The Mechanism involved in the Reversals of the Sun's Polar Magnetic Fields

Durrant, C. J.; Turner, J.; Wilson, P. R.

doi:10.1023/b:sola.0000043577.33961.82

Cited by 32 publications

(30 citation statements)

References 21 publications

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“…These ideas were used to develop a kinematic model for photospheric flux transport and the solar cycle by Leighton (1964Leighton ( , 1969, initiating a class of models known as "Babcock-Leighton" models (see Sections 3.1 and 3.2). Durrant et al (2004) found evidence of a net transport of leadingpolarity flux across the equator, corresponding to a poleward flow of net following-polarity flux across latitudes ±60 ∘ , consistent with the above picture.…”

Section: Chromospheric Synoptic Maps and Potential-field Extrapolationssupporting

confidence: 79%

Solar Magnetism in the Polar Regions

Petrie

2015

Living Rev. Sol. Phys.

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This review describes observations of the polar magnetic fields, models for the cyclical formation and decay of these fields, and evidence of their great influence in the solar atmosphere. The polar field distribution dominates the global structure of the corona over most of the solar cycle, supplies the bulk of the interplanetary magnetic field via the polar coronal holes, and is believed to provide the seed for the creation of the activity cycle that follows. A broad observational knowledge and theoretical understanding of the polar fields is therefore an essential step towards a global view of solar and heliospheric magnetic fields. Analyses of both high-resolution and long-term synoptic observations of the polar fields are summarized. Models of global flux transport are reviewed, from the initial phenomenological and kinematic models of Babcock and Leighton to present-day attempts to produce time-dependent maps of the surface magnetic field and to explain polar field variations, including the weakness of the cycle 23 polar fields. The relevance of the polar fields to solar physics extends far beyond the surface layers from which the magnetic field measurements usually derive. As well as discussing the polar fields' role in the interior as seed fields for new solar cycles, the review follows their influence outward to the corona and heliosphere. The global coronal magnetic structure is determined by the surface magnetic flux distribution, and is dominated on large scales by the polar fields. We discuss the observed effects of the polar fields on the coronal hole structure, and the solar wind and ejections that travel through the atmosphere. The review concludes by identifying gaps in our knowledge, and by pointing out possible future sources of improved observational information and theoretical understanding of these fields. Article RevisionsLiving Reviews supports two ways of keeping its articles up-to-date:Fast-track revision. A fast-track revision provides the author with the opportunity to add short notices of current research results, trends and developments, or important publications to the article. A fast-track revision is refereed by the responsible subject editor. If an article has undergone a fast-track revision, a summary of changes will be listed here.Major update. A major update will include substantial changes and additions and is subject to full external refereeing. It is published with a new publication number.For detailed documentation of an article's evolution, please refer to the history document of the article's online version at http://dx

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Section: Chromospheric Synoptic Maps and Potential-field Extrapolationssupporting

confidence: 79%

Solar Magnetism in the Polar Regions

Petrie

2015

Living Rev. Sol. Phys.

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“…The calculation of the cross-equatorial flux transport of magnetic flux is discussed by Durrant et al (2004), who also estimated the diffusive component of the cross-equatorial flux transport during cycle 22. The net (signed) magnetic flux in the northern hemisphere is given by…”

Section: Measuring the Cross-equatorial Fluxesmentioning

confidence: 99%

“…A signature of the reversal is the change of sign of the net flux in the southern and northern hemispheres, and the reversal of the polarity orientation of emerging bipoles in each hemisphere according to Hale's law. Application of Stokes' theorem to the induction equation shows that the change in the net magnetic flux through the solar surface of the northern hemisphere is equal to the amount of magnetic flux transported across the equator -being the boundary of the northern hemisphere surface (Durrant et al 2004).…”

Section: Introductionmentioning

confidence: 99%

Limits to solar cycle predictability: Cross-equatorial flux plumes

Cameron

Dasi-Espuig

Jiang

et al. 2013

A&A

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Context. Within the Babcock-Leighton framework for the solar dynamo, the strength of a cycle is expected to depend on the strength of the dipole moment or net hemispheric flux during the preceding minimum, which depends on how much flux was present in each hemisphere at the start of the previous cycle and how much net magnetic flux was transported across the equator during the cycle. Some of this transport is associated with the random walk of magnetic flux tubes subject to granular and supergranular buffeting, some of it is due to the advection caused by systematic cross-equatorial flows such as those associated with the inflows into active regions, and some crosses the equator during the emergence process. Aims. We aim to determine how much of the cross-equatorial transport is due to small-scale disorganized motions (treated as diffusion) compared with other processes such as emergence flux across the equator. Methods. We measure the cross-equatorial flux transport using Kitt Peak synoptic magnetograms, estimating both the total and diffusive fluxes. Results. Occasionally a large sunspot group, with a large tilt angle emerges crossing the equator, with flux from the two polarities in opposite hemispheres. The largest of these events carry a substantial amount of flux across the equator (compared to the magnetic flux near the poles). We call such events cross-equatorial flux plumes. There are very few such large events during a cycle, which introduces an uncertainty into the determination of the amount of magnetic flux transported across the equator in any particular cycle. As the amount of flux which crosses the equator determines the amount of net flux in each hemisphere, it follows that the cross-equatorial plumes introduce an uncertainty in the prediction of the net flux in each hemisphere. This leads to an uncertainty in predictions of the strength of the following cycle.

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“…Flux transport models describe the evolution of the flux distribution at the solar surface as a result of the emergence of bipolar magnetic regions and the transport of the corresponding radial magnetic flux by the horizontal flows due to convection, differential rotation and meridional circulation (e.g., Leighton 1964;DeVore et al 1984;Wang et al 1989b;van Ballegooijen et al 1998;Schrijver 2001;Mackay et al 2002;Durrant et al 2004;Baumann et al 2004). When applying the model to multiple solar cycles, Schrijver et al (2002) and Wang et al (2002) noticed that the secular evolution resulting from the simulations is not in agreement with the observations: activity cycles of varying strength lead to a drift of the polar fields and even to the disappearance of polar reversals.…”

Section: Introductionmentioning

confidence: 99%

A necessary extension of the surface flux transport model

Baumann¹,

Schmitt²,

Schüßler³

2006

A&A

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Customary two-dimensional flux transport models for the evolution of the magnetic field at the solar surface do not account for the radial structure and the volume diffusion of the magnetic field. When considering the long-term evolution of magnetic flux, this omission can lead to an unrealistic long-term memory of the system and to the suppression of polar field reversals. In order to avoid such effects, we propose an extension of the flux transport model by a linear decay term derived consistently on the basis of the eigenmodes of the diffusion operator in a spherical shell. A decay rate for each eigenmode of the system is determined and applied to the corresponding surface part of the mode evolved in the flux transport model. The value of the volume diffusivity associated with this decay term can be estimated to be in the range 50-100 km 2 s −1 by considering the reversals of the polar fields in comparison of flux transport simulations with observations. We show that the decay term prohibits a secular drift of the polar field in the case of cycles of varying strength, like those exhibited by the historical sunspot record.

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The Mechanism involved in the Reversals of the Sun's Polar Magnetic Fields

Cited by 32 publications

References 21 publications

Solar Magnetism in the Polar Regions

Solar Magnetism in the Polar Regions

Limits to solar cycle predictability: Cross-equatorial flux plumes

A necessary extension of the surface flux transport model

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