Grand Minima and Equatorward Propagation in a Cycling Stellar Convective Dynamo

Augustson, Kyle; Brun, Allan Sacha; Miesch, Mark S.; Toomre, J.

doi:10.1088/0004-637x/809/2/149

Cited by 114 publications

(168 citation statements)

References 86 publications

(196 reference statements)

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“…It is remarkable that this rule also seems to apply to the fully nonlinear convective dynamo simulations (Warnecke et al , 2016b. The current simulations can produce equatorward migration only in cases where a region with a negative radial gradient of Ω occurs at mid-latitudes (e.g., Käpylä et al 2012Käpylä et al , 2013Augustson et al 2015) or if the sign of the kinetic helicity, which is a proxy of the α effect, is inverted in the bulk of the convection zone (Duarte et al 2016).…”

Section: Differential Rotation and Meridional Circulationmentioning

confidence: 96%

“…Simulations of convection-driven dynamos have recently reached a level of sophistication where they capture effects observed in the Sun such as equatorward migration of activity belts (Schrinner et al 2011;Käpylä et al 2012;Augustson et al 2015;Duarte et al 2016) and irregular cycle variations such as grand minima and long-term modulations (Passos & Charbonneau 2014;Käpylä et al 2016a). Most of these simulations are individual numerical experiments, and it is not clear how they are situated in parameter space in relation to each other.…”

Section: Introductionmentioning

confidence: 99%

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Convection-driven spherical shell dynamos at varying Prandtl numbers

Käpylä

Olspert

et al. 2017

A&A

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Context. Stellar convection zones are characterized by vigorous high-Reynolds number turbulence at low Prandtl numbers. Aims. We study the dynamo and differential rotation regimes at varying levels of viscous, thermal, and magnetic diffusion. Methods. We perform three-dimensional simulations of stratified fully compressible magnetohydrodynamic convection in rotating spherical wedges at various thermal and magnetic Prandtl numbers (from 0.25 to 2 and from 0.25 to 5, respectively). Differential rotation and large-scale magnetic fields are produced self-consistently. Results. We find that for high thermal diffusivity, the rotation profiles show a monotonically increasing angular velocity from the bottom of the convection zone to the top and from the poles toward the equator. For sufficiently rapid rotation, a region of negative radial shear develops at mid-latitudes as the thermal diffusivity is decreased, corresponding to an increase of the Prandtl number. This coincides with and results in a change of the dynamo mode from poleward propagating activity belts to equatorward propagating ones. Furthermore, the clearly cyclic solutions disappear at the highest magnetic Reynolds numbers and give way to irregular sign changes or quasi-stationary states. The total (mean and fluctuating) magnetic energy increases as a function of the magnetic Reynolds number in the range studied here (5-151), but the energies of the mean magnetic fields level off at high magnetic Reynolds numbers. The differential rotation is strongly affected by the magnetic fields and almost vanishes at the highest magnetic Reynolds numbers. In some of our most turbulent cases, however, we find that two regimes are possible, where either differential rotation is strong and mean magnetic fields are relatively weak, or vice versa. Conclusions. Our simulations indicate a strong nonlinear feedback of magnetic fields on differential rotation, leading to qualitative changes in the behaviors of large-scale dynamos at high magnetic Reynolds numbers. Furthermore, we do not find indications of the simulations approaching an asymptotic regime where the results would be independent of diffusion coefficients in the parameter range studied here.

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Section: Differential Rotation and Meridional Circulationmentioning

confidence: 96%

Section: Introductionmentioning

confidence: 99%

Convection-driven spherical shell dynamos at varying Prandtl numbers

Käpylä

Olspert

et al. 2017

A&A

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“…Thus the natural way of studying the solar dynamo is by solving the basic magnetohydrodynamic (MHD) equations in a rotating spherical shell, encompassing the SCZ. However, though substantial progress has been made in recent years in studying fundamental dynamo mechanisms (e.g., Charbonneau 2014; Augustson et al 2015;Featherstone & Miesch 2015;Hotta et al 2016;Käpylä et al 2016;, MHD simulations still cannot capture all processes relevant to the solar dynamo and the solar cycle (Fan & Fang 2014;Karak et al 2015). One reason could be that these simulations do not produce sufficient flux emergence in the form of tilted bipolar magnetic regions (BMRs) that we see in the solar observations (e.g., Wang & Sheeley 1989).…”

Section: Introductionmentioning

confidence: 93%

Solar Cycle Variability Induced by Tilt Angle Scatter in a Babcock–Leighton Solar Dynamo Model

Karak

Miesch

2017

ApJ

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We present results from a three-dimensional Babcock-Leighton dynamo model that is sustained by the explicit emergence and dispersal of bipolar magnetic regions (BMRs). On average, each BMR has a systematic tilt given by Joy's law. Randomness and nonlinearity in the BMR emergence of our model produce variable magnetic cycles. However, when we allow for a random scatter in the tilt angle to mimic the observed departures from Joy's law, we find more variability in the magnetic cycles. We find that the observed standard deviation in Joy's law of σ δ = 15• produces a variability comparable to observed solar cycle variability of ∼ 32%, as quantified by the sunspot number maxima between 1755-2008. We also find that tilt angle scatter can promote grand minima and grand maxima. The time spent in grand minima for σ δ = 15• is somewhat less than that inferred for the Sun from cosmogenic isotopes (about 9% compared to 17%). However, when we double the tilt scatter to σ δ = 30• , the simulation statistics are comparable to the Sun (∼18% of the time in grand minima and ∼ 10% in grand maxima). Though the Babcock-Leighton mechanism is the only source of poloidal field, we find that our simulations always maintain magnetic cycles even at large fluctuations in the tilt angle. We also demonstrate that tilt quenching is a viable and efficient mechanism for dynamo saturation; a suppression of the tilt by only 1-2• is sufficient to limit the dynamo growth. Thus, any potential observational signatures of tilt quenching in the Sun may be subtle.

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“…We employ the magnetohydrodynamic equations modified under the anelastic approximation (Lantz & Fan 1999). This approach is now widely used for simulating subsonic convection in the interiors of stars and planets (e.g., see Browning 2008;Gastine et al 2013;Augustson et al 2015;Duarte et al 2016). To model the convection zone of a star we consider a spherical shell that is bounded by inner radius r i and outer radius r o .…”

Section: Simulation Setupmentioning

confidence: 99%

Magnetic Cycles in a Dynamo Simulation of Fully Convective M-Star Proxima Centauri

Yadav

Christensen

Wolk

et al. 2016

ApJL

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The recent discovery of an Earth-like exoplanet around Proxima Centauri has shined a spot light on slowly rotating fully convective M-stars. When such stars rotate rapidly (period 20 days), they are known to generate very high levels of activity that is powered by a magnetic field much stronger than the solar magnetic field. Recent theoretical efforts are beginning to understand the dynamo process that generates such strong magnetic fields. However, the observational and theoretical landscape remains relatively uncharted for fully convective M-stars that rotate slowly. Here, we present an anelastic dynamo simulation designed to mimic some of the physical characteristics of Proxima Centauri, a representative case for slowly rotating fully convective M-stars. The rotating convection spontaneously generates differential rotation in the convection zone that drives coherent magnetic cycles where the axisymmetric magnetic field repeatedly changes polarity at all latitudes as time progress. The typical length of the "activity" cycle in the simulation is about nine years, in good agreement with the recently proposed activity cycle length of about seven years for Proxima Centauri. Comparing our results with earlier work, we hypothesis that the dynamo mechanism undergoes a fundamental change in nature as fully convective stars spin down with age.

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Grand Minima and Equatorward Propagation in a Cycling Stellar Convective Dynamo

Cited by 114 publications

References 86 publications

Convection-driven spherical shell dynamos at varying Prandtl numbers

Convection-driven spherical shell dynamos at varying Prandtl numbers

Solar Cycle Variability Induced by Tilt Angle Scatter in a Babcock–Leighton Solar Dynamo Model

Magnetic Cycles in a Dynamo Simulation of Fully Convective M-Star Proxima Centauri

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