A new dynamo pattern revealed by solar helical magnetic fields

Zhang, Hongqi; Sakurai, Takashi; Pevtsov, Alexei A.; Gao, Yu; Xu, Haiqing; Sokoloff, Dmitry; Kuzanyan, K. M.

doi:10.1111/j.1745-3933.2009.00793.x

Cited by 94 publications

(119 citation statements)

References 32 publications

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“…They found that the latter is generally stronger, except during the early years of a solar cycle. Further evidence for the dependence of the chirality of features in the solar atmosphere on the phase of the solar cycle comes from SXR loops (Zhang et al 2010b) and active-region magnetic fields (Zhang et al 2010a).…”

Section: Hemispheric Trendsmentioning

confidence: 99%

The magnetic field in the solar atmosphere

Wiegelmann

Thalmann

Solanki

2014

Astron Astrophys Rev

182

125

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This publication provides an overview of magnetic fields in the solar atmosphere with the focus lying on the corona. The solar magnetic field couples the solar interior with the visible surface of the Sun and with its atmosphere. It is also responsible for all solar activity in its numerous manifestations. Thus, dynamic phenomena such as coronal mass ejections and flares are magnetically driven. In addition, the field also plays a crucial role in heating the solar chromosphere and corona as well as in accelerating the solar wind. Our main emphasis is the magnetic field in the upper solar atmosphere so that photospheric and chromospheric magnetic structures are mainly discussed where relevant for higher solar layers. Also, the discussion of the solar atmosphere and activity is limited to those topics of direct relevance to the magnetic field. After giving a brief overview about the solar magnetic field in general and its global structure, we discuss in more detail the magnetic field in active regions, the quiet Sun and coronal holes.

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Section: Hemispheric Trendsmentioning

confidence: 99%

The magnetic field in the solar atmosphere

Wiegelmann

Thalmann

Solanki

2014

Astron Astrophys Rev

182

125

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“…During the minimum phase of the solar activity cycle only these signs sometimes occur. Zhang et al (2010) give the approximative value 10 −5 G 2 /cm for the observed smallscale current helicity. With the radial scale of 100 Mm and with B rBφ 10 4 G 2 , one only needs a magnetic Reynolds number Rm of the fluctuations of order 10 to fulfill the numerical constraints.…”

Section: Discussionmentioning

confidence: 99%

“…at the solar surface, all showing that it is negative (positive) in the northern (southern) hemisphere (Hale 1927;Seehafer 1990;Rust & Kumar 1994;Abramenko et al 1996: Bao & Zhang 1998Pevtsov 2001;Kleeorin et al 2003;Zhang et al 2010Zhang et al , 2012. Here the notation b denotes the fluctuating parts of the total magnetic field in the form B =B + b.…”

Section: Introductionmentioning

confidence: 99%

The influence of helical background fields on current helicity and electromotive force of magnetoconvection

Rüdiger

Küker

2016

A&A

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Motivated by the empirical finding that the known hemispheric rules for the current helicity at the solar surface are not strict, we demonstrate the excitation of small-scale current helicity by the influence of large-scale helical magnetic background fields on nonrotating magnetoconvection. This is shown within a quasilinear analytic theory of driven turbulence and by nonlinear simulations of magnetoconvection that the resulting small-scale current helicity has the same sign as the large-scale current helicity, while the ratio of both pseudoscalars is of the order of the magnetic Reynolds number of the convection. The same models do not provide finite values of the small-scale kinetic helicity. On the other hand, a turbulence-induced electromotive force is produced including the diamagnetic pumping term, as well as the eddy diffusivity but, however, no α effect. It has thus been argued that the relations for the simultaneous existence of small-scale current helicity and α effect do not hold for the model of nonrotating magnetoconvection under consideration. Calculations for various values of the magnetic Prandtl number demonstrate that, for the considered diffusivities, the current helicity increases for growing magnetic Reynolds number, which is not true for the velocity of the diamagnetic pumping, which is in agreement with the results of the quasilinear analytical approximation.

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“…Zhang et al 2010) knowledge of its radial and colatitudinal distributions, here expressed through the functions f 1 (r) and f 2 (θ), is quite preliminary. Therefore, we adopt only simple parametrizations and explore a number of options to investigate the sensitivity of butterfly diagrams to the underlying assumptions.…”

Section: Mean-field Dynamosmentioning

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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A new dynamo pattern revealed by solar helical magnetic fields

Cited by 94 publications

References 32 publications

The magnetic field in the solar atmosphere

The magnetic field in the solar atmosphere

The influence of helical background fields on current helicity and electromotive force of magnetoconvection

Polar branches of stellar activity waves: dynamo models and observations

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