Observing and modeling the poloidal and toroidal fields of the solar dynamo

Context. The solar dynamo consists of a process that converts poloidal magnetic field to toroidal magnetic field followed by a process that creates new poloidal field from the toroidal field. Aims. Our aim is to observe the poloidal and toroidal fields relevant to the global solar dynamo and to see if their evolution is captured by a Babcock-Leighton dynamo. Methods. We used synoptic maps of the surface radial field from the KPNSO/VT and SOLIS observatories, to construct the poloidal field as a function of time and latitude; we also used full disk images from Wilcox Solar Observatory and SOHO/MDI to infer the longitudinally averaged surface azimuthal field. We show that the latter is consistent with an estimate of the longitudinally averaged surface azimuthal field due to flux emergence and therefore is closely related to the subsurface toroidal field. Results. We present maps of the poloidal and toroidal magnetic fields of the global solar dynamo. The longitude-averaged azimuthal field observed at the surface results from flux emergence. At high latitudes this component follows the radial component of the polar fields with a short time lag of between 1−3 years. The lag increases at lower latitudes. The observed evolution of the poloidal and toroidal magnetic fields is described by the (updated) Babcock-Leighton dynamo model.

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An Insight into the Solar Dynamo Theory

International Annals of Science ◽

10.21467/ias.3.1.27-36 ◽

2017 ◽

Vol 3 (1) ◽

pp. 27-36

Author(s):

Babu Ram Tiwari ◽

Mukul Kumar

Keyword(s):

Magnetic Field ◽

Toroidal Field ◽

Solar Dynamo ◽

Dynamo Model ◽

The Sun ◽

Poloidal Field ◽

Dynamo Theory ◽

Dynamo Action ◽

Flux Transport ◽

Alpha Omega

The Sun manifests its magnetic field in form of the solar activities, being observed on the surface of the Sun. The dynamo action is responsible for the evolution of the magnetic field in the Sun. The present article aims to present an overview of the studies have been carried on the theory and modelling of the solar dynamo. The article describes the alpha-omega dynamo model. Generally, the dynamo model involves the cyclic conversion between the poloidal field and the toroidal field. In case of alpha-omega dynamo model, the strong differential rotation generates a toroidal field near the base of the convection zone. On the other hand, the turbulent helicity leads to the generation of the poloidal field near the surface. The turbulent diffusion and the meridional circulation are considered as the two important flux transport agents in this model. The article briefly describes the theory of solar dynamo and mean field dynamo model.

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A Behavioural Model of the Solar Magnetic Cycle

Proceedings of the International Astronomical Union ◽

10.1017/s174392131800056x ◽

2017 ◽

Vol 13 (S335) ◽

pp. 94-97

Author(s):

Milton Munroe

Keyword(s):

Magnetic Field ◽

Conceptual Model ◽

Differential Rotation ◽

Toroidal Field ◽

Solar Dynamo ◽

The Sun ◽

Poloidal Field ◽

Magnetic Cycle ◽

Poloidal Magnetic Field ◽

Solar Magnetic Cycle

All recent models of solar magnetic cycle behaviour assume that the Ω-effect stretches an existing poloidal magnetic field into a toroidal field using differential rotation (Featherstone and Miesch 2015). The α-effect recycles the toroidal field back to a poloidal field by convection and rotation and this is repeated throughout the cycle. Computer simulations based on that conceptual model still leave many questions unanswered. It has not resolved where the solar dynamo is located, what it is or what causes the differential rotation which it takes for granted. Does this paradigm need changing? The conceptual model presented here examines the sun in horizontal sections, analyses its internal structure, presents new characterizations for the solar wind and structures found and shows how their interaction creates rotation, differential rotation, the solar dynamo and the magnetic cycle.

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Solar Dynamo

Oxford Research Encyclopedia of Physics ◽

10.1093/acrefore/9780190871994.013.11 ◽

2020 ◽

Author(s):

Robert Cameron

Keyword(s):

Magnetic Field ◽

Magnetic Fields ◽

Magnetic Flux ◽

Differential Rotation ◽

Large Scale ◽

Toroidal Field ◽

Solar Dynamo ◽

Small Scale ◽

The Sun ◽

The Magnetic Field

The solar dynamo is the action of flows inside the Sun to maintain its magnetic field against Ohmic decay. On small scales the magnetic field is seen at the solar surface as a ubiquitous “salt-and-pepper” disorganized field that may be generated directly by the turbulent convection. On large scales, the magnetic field is remarkably organized, with an 11-year activity cycle. During each cycle the field emerging in each hemisphere has a specific East–West alignment (known as Hale’s law) that alternates from cycle to cycle, and a statistical tendency for a North-South alignment (Joy’s law). The polar fields reverse sign during the period of maximum activity of each cycle. The relevant flows for the large-scale dynamo are those of convection, the bulk rotation of the Sun, and motions driven by magnetic fields, as well as flows produced by the interaction of these. Particularly important are the Sun’s large-scale differential rotation (for example, the equator rotates faster than the poles), and small-scale helical motions resulting from the Coriolis force acting on convective motions or on the motions associated with buoyantly rising magnetic flux. These two types of motions result in a magnetic cycle. In one phase of the cycle, differential rotation winds up a poloidal magnetic field to produce a toroidal field. Subsequently, helical motions are thought to bend the toroidal field to create new poloidal magnetic flux that reverses and replaces the poloidal field that was present at the start of the cycle. It is now clear that both small- and large-scale dynamo action are in principle possible, and the challenge is to understand which combination of flows and driving mechanisms are responsible for the time-dependent magnetic fields seen on the Sun.

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Stellar dynamos with solar and antisolar differential rotations: Implications to magnetic cycles of slowly rotating stars

Monthly Notices of the Royal Astronomical Society ◽

10.1093/mnras/stz3220 ◽

2019 ◽

Vol 491 (3) ◽

pp. 3155-3164 ◽

Cited By ~ 5

Author(s):

Bidya Binay Karak ◽

Aparna Tomar ◽

Vindya Vashishth

Keyword(s):

Magnetic Field ◽

Magnetic Fields ◽

Large Scale ◽

Mean Field ◽

Rotation Period ◽

Toroidal Field ◽

Dynamo Model ◽

Cycle Period ◽

Rotating Stars ◽

Magnetic Cycles

ABSTRACT Simulations of magnetohydrodynamics convection in slowly rotating stars predict antisolar differential rotation (DR) in which the equator rotates slower than poles. This antisolar DR in the usual αΩ dynamo model does not produce polarity reversal. Thus, the features of large-scale magnetic fields in slowly rotating stars are expected to be different than stars having solar-like DR. In this study, we perform mean-field kinematic dynamo modelling of different stars at different rotation periods. We consider antisolar DR for the stars having rotation period larger than 30 d and solar-like DR otherwise. We show that with particular α profiles, the dynamo model produces magnetic cycles with polarity reversals even with the antisolar DR provided, the DR is quenched when the toroidal field grows considerably high and there is a sufficiently strong α for the generation of toroidal field. Due to the antisolar DR, the model produces an abrupt increase of magnetic field exactly when the DR profile is changed from solar-like to antisolar. This enhancement of magnetic field is in good agreement with the stellar observational data as well as some global convection simulations. In the solar-like DR branch, with the decreasing rotation period, we find the magnetic field strength increases while the cycle period shortens. Both of these trends are in general agreement with observations. Our study provides additional support for the possible existence of antisolar DR in slowly rotating stars and the presence of unusually enhanced magnetic fields and possibly cycles that are prone to production of superflare.

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An 84-μG Magnetic Field in a Galaxy at Z=0.692?

Proceedings of the International Astronomical Union ◽

10.1017/s1743921308027439 ◽

2008 ◽

Vol 4 (S254) ◽

pp. 95-96

Author(s):

Arthur M. Wolfe ◽

Regina A. Jorgenson ◽

Timothy Robishaw ◽

Carl Heiles ◽

Jason X. Prochaska

Keyword(s):

Magnetic Field ◽

Magnetic Fields ◽

Faraday Rotation ◽

Large Scale ◽

Dynamo Model ◽

Magnetic Field Generation ◽

Interstellar Gas ◽

Splitting Technique ◽

Average Value ◽

The Magnetic Field

AbstractThe magnetic field pervading our Galaxy is a crucial constituent of the interstellar medium: it mediates the dynamics of interstellar clouds, the energy density of cosmic rays, and the formation of stars (Beck 2005). The field associated with ionized interstellar gas has been determined through observations of pulsars in our Galaxy. Radio-frequency measurements of pulse dispersion and the rotation of the plane of linear polarization, i.e., Faraday rotation, yield an average value B ≈ 3 μG (Han et al. 2006). The possible detection of Faraday rotation of linearly polarized photons emitted by high-redshift quasars (Kronberg et al. 2008) suggests similar magnetic fields are present in foreground galaxies with redshifts z > 1. As Faraday rotation alone, however, determines neither the magnitude nor the redshift of the magnetic field, the strength of galactic magnetic fields at redshifts z > 0 remains uncertain.Here we report a measurement of a magnetic field of B ≈ 84 μG in a galaxy at z =0.692, using the same Zeeman-splitting technique that revealed an average value of B = 6 μG in the neutral interstellar gas of our Galaxy (Heiles et al. 2004). This is unexpected, as the leading theory of magnetic field generation, the mean-field dynamo model, predicts large-scale magnetic fields to be weaker in the past, rather than stronger (Parker 1970).The full text of this paper was published in Nature (Wolfe et al. 2008).

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The magnetic helicity density patterns from non-axisymmetric solar dynamo

Journal of Plasma Physics ◽

10.1017/s0022377820001609 ◽

2021 ◽

Vol 87 (1) ◽

Author(s):

Valery V. Pipin

Keyword(s):

Magnetic Field ◽

Differential Rotation ◽

Conservation Law ◽

Large Scale ◽

Solar Dynamo ◽

Magnetic Helicity ◽

Dynamo Model ◽

Density Distributions ◽

Large Scale Magnetic Field ◽

Helicity Conservation

We study the helicity density patterns which can result from the emerging bipolar regions. Using the relevant dynamo model and the magnetic helicity conservation law we find that the helicity density patterns around the bipolar regions depend on the configuration of the ambient large-scale magnetic field, and in general they show a quadrupole distribution. The position of this pattern relative to the equator can depend on the tilt of the bipolar region. We compute the time–latitude diagrams of the helicity density evolution. The longitudinally averaged effect of the bipolar regions shows two bands of sign for the density distributions in each hemisphere. Similar helicity density patterns are provided by the helicity density flux from the emerging bipolar regions subjected to surface differential rotation.

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Observations of magnetic and kinetic helicity proxies

Proceedings of the International Astronomical Union ◽

10.1017/s1743921313002160 ◽

2012 ◽

Vol 8 (S294) ◽

pp. 13-24

Author(s):

Hongqi Zhang

Keyword(s):

Magnetic Field ◽

Velocity Field ◽

Magnetic Fields ◽

Solar Atmosphere ◽

Active Regions ◽

Solar Dynamo ◽

Magnetic Helicity ◽

Vector Magnetograms ◽

Topological Configuration ◽

The Relationship

AbstractThe helicity is important to present the basic topological configuration of magnetic field in solar atmosphere. The distribution of magnetic helicity in solar atmosphere is presented by means of the observational (vector) magnetograms. As the kinetic helicity in the solar subatmosphere can be inferred from the velocity field based on the technique of the helioseismology and used to compare with the magnetic helicity in the solar atmosphere, the observational helicities provide the important chance for the confirmation on the generation of magnetic fields in the subatmosphere and solar dynamo models also. In this paper, we present the observational magnetic and kinetic helicity in solar active regions and corresponding questions, except the relationship with solar eruptive phenomena.

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Does the mean-field α effect have any impact on the memory of the solar cycle?

Astronomy and Astrophysics ◽

10.1051/0004-6361/201937287 ◽

2020 ◽

Vol 642 ◽

pp. A51

Author(s):

Soumitra Hazra ◽

Allan Sacha Brun ◽

Dibyendu Nandy

Keyword(s):

Solar Cycle ◽

Mean Field ◽

Solar Dynamo ◽

Dynamo Model ◽

Solar Cycles ◽

Poloidal Field ◽

Flux Transport ◽

Solar Convection ◽

The Mean ◽

And Diffusion

Context. Predictions of solar cycle 24 obtained from advection-dominated and diffusion-dominated kinematic dynamo models are different if the Babcock–Leighton mechanism is the only source of the poloidal field. Some previous studies argue that the discrepancy arises due to different memories of the solar dynamo for advection- and diffusion-dominated solar convection zones. Aims. We aim to investigate the differences in solar cycle memory obtained from advection-dominated and diffusion-dominated kinematic solar dynamo models. Specifically, we explore whether inclusion of Parker’s mean-field α effect, in addition to the Babcock–Leighton mechanism, has any impact on the memory of the solar cycle. Methods. We used a kinematic flux transport solar dynamo model where poloidal field generation takes place due to both the Babcock–Leighton mechanism and the mean-field α effect. We additionally considered stochastic fluctuations in this model and explored cycle-to-cycle correlations between the polar field at minima and toroidal field at cycle maxima. Results. Solar dynamo memory is always limited to only one cycle in diffusion-dominated dynamo regimes while in advection-dominated regimes the memory is distributed over a few solar cycles. However, the addition of a mean-field α effect reduces the memory of the solar dynamo to within one cycle in the advection-dominated dynamo regime when there are no fluctuations in the mean-field α effect. When fluctuations are introduced in the mean-field poloidal source a more complex scenario is evident, with very weak but significant correlations emerging across a few cycles. Conclusions. Our results imply that inclusion of a mean-field α effect in the framework of a flux transport Babcock–Leighton dynamo model leads to additional complexities that may impact memory and predictability of predictive dynamo models of the solar cycle.

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Mid-infrared Observations of Magnetic Fields in the Disks of Bi-Polar Outflow Sources

International Astronomical Union Colloquium ◽

10.1017/s0252921100044067 ◽

1997 ◽

Vol 163 ◽

pp. 799-800

Author(s):

Craig H. Smith ◽

Christopher M. Wright ◽

David K. Aitken ◽

Patrick F. Roche

Keyword(s):

Magnetic Field ◽

Magnetic Fields ◽

Field Configuration ◽

Field Direction ◽

Magnetic Field Direction ◽

Poloidal Field ◽

Mid Infrared ◽

Infrared Observations ◽

Polarization Mechanism ◽

The Magnetic Field

AbstractWe present the results from mid-infrared spectro-polarimetric observations of a number of bi-polar outflow sources. The specto-polarimetric data provides information on the polarization mechanism and the magnetic field direction. The field direction in the disks of the observed sources is most often normal to the ambient field direction and lies in the plane of the disk, indicating a toroidal rather than poloidal field configuration.

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Stability of H3O at extreme conditions and implications for the magnetic fields of Uranus and Neptune

Proceedings of the National Academy of Sciences ◽

10.1073/pnas.1921811117 ◽

2020 ◽

Vol 117 (11) ◽

pp. 5638-5643 ◽

Cited By ~ 4

Author(s):

Peihao Huang ◽

Hanyu Liu ◽

Jian Lv ◽

Quan Li ◽

Chunhong Long ◽

...

Keyword(s):

Magnetic Field ◽

Magnetic Fields ◽

Thin Shell ◽

Underlying Mechanism ◽

Giant Planets ◽

Dynamo Model ◽

Extreme Conditions ◽

Material Basis ◽

The Magnetic Field ◽

Planetary Dynamos

The anomalous nondipolar and nonaxisymmetric magnetic fields of Uranus and Neptune have long challenged conventional views of planetary dynamos. A thin-shell dynamo conjecture captures the observed phenomena but leaves unexplained the fundamental material basis and underlying mechanism. Here we report extensive quantum-mechanical calculations of polymorphism in the hydrogen–oxygen system at the pressures and temperatures of the deep interiors of these ice giant planets (to >600 GPa and 7,000 K). The results reveal the surprising stability of solid and fluid trihydrogen oxide (H3O) at these extreme conditions. Fluid H3O is metallic and calculated to be stable near the cores of Uranus and Neptune. As a convecting fluid, the material could give rise to the magnetic field consistent with the thin-shell dynamo model proposed for these planets. H3O could also be a major component in both solid and superionic forms in other (e.g., nonconvecting) layers. The results thus provide a materials basis for understanding the enigmatic magnetic-field anomalies and other aspects of the interiors of Uranus and Neptune. These findings have direct implications for the internal structure, composition, and dynamos of related exoplanets.

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