Reconciling solar and stellar magnetic cycles with nonlinear dynamo simulations

Strugarek, Antoine; Beaudoin, Patrice; Charbonneau, Paul; Brun, Allan Sacha; Nascimento, J. D. Do

doi:10.1126/science.aal3999

Cited by 85 publications

(116 citation statements)

References 42 publications

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“…The other, "inactive" branch, however, remains robust in these same studies, and more importantly this cycle branch covers the solar activity-Rossby parameter regime. Both Babcock-Leighton flux transport dynamos (Jouve et al 2010;) and recent global MHD simulations (Strugarek et al 2017(Strugarek et al , 2018) are unable to reproduce this observed cycle period trend (however see Hazra et al 2019, who find some correct trend in a pumping-dominated dynamo model). By necessity, our study included only targets with high-quality cycles, which are the same set of stars comprising the inactive branch.…”

Section: Conclusion and Discussionmentioning

confidence: 98%

Waldmeier Effect in Stellar Cycles

Garg

Karak

Egeland

et al. 2019

ApJ

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One of the most robust features of the solar magnetic cycle is that the stronger cycles rise faster than the weaker ones. This is popularly known as the Waldmeier Effect, which is known for more than 80 years. This fundamental feature of the solar cycle has not only practical implications, e,g., in predicting the solar cycle, but also implications in understanding the solar dynamo. Here we ask the question whether the Waldmeier Effect exists in other Sun-like stars. To answer this question, we analyze the Ca II H & K S-index from Mount Wilson Observatory for 21 Sun-like G-K stars. We specifically check two aspects of Waldmeier Effect, namely, WE1: the anti-correlation between the rise times and the peaks and WE2: the positive correlation between rise rates and amplitudes. We show that except HD 16160, HD 81809, HD 155886 and HD 161239, all stars considered in the analysis show WE2. While WE1 is found to be present only in some of the stars studied. Further, the WE1 correlation is weaker than the WE2. Both WE1 and WE2 exist in the solar S-index as well. Similar to the solar cycles, the magnetic cycles of many stars are asymmetric about their maxima. The existence of the Waldmeier Effect and asymmetric cycles in Sun-like stars suggests that the dynamo mechanism which operates in the Sun is also operating in other stars.

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Section: Conclusion and Discussionmentioning

confidence: 98%

Waldmeier Effect in Stellar Cycles

Garg

Karak

Egeland

et al. 2019

ApJ

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“…6.3, we will discuss how in a classical α − ω dynamo (and in the equivalent flux transport Babcock-Leighton alternative) there exists a simple link between the Rossby number and the Dynamo number D, that can explain the observed positive linear scaling between rotation period and magnetic cycle period (e.g., Durney and Latour 1978;Noyes et al 1984b;Baliunas et al 1996;Tobias 1998;Montesinos et al 2001;Jouve et al 2010). We will also discuss a subset of recent simulations that show that the magnetic cycle length could also increase rather than decrease with shorter rotation period (Jouve et al 2010;Strugarek et al 2017). …”

Section: Main Sequence Solar-like Starsmentioning

confidence: 99%

Magnetism, dynamo action and the solar-stellar connection

Brun

Browning

2017

Living Rev Sol Phys

252

182

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The Sun and other stars are magnetic: magnetism pervades their interiors and affects their evolution in a variety of ways. In the Sun, both the fields themselves and their influence on other phenomena can be uncovered in exquisite detail, but these observations sample only a moment in a single star's life. By turning to observations of other stars, and to theory and simulation, we may infer other aspects of the magnetism-e.g., its dependence on stellar age, mass, or rotation rate-that would be invisible from close study of the Sun alone. Here, we review observations and theory of magnetism in the Sun and other stars, with a partial focus on the "Solar-stellar connection": i.e., ways in which studies of other stars have influenced our understanding of the Sun and vice versa. We briefly review techniques by which magnetic fields can be measured (or their presence otherwise inferred) in stars, and then highlight some key observational findings uncovered by such measurements, focusing (in many cases) on those that offer particularly direct constraints on theories of how the fields are built and maintained. We turn then to a discussion of how the fields arise in different objects: first, we summarize some essential elements of convection and dynamo theory, including a very brief discussion of mean-field theory and related concepts. Next we turn to simulations of convection and magnetism in stellar interiors, highlighting both some peculiarities of field generation in different types of stars and some unifying physical processes that likely influence dynamo action in general. We conclude with a brief

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“…The absence of a self-consistent and realistically thin tachocline in these models may explain why they have difficulty producing solar-like magnetic cycles (e.g., Browning et al 2006). Although some models do generate dipolar magnetic fields with regular reversals (e.g., Käpylä et al 2012;Passos & Charbonneau 2014;Strugarek et al 2017), they lack the smaller-scale features that characterise the solar dynamo, such as the coherent magnetic flux tubes that form sunspots and magnetic prominences, which are generally believed to originate in the tachocline (Parker 1955). The tachocline may also influence the dynamics of the convection zone in other ways.…”

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confidence: 99%

A Self-consistent Model of the Solar Tachocline

Wood

Brummell²

2018

ApJ

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We present a local but fully nonlinear model of the solar tachocline, using three-dimensional direct numerical simulations. The tachocline forms naturally as a statistically steady balance between Coriolis, pressure, buoyancy and Lorentz forces beneath a turbulent convection zone. Uniform rotation is maintained in the radiation zone by a primordial magnetic field, which is confined by meridional flows in the tachocline and convection zone. Such balanced dynamics has previously been found in idealised laminar models, but never in fully self-consistent numerical simulations.

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Reconciling solar and stellar magnetic cycles with nonlinear dynamo simulations

Cited by 85 publications

References 42 publications

Waldmeier Effect in Stellar Cycles

Waldmeier Effect in Stellar Cycles

Magnetism, dynamo action and the solar-stellar connection

A Self-consistent Model of the Solar Tachocline

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