Solar active regions: a nonparametric statistical analysis

Pelt, J.; Korpi, M. J.; Tuominen, I.

doi:10.1051/0004-6361/200913193

Cited by 18 publications

(16 citation statements)

References 23 publications

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“…The typical correlation time of the nonaxisymmetric modes is estimated to be around 1 year. This agrees with results of the recent analysis of rotation of solar active regions made by Pelt et al (2010). They, following Krause & Rädler (1980), suggested the existence of non-oscillatory nonaxisymmetric modes which rotate rigidly with an angular velocity that is different from the overall rotation period.…”

Section: Discussionsupporting

confidence: 90%

Reversals of the solar magnetic dipole in the light of observational data and simple dynamo models

Пипин

Moss

Sokoloff

et al. 2014

A&A

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Context. Observations show that the photospheric solar magnetic dipole usually does not vanish during the reversal of the solar magnetic field, which occurs in each solar cycle. In contrast, mean-field solar dynamo models predict that the dipole field does become zero. In a recent paper it was suggested that this contradiction could be explained as a large-scale manifestation of small-scale magnetic fluctuations of the surface poloidal field. Aims. Our aim is to confront this interpretation with the available observational data. Methods. Here we compare this interpretation with Wilcox Solar Observatory (WSO) photospheric magnetic field data in order to determine the amplitude of magnetic fluctuations required to explain the phenomenon and to compare the results with predictions from a simple dynamo model which takes these fluctuations into account. Results. We demonstrate that the WSO data concerning the magnetic dipole reversals are very similar to the predictions from our very simple solar dynamo model, which includes both mean magnetic field and fluctuations. The ratio between the rms value of the magnetic fluctuations and the mean field is estimated to be about 2, in reasonable agreement with estimates from sunspot data. The reversal epoch, during which the fluctuating contribution to the dipole is larger than that from the mean field, is about 4 months. The memory time of the fluctuations is about 2 months. Observations demonstrate that the rms of the magnetic fluctuations is strongly modulated by the phase of the solar cycle. This gives additional support to the concept that the solar magnetic field is generated by a single dynamo mechanism rather than also by independent small-scale dynamo action. A suggestion of a weak nonaxisymmetric magnetic field of a fluctuating nature arises from the analysis, with a lifetime of about 1 year. Conclusions. The behaviour of the magnetic dipole during the reversal epoch gives valuable information about details of solar dynamo action.

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Section: Discussionsupporting

confidence: 90%

Reversals of the solar magnetic dipole in the light of observational data and simple dynamo models

Пипин

Moss

Sokoloff

et al. 2014

A&A

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“…To detect possible cycles and to estimate their average lengths, we have chosen to use D 2 phase dispersion statistic (Pelt 1983). It has recently been applied to irregularly spaced long-term photometry of solar-like stars (Lindborg et al 2013;Olspert et al 2015) as well as to more regularly sampled magnetoconvection simulation data (Karak et al 2015;Käpylä et al 2016a).…”

Section: Data Analysis: D 2 Statisticmentioning

confidence: 99%

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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“…However, even by visual inspection of the butterfly diagrams, it is evident that these variations are not strictly harmonic, and therefore Fourier transform is not useful for the analysis. Instead, we use the phase dispersion method (Pelt 1983;Lindborg et al 2013). It is based on the statistic…”

Section: Diagnostic Stellar Activity Diagramsmentioning

confidence: 99%

Magnetically controlled stellar differential rotation near the transition from solar to anti-solar profiles

Karak

Käpylä

et al. 2015

A&A

Self Cite

149

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Context. Late-type stars rotate differentially owing to anisotropic turbulence in their outer convection zones. The rotation is called solar-like (SL) when the equator rotates fastest and anti-solar (AS) otherwise. Hydrodynamic simulations show a transition from SL to AS rotation as the influence of rotation on convection is reduced, but the opposite transition occurs at a different point in the parameter space. The system is bistable, i.e., SL and AS rotation profiles can both be stable. Aims. We study the effect of a dynamo-generated magnetic field on the large-scale flows, particularly on the possibility of bistable behaviour of differential rotation. Methods. We solve the hydromagnetic equations numerically in a rotating spherical shell that typically covers ±75• latitude (wedge geometry) for a set of different radiative conductivities controlling the relative importance of convection. We analyse the resulting differential rotation, meridional circulation, and magnetic field and compare the corresponding modifications of the Reynolds and Maxwell stresses. Results. In agreement with earlier findings, our models display SL rotation profiles when the rotational influence on convection is strong and a transition to AS when the rotational influence decreases. We find that dynamo-generated magnetic fields help to produce SL differential rotation compared to the hydrodynamic simulations. We do not observe any bistable states of differential rotation. In the AS cases we find coherent single-cell meridional circulation, whereas in SL cases we find multi-cellular patterns. In both cases, we obtain poleward circulation near the surface with a magnitude close to that observed in the Sun. In the slowly rotating cases, we find activity cycles, but no clear polarity reversals, whereas in the more rapidly rotating cases irregular variations are obtained. Moreover, both differential rotation and meridional circulation have significant temporal variations that are similar in strength to those of the Sun. Conclusions. Purely hydrodynamic simulations of differential rotation and meridional circulation are shown to be of limited relevance as magnetic fields, self-consistently generated by dynamo action, significantly affect the flows.

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Solar active regions: a nonparametric statistical analysis

Cited by 18 publications

References 23 publications

Reversals of the solar magnetic dipole in the light of observational data and simple dynamo models

Reversals of the solar magnetic dipole in the light of observational data and simple dynamo models

Convection-driven spherical shell dynamos at varying Prandtl numbers

Magnetically controlled stellar differential rotation near the transition from solar to anti-solar profiles

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