Exploring Bistability in the Cycles of the Solar Dynamo through Global Simulations

Abstract

The calling card of solar magnetism is the sunspot cycle, during which sunspots regularly reverse their polarity sense every 11 years. However, a number of more complicated time-dependent behaviors have also been identified. In particular, there are temporal modulations associated with active longitudes and hemispheric asymmetry, when sunspots appear at certain solar longitudes or else in one hemisphere preferentially. So far, a direct link between between this asymmetric temporal behavior and the underlying solar dynamo has remained elusive. In this work, we present results from global, 3D magnetohydrodynamic (MHD) simulations, which for the first time display both behavior reminiscent of the sunspot cycle (regular polarity reversals and equatorward migration of internal magnetic field) and asymmetric, irregular behavior that in the simulations we interpret as active longitudes and hemispheric asymmetry. The simulations are thus bistable, in that the turbulent convection can stably support two distinct flavors of magnetism at different times, in superposition, or with smooth transitions from one state to the other. We discuss this new family of dynamo models in the context of the extensive observations of the Sun's surface magnetic field with the Solar and Heliospheric Observatory (SOHO) and the Solar Dynamics Observatory (SDO), as well as earlier observations of sunspot number and synoptic maps. We suggest that the solar dynamo itself may be bistable in nature, exhibiting two types of temporal behavior in the magnetic field.

Effects of Shell Thickness on Cross-Helicity Generation in Convection-Driven Spherical Dynamos

“…The Boussinesq approximation is justified and generally used for modelling convection in Earth's inner core where density variation between the inner-outer core boundary and the core mantle boundary is small [13,14,38,47]. The density contrast between the bottom (ρ i ) and the top (ρ o ) of the Solar convection zone is five orders of magnitude giving a density scale number of log(ρ i /ρ o ) ≈ 12 [62], and the anelastic approximation is more appropriate and commonly used in global solar convection models, for example, [11,33,63]. However, anelastic and Boussinesq simulations show many similarities [63], with Boussinesq models able to mimic solar periodicity and active longitude phenomena [25,42].…”

“…It is reasonable to expect that the two branches will offer different mechanisms of helicity and cross-helicity generation and thus in this paper we proceed to study both branches. Bistability, in itself, may play a role in aperiodic magnetic field polarity reversals, a notable feature of the geodynamo [32], as well as in the regular cycle of the solar dynamo [33]. We have previously investigated the hysteretic transitions between the coexisting dynamo branches with variation of the Rayleigh, Prandtl and Coriolis numbers (defined further below).…”

Effects of shell thickness on cross-helicity generation in convection-driven spherical dynamos

“…The density contrast between the bottom (ρ i ) and the top (ρ o ) of the Solar convection zone is five orders of magnitude giving a density scale number of log(ρ i /ρ o ) ≈ 12 [62], and the anelastic approximation is more appropriate and commonly used in global solar convection models, e.g. [11,33,63]. However, anelastic and Boussinesq simulations show many similarities [63], with Boussinesq models able to mimic solar periodicity and active longitude phenomena [25,42].…”

Powering Stellar Magnetism: Energy Transfers in Cyclic Dynamos of Sun-like Stars

“…Nevertheless, with the arrival of more powerful supercomputers, other authors have used instead global 3D MHD simulations to model differential rotation and stellar magnetism in the convection zone of solar-like stars (Glatzmaier & Gilman 1982;Miesch et al 2000;Brun et al 2004;Miesch et al 2006;Brown et al 2008;Ghizaru et al 2010;Brown et al 2010;Brun et al 2011;Käpylä et al 2011Käpylä et al , 2014Gastine et al 2014;Augustson et al 2015;Karak et al 2015). These studies pointed out the large magnetic temporal variability and the critical effect of stellar rotation and mass on magnetic field generation through dynamo mechanism, leading in some parameter regimes to configurations with cyclic activity (Gilman & Miller 1981;Gilman 1983;Glatzmaier 1985a;Racine et al 2011;Brown et al 2011;Nelson et al 2013;Käpylä et al 2013;Augustson et al 2013Augustson et al , 2015Beaudoin et al 2016;Guerrero et al 2016;Strugarek et al 2017Strugarek et al , 2018Warnecke 2018;Viviani et al 2018Viviani et al , 2019Guerrero et al 2019;Matilsky & Toomre 2020). Several studies pointed out the positive effect of a stable region underneath the convection zone (Parker 1993) on the efficient storage of intense toroidal field and the lengthening of the stellar dynamo cycle period (Glatzmaier 1985b;Browning et al 2006;Lawson et al 2015;...…”