with the dynamo transitioning from periodic states to chaotic states via multiply periodic solutions. Similar transitions can be observed in the full model, with the chaotically modulated solutions exhibiting solar-like behaviour. Conclusions. Competing α-effects could explain the observed modulation in the solar cycle.