Oscillatory dynamos and their induction mechanisms

Schrinner, M.; Petitdemange, Ludovic; Dormy, Emmanuel

doi:10.1051/0004-6361/201016372

Cited by 59 publications

(36 citation statements)

References 30 publications

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“…It is then a daunting possibility that we could be forced to stretch our simulations over very long temporal base-lines to find the actual long-lasting field configuration. More positively, our result, while in a different geometry, increases the importance of recent work on nonoscillatory αΩ and oscillatory α 2 modes in spherical shells for the solar dynamo (Mitra et al 2010;Schrinner et al 2011).…”

Section: Discussionmentioning

confidence: 49%

The fratricide ofαΩ dynamos by theirα²siblings

Hubbard¹,

Rheinhardt²,

Brandenburg³

2011

A&A

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Context. Helically forced magneto-hydrodynamic shearing-sheet turbulence can support different large-scale dynamo modes, although the αΩ mode is generally expected to dominate because it is the fastest growing one. In an αΩ dynamo, most of the field amplification is produced by the shear. As differential rotation is an ubiquitous source of shear in astrophysics, such dynamos are believed to be the source of most astrophysical large-scale magnetic fields. Aims. We study the stability of oscillatory migratory αΩ type dynamos in turbulence simulations. Methods. We use shearing-sheet simulations of hydromagnetic turbulence that is helically forced at a wavenumber that is about three times larger than the lowest wavenumber in the domain so that both αΩ and α 2 dynamo action is possible. Results. After initial dominance and saturation, the αΩ mode is found to be destroyed by an orthogonal α 2 mode sustained by the helical turbulence alone. We show that there are at least two processes through which this transition can occur. Conclusions. The fratricide of αΩ dynamos by its α 2 sibling is discussed in the context of grand minima of stellar activity. However, the genesis of αΩ dynamos from an α 2 dynamo has not yet been found.

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Section: Discussionmentioning

confidence: 49%

The fratricide ofαΩ dynamos by theirα²siblings

Hubbard¹,

Rheinhardt²,

Brandenburg³

2011

A&A

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show abstract

“…A butterfly diagram (not shown) of the azimuthally averaged radial field also shows poleward-propagating features. Such travelling nonaxisymmetric dynamo solutions have previously been observed in dynamos with free-slip boundary conditions (Schrinner et al 2011(Schrinner et al , 2012Käpylä et al 2013) and can be described in terms of the classical Parker-waves (Busse & Simitev 2006;Goudard & Dormy 2008;Schrinner et al 2012).…”

Section: Hydrodynamical Convectionmentioning

confidence: 99%

Formation of starspots in self-consistent global dynamo models: Polar spots on cool stars

Yadav

Gastine

Christensen

et al. 2014

A&A

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Context. Observations of cool stars reveal dark spot-like features on their surfaces. These starspots can be more extended than sunspots and cover a large area of the stellar surface. While sunspots appear only at low latitudes, starspots are also found in polar regions, in particular on rapidly rotating stars. Conventional flux-tube models have been invoked to explain starspot properties. However, these models use several simplifications, and so far, neither sunspots nor starspots have been generated in a self-consistent simulation of stellar magnetic convection. Aims. We aim to clarify the conditions necessary for the spontaneous formation of dark spots in numerical models of convectiondriven stellar dynamos. Methods. We simulated convection and magnetic field generation in rapidly rotating spherical shells assuming anelastic approximation. The high-resolution simulations were performed using a fully spectral magnetohydrodynamic code. Results. We demonstrate for the first time that a self-consistent distributed dynamo can spontaneously generate high-latitude dark spots. Dark spots are generated when a large-scale magnetic field, generated in the bulk of the convection zone, interacts with and locally quenches flow near the surface. Sufficiently strong density stratification and rapid rotation are prerequisites for the formation of sizeable dark spots in the model. Conclusions. Our models present an alternative scenario for starspot formation by distributed dynamo action. Our results also lend strong support to the idea that dynamos in the interiors of rapidly rotating stars might be fundamentally different from the solar one.

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“…The first solar-like solutions, however, were obtained only recently (Schrinner et al 2011;Käpylä et al 2012;Augustson et al 2015), and only a couple of them have been run up to time scales of interest for detecting the irregular variations: the EULAG code Millennium simulation at solar rotation Norton et al 2014, hereafter EULAG-MHD) covers roughly 20 magnetic cycles of 80 years cycle length, while the ASH code simulation (Augustson et al 2015, hereafter ABMT), rotating at three times the solar rate, covers roughly 24 cycles with a cycle length of 6.2 years. The former does not exhibit significant irregular behavior and only very weak latitudinal migration of the magnetic field, while the latter produces a clearer equatorward migrating branch at lower latitudes and a poleward migrating branch at higher latitudes, especially pronounced in the radial field.…”

Section: Introductionmentioning

confidence: 99%

Multiple dynamo modes as a mechanism for long-term solar activity variations

Käpylä

Olspert

et al. 2016

A&A

104

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Context. Solar magnetic activity shows both smooth secular changes, such as the modern Grand Maximum, and quite abrupt drops that are denoted as grand minima, such as the Maunder Minimum. Direct numerical simulations (DNS) of convection-driven dynamos offer one way of examining the mechanisms behind these events. Aims.In this work, we analyze a solution of a solar-like DNS that was evolved for roughly 80 magnetic cycles of 4.9 years and where epochs of irregular behavior are detected. The emphasis of our analysis is to find physical causes for such behavior. Methods. The DNS employed is a semi-global (wedge-shaped) magnetoconvection model. For the data analysis tasks we use Ensemble Empirical Mode Decomposition and phase dispersion methods, as they are well suited for analyzing cyclic (non-periodic) signals. Results. A special property of the DNS is the existence of multiple dynamo modes at different depths and latitudes. The dominant mode is solar-like (equatorward migration at low latitudes and poleward at high latitudes). This mode is accompanied by a higher frequency mode near the surface and at low latitudes, showing poleward migration, and a low-frequency mode at the bottom of the convection zone. The low-frequency mode is almost purely antisymmetric with respect to the equator, while the dominant mode has strongly fluctuating mixed parity. The overall behavior of the dynamo solution is extremely complex, exhibiting variable cycle lengths, epochs of disturbed and even ceased surface activity, and strong short-term hemispherical asymmetries. Surprisingly, the most prominent suppressed surface activity epoch is actually a global magnetic energy maximum; during this epoch the bottom toroidal magnetic field obtains a maximum, demonstrating that the interpretation of grand minima-type events is non-trivial. The hemispherical asymmetries are seen only in the magnetic field, while the velocity field exhibits considerably weaker asymmetry. Conclusions. We interpret the overall irregular behavior as being due to the interplay of the different dynamo modes showing different equatorial symmetries, especially the smoother part of the irregular variations being related to the variations of the mode strengths, evolving with different and variable cycle lengths. The abrupt low-activity epoch in the dominant dynamo mode near the surface is related to a strong maximum of the bottom toroidal field strength, which causes abrupt disturbances especially in the differential rotation profile via the suppression of the Reynolds stresses.

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Oscillatory dynamos and their induction mechanisms

Cited by 59 publications

References 30 publications

The fratricide ofαΩ dynamos by theirα²siblings

The fratricide ofαΩ dynamos by theirα²siblings

Formation of starspots in self-consistent global dynamo models: Polar spots on cool stars

Multiple dynamo modes as a mechanism for long-term solar activity variations

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Oscillatory dynamos and their induction mechanisms

Cited by 59 publications

References 30 publications

The fratricide ofαΩ dynamos by theirα2siblings

The fratricide ofαΩ dynamos by theirα2siblings

Formation of starspots in self-consistent global dynamo models: Polar spots on cool stars

Multiple dynamo modes as a mechanism for long-term solar activity variations

Contact Info

Product

Resources

About

The fratricide ofαΩ dynamos by theirα²siblings

The fratricide ofαΩ dynamos by theirα²siblings