The Magnetic Field of Sunspots

Cowling, T. G.

doi:10.1093/mnras/94.1.39

Cited by 669 publications

(272 citation statements)

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“…However, the equation for the poloidal field has only an advection term that can neither create nor destroy magnetic flux, and a diffusion term that can only destroy magnetic flux. Thus whatever an axisymmetric flow field does, the poloidal field must ultimately decay away, and the toroidal field must also decay as a consequence (Cowling, 1933).…”

Section: Chromospheric Synoptic Maps and Potential-field Extrapolationsmentioning

confidence: 99%

Solar Magnetism in the Polar Regions

Petrie

2015

Living Rev. Sol. Phys.

100

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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 Extrapolationsmentioning

confidence: 99%

Solar Magnetism in the Polar Regions

Petrie

2015

Living Rev. Sol. Phys.

100

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“…to support the destabilizing magnetic field via its own dynamo (Brandenburg et al 1995;Hawley et al 1996). Deviations from the axial symmetry are necessary for any dynamo (Cowling 1933;Elsasser 1946) which in this case have to be produced by the MRI itself. An excitation of nonaxisymmetric modes of MRI is thus necessary for the self-sustained turbulence.…”

Section: Introductionmentioning

confidence: 99%

Nonaxisymmetric modes of MRI in dissipative Keplerian disks

Kitchatinov

Rüdiger

2010

A&A

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Aims. Deviations from the axial symmetry are necessary to maintain self-sustained MRI-turbulence by a dynamo mechanism. We define the parameter region where the nonaxisymmetric MRI modes are excited and study their geometries and growth rates. Methods. The linear eigenvalue problem for global nonaxisymmetric modes of standard-MRI in Keplerian disks is solved numerically with allowance for finite diffusion. Results. For small magnetic Prandtl numbers the microscopic viscosity completely drops out of the analysis so that the stability maps and the growth rates expressed in terms of the magnetic Reynolds number Rm and the Lundquist number S do not depend on the magnetic Prandtl number Pm. The minimum magnetic field for the onset of nonaxisymmetric MRI grows with the rotation rate. For a given S all nonaxisymmetric modes disappear for a sufficiently large Rm. This is a consequence of the radial fine-structure of the nonaxisymmetric modes resulting from the winding effect of differential rotation. It is this fine-structure which also provides serious resolution problems for the numerical simulation of MRI at large Rm. Conclusions. For weak magnetic fields slightly above the critical value for the onset of MRI only axisymmetric modes are unstable. Nonaxisymmetric modes need stronger fields and not too large Rm. If Pm is small its real value does not play any role in MRI.

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“…Classical fluid dynamo theory [45] does not allow to maintain a field with perfect rotational symmetry, but new models do allow for the dipole tilt to be as small as 0.5°. Saturn's dynamo is thought to be produced by liquid 'metallic' hydrogen in its outer core, combined with strong convective motions and the rapid rotation of the planet itself (e.g.…”

Section: Magnetic Dynamo Magnetosphere and Radiation Environmentmentioning

confidence: 99%

Kronos: exploring the depths of Saturn with probes and remote sensing through an international mission

et al. 2008

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Kronos is a mission aimed to measure in situ the chemical and isotopic compositions of the Saturnian atmosphere with two probes and also by remote sensing, in order to understand the origin, formation, and evolution of giant planets in general, including extrasolar planets. Xe/ 129 Xe isotopic ratios will be measured by mass spectrometry on two probes entering the atmosphere of Saturn at two different locations near mid-latitudes, down to a pressure of 10 Bar. The global composition of Saturn will be investigated through these measurements, together with microwave radiometry determination of H 2 O and NH 3 and their 3D variations. The dynamics of Saturn's atmosphere will be investigated from: (1) measurements of pressure, temperature, vertical distribution of clouds and wind speed along the probes' descent trajectories, and (2) determination of deep winds, differential rotation and convection with combined probe, gravity and radiometric measurements. Besides these primary goals, Kronos will also measure the intensities and characteristics of Saturn's magnetic field inside the D ring as well as Saturn's gravitational field, in order to constrain the abundance of heavy elements in Saturn's interior and in its central core. Depending on the preferred architecture (flyby versus orbiter), Kronos will be in a position to measure the properties of Saturn's innermost magnetosphere and to investigate the ring structure in order to understand how these tiny structures could have formed and survived up to the present times.

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The Magnetic Field of Sunspots

Cited by 669 publications

References 0 publications

Solar Magnetism in the Polar Regions

Solar Magnetism in the Polar Regions

Nonaxisymmetric modes of MRI in dissipative Keplerian disks

Kronos: exploring the depths of Saturn with probes and remote sensing through an international mission

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