Evidence for Current-carrying Emerging Flux

Leka, K. D.; Canfield, RC; McClymont, AN; Driel‐Gesztelyi, L. van

doi:10.1086/177171

Cited by 384 publications

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“…Magnetic flux emerges in a non-potential state (Leka et al, 1996), i.e., twisted and carrying magnetic helicity (Wang, 1996). This inherent global twist has its signatures in the longitudinal magnetic field distribution of the emerging flux: asymmetric magnetic tongues or tails, due to the azimuthal field component in the flux rope, develop.…”

Section: Main Laws and Intrinsic Characteristics Of Large-scale Magnementioning

confidence: 99%

“…Leka et al (1996) were the first to provide observational evidence for twisted flux emergence, inspiring research contributing to a revival of interest in magnetic helicity. Magnetic helicity is a measure of the shear, twist and linkage of the magnetic field.…”

Section: Magnetic Helictymentioning

confidence: 99%

“…The widest-documented example of twisted flux emergence was published by Leka et al (1996), who presented a careful analysis of flux emergence in AR 7260 utilising multi-wavelength data including Hawaii IVM vector magnetograms. They showed that: the emerging bipoles were already co-spatial with significant vertical currents at a very early stage and the currents increased together with the flux; Hα and X-ray morphology, proper motions and measured currents of the bipoles were all consistent, implying the same sense of twist the increase of currents, as the new flux emerged, was not consistent with their generation by photospheric motions.…”

Section: Magnetic Helictymentioning

confidence: 99%

“…Prior to the observational evidence by Leka et al (1996) theoretical arguments had been raised in favour of non-potential flux emergence from considerations of energy available for flaring (McClymont and Fisher, 1989;Melrose, 1992). Furthermore, Schüssler (1979) and later , through MHD simulations, showed that non-twisted flux cannot even make it through the convection zone due to a strong tendency for fragmentation.…”

Section: Magnetic Helictymentioning

confidence: 99%

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Evolution of Active Regions

Driel-Gesztelyi

Green

2015

Living Rev. Sol. Phys.

265

131

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The evolution of active regions (AR) from their emergence through their long decay process is of fundamental importance in solar physics. Since large-scale flux is generated by the deepseated dynamo, the observed characteristics of flux emergence and that of the subsequent decay provide vital clues as well as boundary conditions for dynamo models. Throughout their evolution, ARs are centres of magnetic activity, with the level and type of activity phenomena being dependent on the evolutionary stage of the AR. As new flux emerges into a pre-existing magnetic environment, its evolution leads to re-configuration of small-and large-scale magnetic connectivities. The decay process of ARs spreads the once-concentrated magnetic flux over an ever-increasing area. Though most of the flux disappears through small-scale cancellation processes, it is the remnant of large-scale AR fields that is able to reverse the polarity of the poles and build up new polar fields. In this Living Review the emphasis is put on what we have learned from observations, which is put in the context of modelling and simulation efforts when interpreting them. For another, modelling-focused Living Review on the sub-surface evolution and emergence of magnetic flux see Fan (2009). In this first version we focus on the evolution of dominantly bipolar ARs. 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: Main Laws and Intrinsic Characteristics Of Large-scale Magnementioning

confidence: 99%

Section: Magnetic Helictymentioning

confidence: 99%

Section: Magnetic Helictymentioning

confidence: 99%

Section: Magnetic Helictymentioning

confidence: 99%

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Evolution of Active Regions

Driel-Gesztelyi

Green

2015

Living Rev. Sol. Phys.

265

131

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“…These properties can be seen in Figure 7 showing a vector magnetogram of a δ-spot region taken with the HSP instrument (Mickey, 1985) at the University of Hawaii's Mees Solar Observatory. Analysis of the observed development of δ-spot regions over time have convinced many observers that the magnetic field has a kinked or knotted geometry below the photosphere (Tanaka, 1991;Kurokawa, 1991;Leka et al, 1996).…”

Section: δ-Spot Active Regionsmentioning

confidence: 99%

The Solar Dynamo and Emerging Flux (Invited Review)

Fisher¹,

Fan²,

Longcope³

et al. 2000

Helioseismic Diagnostics of Solar Convection and Activity

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Abstract. The largest concentrations of magnetic flux on the Sun occur in active regions. In this paper, the properties of active regions are investigated in terms of the dynamics of magnetic flux tubes which emerge from the base of the solar convection zone, where the solar cycle dynamo is believed to operate, to the photosphere. Flux tube dynamics are computed using the 'thin flux tube' approximation, and by using MHD simulation. Simulations of active region emergence and evolution, when compared with the known observed properties of active regions, have yielded the following results:(1) The magnetic field at the base of the convection zone is confined to an approximately toroidal geometry with a field strength in the range (3-10) × 10 4 G. The latitude distribution of the toroidal field at the base of the convection zone is more or less mirrored by the observed active latitudes; there is not a large poleward drift of active regions as they emerge. The time scale for emergence of an active region from the base of the convection zone to the surface is typically 2 -4 months. The equatorial gap in the distribution of active regions has two possible origins; if the toroidal field strength is close to 10 5 G, it is due to the lack of equilibrium solutions at low latitude; if it is closer to 3 × 10 4 G, it may be due to modest poleward drift during emergence. (2) The tilt of active regions is due primarily to the Coriolis force acting to twist the diverging flows of the rising flux loops. The dispersion in tilts is caused primarily by the buffeting of flux tubes by convective motions as they rise through the interior. (3) The Coriolis force also bends the active region flux tube shape toward the following (i.e., anti-rotational) direction, resulting in a steeper leg on the following side as compared to the leading side of an active region. When the active region emerges through the photosphere, this results in a more rapid separation of the leading spots away from the magnetic neutral line as compared to the following spots. This bending motion also results in the neutral line being closer to the following magnetic polarity. (4) Active regions behave kinematically after they emerge because of 'dynamic disconnection', which occurs because of the lack of a solution to the hydrostatic equilibrium equation once the flux loop has emerged. This could explain why active regions decay once they have emerged, and why the advection-diffusion description of active regions works well after emergence. Smaller flux tubes may undergo 'flux tube explosion', a similar process, and provide a source for the constant emergence of small-scale magnetic fields. positive twist in the south can be accounted for by the action of Coriolis forces on convective eddies, which ultimately writhes active region flux tubes to produce a magnetic twist of the correct sign and amplitude to explain the observations. (6) The properties of the strongly sheared, flare productive δ-spot active regions can be accounted for by the dynamics of highly twisted ...

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Current‐driven flare and CME models

Melrose

2017

JGR Space Physics

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Roles played by the currents in the impulsive phase of a solar flare and in a coronal mass ejection (CME) are reviewed. Solar flares are magnetic explosions: magnetic energy stored in unneutralized currents in coronal loops is released into energetic electrons in the impulsive phase and into mass motion in a CME. The energy release is due to a change in current configuration effectively reducing the net current path. A flare is driven by the electromotive force (EMF) due to the changing magnetic flux. The EMF drives a flare‐associated current whose cross‐field closure is achieved by redirection along field lines to the chromosphere and back. The essential roles that currents play are obscured in the “standard” model and are described incorrectly in circuit models. A semiquantitative treatment of the energy and the EMF is provided by a multicurrent model, in which the currents are constant and the change in the current paths is described by time‐dependent inductances. There is no self‐consistent model that includes the intrinsic time dependence, the EMF, the flare‐associated current, and the internal energy transport during a flare. The current, through magnetic helicity, plays an important role in a CME, with twist converted into writhe allowing the kink instability plus reconnection to lead to a new closed loop and with the current‐current force accelerating the CME through the torus instability.

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Evidence for Current-carrying Emerging Flux

Cited by 384 publications

References 0 publications

Evolution of Active Regions

Evolution of Active Regions

The Solar Dynamo and Emerging Flux (Invited Review)

Current‐driven flare and CME models

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