Evidence of Twisted Flux-Tube Emergence in Active Regions

We model thin magnetic flux tubes as they rise from the bottom of a stellar convection zone to the photosphere. On emergence they form active regions, i.e. star spots. This model was very successfully applied to the solar case, where the simulations where in agreement with the butterfly diagram, Joy's law, and Hale's law. We propose the use of a similar model to describe stellar activity in the more extreme form found on active stars. A comparison between Doppler-images of well-observed pre-MS stars and a theoretically derived probability of star-spot formation as a function of latitude is presented.

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Properties of Faculae from Observations Near the Opacity Minimum

Symposium - International Astronomical Union ◽

10.1017/s0074180900124210 ◽

1994 ◽

Vol 154 ◽

pp. 23-27

Author(s):

P Foukal ◽

T Moran

Keyword(s):

Magnetic Flux ◽

Brightness Temperature ◽

Flux Density ◽

Flux Tube ◽

Angular Resolution ◽

Negative Contrast ◽

Active Regions ◽

Disk Center ◽

Quantitative Comparison ◽

Magnetic Flux Density

Imaging of active regions in continuum around 1.6 μm shows that many facular regions are less bright than the photosphere when observed nearer to disk center than μ = cos θ ~ 0.75. The contrast of these dark faculae increases with magnetic flux above a threshold of approximately 2 × 1018 Mx. This explains why not all faculae are dark at 1.6 μm, since the magnetic flux density in many regions of bright Ca K plage emission falls below this threshold. After correction for blurring, the typical contrast value is about 4-5%, so the brightness temperature deficit is about 130 K. Faculae are brighter than the photosphere at 1.63 μm nearer to the limb than μ ~ 0.5. The negative contrast of dark faculae may arise from cooling of the surrounding photosphere, or from increased visibility of cool layers of the facular flux tube itself. Quantitative comparison of these IR data with MHD models awaits calculation of flux tube contrasts at realistic angular resolution.

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Interaction of magnetic field systems leading to an X1.7 flare due to large-scale flux tube emergence

Astronomy and Astrophysics ◽

10.1051/0004-6361:20077500 ◽

2007 ◽

Vol 475 (3) ◽

pp. 1081-1091 ◽

Cited By ~ 24

Author(s):

H. Li ◽

B. Schmieder ◽

M. T. Song ◽

V. Bommier

Keyword(s):

Magnetic Field ◽

Flux Tube ◽

Large Scale ◽

Tube Emergence ◽

Field Systems

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Twist and writhe of δ-island active regions

Proceedings of the International Astronomical Union ◽

10.1017/s174392131101516x ◽

2010 ◽

Vol 6 (S273) ◽

pp. 153-156

Author(s):

M. C. López Fuentes ◽

C. H. Mandrini ◽

P. Démoulin

Keyword(s):

Magnetic Field ◽

Flux Tube ◽

Active Regions ◽

Magnetic Helicity ◽

Coronal Structure ◽

Flux Tubes ◽

Single Mechanism ◽

Magnetic Flux Tubes ◽

Kink Instability ◽

The Magnetic Field

AbstractWe study the magnetic helicity properties of a set of peculiar active regions (ARs) including δ-islands and other high-tilt bipolar configurations. These ARs are usually identified as the most active in terms of flare and CME production. Due to their observed structure, they have been associated with the emergence of magnetic flux tubes that develop a kink instability. Our main goal is to determine the chirality of the twist and writhe components of the AR magnetic helicity in order to set constrains on the possible mechanisms producing the flux tube deformations. We determine the magnetic twist comparing observations of the AR coronal structure with force-free models of the magnetic field. We infer the flux-tube writhe from the rotation of the main magnetic bipole during the observed evolution. From the relation between the obtained twist and writhe signs we conclude that the development of the kink instability cannot be the single mechanism producing deformed flux-tubes.

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Magnetic Helicity Propagation from Inside the Sun

Highlights of Astronomy ◽

10.1017/s1539299600015185 ◽

2005 ◽

Vol 13 ◽

pp. 97-100

Author(s):

Dana Longcope

Keyword(s):

Active Region ◽

Flux Tube ◽

Convection Zone ◽

Active Regions ◽

Magnetic Helicity ◽

The Sun ◽

Solar Convection Zone ◽

Solar Convection ◽

Region Emergence

AbstractModels of twisted flux tube evolution provide a picture of how magnetic helicity is propagated through the solar convection zone into the corona. According to the models, helicity tends toward an approximately uniform length-density along a tube, rather than concentrating at wider portions. Coronal fields lengthen rapidly during active region emergence, requiring additional helicity to propagate from the submerged flux tube. Recent observations of emerging active regions show an evolution consistent with this prediction, and no evidence of helicity concentrating in wider sections.

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Eruptions and flaring activity in emerging quadrupolar regions

Astronomy and Astrophysics ◽

10.1051/0004-6361/201936246 ◽

2019 ◽

Vol 630 ◽

pp. A134 ◽

Cited By ~ 3

Author(s):

P. Syntelis ◽

E. J. Lee ◽

C. W. Fairbairn ◽

V. Archontis ◽

A. W. Hood

Keyword(s):

Current Sheet ◽

Flux Tube ◽

Flux Rope ◽

Active Regions ◽

Solar Interior ◽

Magnetic Flux Ropes ◽

Solar Observations ◽

Flux Ropes ◽

Polarity Inversion Line ◽

Field Lines

Context. Solar observations suggest that some of the most dynamic active regions are associated with complex photospheric magnetic configurations such as quadrupolar regions, and especially those that have a δ-spot configuration and a strong polarity inversion line (PIL). Aims. We study the formation and eruption of magnetic flux ropes in quadrupolar regions. Methods. We performed 3D magnetohydrodynamics simulations of the partial emergence of a highly twisted flux tube from the solar interior into a non-magnetised stratified atmosphere. We introduced a density deficit at two places along the length of the subphotospheric flux tube to emerge as two Ω-shaped loops, forming a quadrupolar region. Results. At the photosphere, the emerging flux forms two initially separated bipoles, which later come in contact, forming a δ-spot central region. Above the two bipoles, two magnetic lobes expand and interact through a series of current sheets at the interface between them. Two recurrent confined eruptions are produced. In both cases, the reconnection between sheared low-lying field lines forms a flux rope. The reconnection between the two lobes higher in the atmosphere forms field lines that retract down and push against the flux rope, creating a current sheet between them. It also forms field lines that create a third magnetic lobe between the two emerged lobes, that later acts as a strapping field. The flux rope eruptions are triggered when the reconnection between the flux ropes and the field above the ropes becomes efficient enough to remove the tension of the overlying field. These reconnection events occur internally in the quadrupolar system, as the atmosphere is non-magnetised. The flux rope of the first, weaker, eruption almost fully reconnects with the overlying field. The flux rope of the second, more energetic, eruption is confined by the overlying strapping field. During the second eruption, the flux rope is enhanced in size, flux, and twist, similar to confined-flare-to-flux-rope observations. Proxies of the emission reveal the two erupting filaments channels. A flare arcade is only formed in the second eruption owing to the longer lasting and more efficient reconnection at the current sheet below the flux rope.

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The Observational Signature of Flux Tube Waves and an Upper Limit on the Energy Flux Transported by Them

Symposium - International Astronomical Union ◽

10.1017/s007418090004420x ◽

1990 ◽

Vol 138 ◽

pp. 259-262

Author(s):

S.K. Solanki ◽

B. Roberts

Keyword(s):

Energy Flux ◽

Flux Tube ◽

Acoustic Waves ◽

Filling Factor ◽

Active Regions ◽

Upper Limit ◽

Magnetic Elements ◽

Total Luminosity ◽

Observational Signature ◽

Rule Out

The influence of undamped linear longitudinal tube waves on Stokes V profiles is considered. A rough upper limit is set on the energy flux transported by such waves through the photosphere. It is found that this upper limit is larger than the flux in the quiet sun. However, due to the small filling factor of the magnetic elements, the total luminosity of flux tube waves is unlikely to be larger than that of acoustic waves when averaged over the whole sun. Therefore, probably both kinds of waves contribute to chromospheric heating. However, the derived upper limit does not rule out that flux tube waves can significantly enhance the chromospheric brightness in active regions and the supergranular network where the magnetic filling factor is large.

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Average motion of emerging solar active region polarities

Astronomy and Astrophysics ◽

10.1051/0004-6361/201937322 ◽

2020 ◽

Vol 640 ◽

pp. A116 ◽

Cited By ~ 1

Author(s):

H. Schunker ◽

C. Baumgartner ◽

A. C. Birch ◽

R. H. Cameron ◽

D. C. Braun ◽

...

Keyword(s):

Active Region ◽

Flux Tube ◽

Tilt Angle ◽

Active Regions ◽

Field Measurements ◽

Solar Dynamics Observatory ◽

Flux Tubes ◽

Average Motion ◽

Solar Dynamics ◽

East West

Context. The tilt of solar active regions described by Joy’s law is essential for converting a toroidal field to a poloidal field in Babcock-Leighton dynamo models. In thin flux tube models the Coriolis force causes what we observe as Joy’s law, acting on east-west flows as they rise towards the surface. Aims. Our goal is to measure the evolution of the average tilt angle of hundreds of active regions as they emerge, so that we can constrain the origins of Joy’s law. Methods. We measured the tilt angle of the primary bipoles in 153 emerging active regions (EARs) in the Solar Dynamics Observatory Helioseismic Emerging Active Region survey. We used line-of-sight magnetic field measurements averaged over 6 h to define the polarities and measure the tilt angle up to four days after emergence. Results. We find that at the time of emergence the polarities are on average aligned east-west, and that neither the separation nor the tilt depends on latitude. We do find, however, that EARs at higher latitudes have a faster north-south separation speed than those closer to the equator at the emergence time. After emergence, the tilt angle increases and Joy’s law is evident about two days later. The scatter in the tilt angle is independent of flux until about one day after emergence, when we find that higher-flux regions have a smaller scatter in tilt angle than lower-flux regions. Conclusions. Our finding that active regions emerge with an east-west alignment is consistent with earlier observations, but is still surprising since thin flux tube models predict that tilt angles of rising flux tubes are generated below the surface. Previously reported tilt angle relaxation of deeply anchored flux tubes can be largely explained by the change in east-west separation. We conclude that Joy’s law is caused by an inherent north-south separation speed present when the flux first reaches the surface, and that the scatter in the tilt angle is consistent with buffeting of the polarities by supergranulation.

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The Emergence of Magnetic Flux in Active Regions

Symposium - International Astronomical Union ◽

10.1017/s0074180900219153 ◽

2001 ◽

Vol 203 ◽

pp. 225-228

Author(s):

W. P. Abbett ◽

G. H. Fisher ◽

Y. Fan

Keyword(s):

Magnetic Flux ◽

Flux Tube ◽

Active Regions ◽

Large Degree ◽

Solar Interior ◽

Magnetic Flux Tube ◽

Initial Field ◽

Flux Tubes ◽

Mhd Simulations ◽

Magnetic Flux Tubes

Over the past decade, “thin flux tube” models have proven successful in explaining many properties of active regions in terms of magnetic flux tube dynamics in the solar interior. On the other hand, recent 2-D MHD simulations of the emergence of magnetic flux have shown that many of the assumptions adopted in the thin flux tube approximation are invalid. For example, unless the flux tubes exhibit a large degree of initial field line twist — and observations of emerging active regions suggest they do not — they will fragment (break apart) before they are able to emerge through the surface. We attempt to resolve this paradox using a number of 3-D MHD simulations (in the anelastic approximation) that describe the rise and fragmentation of twisted magnetic flux tubes. We find that the degree of fragmentation of an evolving Ω-loop depends strongly on the 3-D geometry of the loop, and that the Coriolis force plays a dynamically important role in the evolution and emergence of magnetic flux.

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