A Time Delay Model for Solar and Stellar Dynamos

Wilmot-Smith, A. L.; Nandy, Dibyendu; Hornig, G.; Martens, Petrus C.

doi:10.1086/508013

Cited by 69 publications

(73 citation statements)

References 38 publications

(35 reference statements)

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“…As a consequence, it is mainly the effect of meridional flow which has been tested in previous models (e.g. Wilmot-Smith et al 2006). However, we showed here that the time delays due to flux tubes rising have in fact a significant effect to modulate the cycle on time scales very large compared to the delays.…”

Section: Discussionmentioning

confidence: 95%

“…Indeed, time delays built up in solar dynamo models have been shown to cause long-term modulation of the dynamo cycle and under some circumstances to lead to a chaotic behaviour (Yoshimura 1978;Charbonneau et al 2005;or Wilmot-Smith et al 2006). In the framework of mean-field dynamo theory, several possibilities have been studied to explain the variability of the solar cycle.…”

Section: Introductionmentioning

confidence: 99%

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Buoyancy-induced time delays in Babcock-Leighton flux-transport dynamo models

Jouve¹,

Proctor²,

Lesur³

2010

A&A

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Context. The Sun is a magnetic star whose cyclic activity is thought to be linked to internal dynamo mechanisms. A combination of numerical modelling with various levels of complexity is an efficient and accurate tool to investigate such intricate dynamical processes. Aims. We investigate the role of the magnetic buoyancy process in 2D Babcock-Leighton dynamo models, by modelling more accurately the surface source term for poloidal field. Methods. To do so, we reintroduce in mean-field models the results of full 3D MHD calculations of the non-linear evolution of a rising flux tube in a convective shell. More specifically, the Babcock-Leighton source term is modified to take into account the delay introduced by the rise time of the toroidal structures from the base of the convection zone to the solar surface. Results. We find that the time delays introduced in the equations produce large temporal modulation of the cycle amplitude even when strong and thus rapidly rising flux tubes are considered. Aperiodic modulations of the solar cycle appear after a sequence of period doubling bifurcations typical of non-linear systems. The strong effects introduced even by small delays is found to be due to the dependence of the delays on the magnetic field strength at the base of the convection zone, the modulation being much less when time delays remain constant. We do not find any significant influence on the cycle period except when the delays are made artificially strong. Conclusions. A possible new origin of the solar cycle variability is here revealed. This modulated activity and the resulting butterfly diagram are then more compatible with observations than what the standard Babcock-Leighton model produces.

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Section: Discussionmentioning

confidence: 95%

Section: Introductionmentioning

confidence: 99%

Buoyancy-induced time delays in Babcock-Leighton flux-transport dynamo models

Jouve¹,

Proctor²,

Lesur³

2010

A&A

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“…Wilmot-Smith et al (2006), but here we concentrate on models that we feel are more directly interpretable physically.…”

Section: Introductionmentioning

confidence: 99%

Polar branches of stellar activity waves: dynamo models and observations

Moss

Sokoloff

Lanza

2011

A&A

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Aims. Stellar activity data provide evidence of activity wave branches propagating polewards rather than equatorwards (the solar case). This evidence is especially pronounced for the well-observed subgiant HR 1099. Stellar dynamo theory allows polewards propagating dynamo waves for certain governing parameters. In this paper, we try to unite observations and theory. Methods. Taking into account the preliminary stage both of observations of polar activity branches and of the determination of the governing parameters for stellar dynamos, we restrict our investigation to the simplest mean-field dynamo models, while recognizing more modern approaches to be an essential development. Results. We suggest a crude preliminary systematization of the reported cases of polar activity branches. Then we present results of dynamo model simulations which contain magnetic structures with polar dynamo waves, and identify the models which look most promising for explaining the latitudinal distribution of spots in dwarf stars. Those models require specific features of stellar rotation laws, and so observations of polar activity branches may constrain internal stellar rotation. Specifically, we find it unlikely that a pronounced poleward branch can be associated with a solar-like internal rotation profile, while it can be more readily reproduced in the case of a cylindrical rotation law appropriate for fast rotators. We stress the case of the subgiant component of the active close binary HR 1099 which, being best investigated, presents the most severe problems for a dynamo interpretation. Our best model requires dynamo action in two layers separated in radius. This interpretation requires some change in the paradigm of stellar magnetic studies, as it explains surface manifestations in a subgiant as a joint effect of shallow and deep layers of the stellar convective zone, rather than of a surface magnetic field only, as appears to be the case in dwarf stars. Conclusions. Observations of polar activity branches provide valuable information for understanding stellar activity mechanisms and internal rotation, and thus deserve intensive observational and theoretical investigation. Current stellar dynamo theory seems sufficiently robust to accommodate the phenomenology.

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“…The MM is the largest of several solar activity minima identified in proxy records of solar irradiance. Current models of the solar dynamo are beginning to identify potential triggers of the MM (e.g., Gómez & Mininni 2006;Wilmot-Smith et al 2006;Brandenburg & Spiegel 2008). However, controversy remains in the interpretation of proxy records (e.g., Frölich & Lean 2004, and references therein) regarding whether the manifestation of this "grand minimum" can be described as a cyclic or stochastic process (Usoskin et al 2007).…”

Section: Introductionmentioning

confidence: 99%

Lithium Abundance in Solar-Type Stars With Low Chromospheric Activity: Application to the Search for Maunder Minimum Analogs

Lubin¹,

Tytler²,

Kirkman³

2010

ApJ

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We use measurements of lithium abundance to examine the evolutionary history of stars frequently believed to be in a Maunder minimum (MM) state due to their low chromospheric activity. In a sample whose main-sequence membership has been verified using Hipparcos parallax data, we find that stars with very low chromospheric activity log R HK −5.0 have substantially depleted lithium compared with the full sample, with half of these lithium abundances lying more than one standard deviation below the sample mean for their range of color index. One interpretation is that these stars are near the end of their main-sequence lifetime, and therefore their low activity does not necessarily signify a transient MM state in a solar-age star. Conversely, using information in published activity time series for some stars, and combined lithium and activity measurements from the Ursa Major moving group and M67, we find limited evidence that a low-activity star having lithium abundance in the normal range for its color index may be a viable MM candidate. Thus, lithium abundance, which can be readily observed or even retrieved from some of the spectroscopic data collected by recent planetsearch surveys, may have value for expanding and refining the program star lists for long-term MM searches. Finally, we find that the use of Hipparcos parallax data to ascertain main-sequence membership sharpens the distinction in sample-mean lithium abundance between stars with planet detections and comparison stars.

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A Time Delay Model for Solar and Stellar Dynamos

Cited by 69 publications

References 38 publications

Buoyancy-induced time delays in Babcock-Leighton flux-transport dynamo models

Buoyancy-induced time delays in Babcock-Leighton flux-transport dynamo models

Polar branches of stellar activity waves: dynamo models and observations

Lithium Abundance in Solar-Type Stars With Low Chromospheric Activity: Application to the Search for Maunder Minimum Analogs

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