On the role of meridional flows in flux transport dynamo models

Abstract

Context. The Sun is a magnetic star whose magnetism and cyclic activity is linked to the existence of an internal dynamo. Aims. We aim to understand the establishment of the solar magnetic 22-yr cycle, its associated butterfly diagram and field parity selection through numerical simulations of the solar global dynamo. Inspired by recent observations and 3D simulations that both exhibit multicellular flows in the solar convection zone, we seek to characterise the influence of various profiles of circulation on the behaviour of solar mean-field dynamo models. We focus our study on a number of specific points: the role played by these flows in setting the cycle period and the shape of the butterfly diagram and their influence on the magnetic field parity selection, namely the field parity switching from an antisymmetric, dipolar field configuration to a symmetric, mostly quadrupolar one, that has been discussed by several authors in the recent literature. Methods. We are using 2D mean field flux transport Babcock-Leighton numerical models in which we test several types of meridional flows: 1 large single cell, 2 cells in radius and 4 cells per hemisphere. Results. We confirm that adding cells in latitude tends to speed up the dynamo cycle whereas adding cells in radius more than triples the period. We find that the cycle period in the four cells model is less sensitive to the flow speed than in the other simpler meridional circulation profiles studied. Moreover, our studies show that adding cells in radius or in latitude seems to favour the parity switching to a quadrupolar solution. Conclusions. According to our numerical models, the observed 22-yr cycle and dipolar parity is easily reproduced by models including multicellular meridional flows. On the contrary, the resulting butterfly diagram and phase relationship between the toroidal and poloidal fields are affected to a point where it is unlikely that such multicellular meridional flows persist for a long period of time inside the Sun, without having to reconsider the model itself.

Supermodulation of the Sun's magnetic activity: the effects of symmetry changes

“…The first (the "Babcock-Leighton" model) relied on a poleward meridional flow that transported poloidal fields generated near the photosphere down to the base of the convection zone, where the flow reversed and strong toroidal fields could be formed by differential rotation. Others have proposed that the meridional flow should have a multicellular structure (Jouve & Brun 2007). The simple Babcock-Leighton picture has meanwhile been demolished by recent observations.…”

Sunspot cycles and Grand Minima

“…Weiss et al (1984) and Tobias et al (2005), while experiments with more realistic solar dynamos show that the preferred symmetry of solutions near marginal excitation can be sensitive to quite small changes in the model (e.g. Moss, 1999, Dikpati & Gilman, 2001, Bonnano et al, 2002, Jouve & Brun, 2007, or at least the excitation conditions for even and odd modes are similar (e.g. Moss & Brooke, 2000).…”

Towards understanding the global magnetism of the Sun and solar-like stars

“…As a direct consequence it is natural to ask how the meridional circulation amplitude and profile change with the rotation rate and how these may influence the magnetic cycle period. Jouve & Brun (2007a) have done a systematic study of the influence on the cycle period of the various parameters of Babcock-Leighton flux transport models. For the reference model possessing only one meridional circulation cell per hemisphere they confirm the results of Dikpati & Charbonneau (1999) that the cycle period is strongly dependent on 164 A. S. Brun the meridional circulation amplitude.…”

“…with v 0 , s 0 and η 0 being respectively, the amplitude of the meridional circulation at the surface, that of the Babcock-Leighton source term and of the magnetic diffusivity (see Jouve & Brun (2007a), Jouve, Brown & Brun (2009) for more details). We can thus expect that as we increase the rotation rate the cycle period will shorten if all the other parameters are kept constant but unfortunately not as fast as what is observed (Noyes et al 1984;Saar 2002).…”

“…Dikpati et al (2004), Bonanno et al (2005) and Jouve & Brun (2007a) have studied in details the influence of various meridional circulation profiles on the butterfly diagram and other large scale magnetic properties of the Sun. In their study Jouve et al (2009) demonstrate that adding cells in latitude may resolve the apparent disagreement between B-L dynamo models and observations of stellar activity cycles.…”