Flux‐Tube Twist Resulting from Helical Turbulence: The Σ‐Effect

Longcope, D. W.; Fisher, G. H.; Pevtsov, Alexei A.

doi:10.1086/306312

Cited by 203 publications

(194 citation statements)

References 36 publications

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“…During its rise through the convection zone, the rope divides into smaller strands. As also suggested by the results of Socas-Navarro (2005) about field torsion, different twists develop for each rope strand owing to the effects of turbulent convective buffeting (Longcope et al 1998) during the flux rise. When they emerge at the surface, the various strands appear first as small flux tubes as in the Cheung et al (2010) simulations.…”

Section: Discussionmentioning

confidence: 60%

Shear and vortex motions in a forming sunspot

González

Kneer²,

Schlichenmaier

2012

A&A

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Aims. We measure proper motions of fine structures in a forming sunspot to infer information about the dynamics of flux emergence at the sub-photospheric level. Methods. The active region NOAA 11024 was observed with the Vacuum Tower Telescope at Observatorio del Teide/Tenerife over several days in July 2009. Here, we concentrate on a two-hour sequence taken on July 4, when the leading spot was at an early stage of its evolution. Speckle reconstructions from Ca ii K images and polarimetric data in Fe i λ6173 allow us to study proper motions of umbral fine structures. Results. We detect three prominent features: (1) A light bridge, divided by a dark lane along its axis, shows proper motions in opposing directions on its sides, with velocities of ∼100-500 m s −1 . The flows are seen in both the Ca ii K and the broadband time sequences. (2) Umbral dots in one umbral region outline a vortex with speeds of up to 550 m s −1 . The direction of the motion of the umbral dots is different from that in the light bridge. (3) At one rim of the umbra, the fine structure of the magnetic field moves horizontally with typical velocities of 250-300 m s −1 , prior to the formation of the penumbra. Conclusions. We report on shear and vortex motions in a forming sunspot and interpret them as tracers of twist relaxation in magnetic flux ropes. We suggest that the forming sunspot contains detached magnetic flux ropes that emerge at the surface with different amounts of twist. As they merge to form a sunspot, they untwist giving rise to the observed shear and vortex motions.

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Section: Discussionmentioning

confidence: 60%

Shear and vortex motions in a forming sunspot

González

Kneer²,

Schlichenmaier

2012

A&A

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“…On the other hand, according to Pevtsov et al (1995) and Longcope, Fisher, & Pevtsov (1998), the statistical average of the force-free fitting is given by ff ¼ À2:7 Â 10 À10 m À1 as a function of the latitude h (in degrees). The value of ff for NOAA 8100, which was located at ¼ À20 , is 5:4 Â 10 À9 m À1 , so that n of all three groups is much larger than that, while the sign of n is consistent with the hemispheric helicity rule.…”

Section: No 1 2002mentioning

confidence: 99%

Measurement of Magnetic Helicity Injection and Free Energy Loading into the Solar Corona

Kusano

Maeshiro

Yokoyama

et al. 2002

ApJ

220

195

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We develop a new methodology that can determine magnetic helicity flux as well as Poynting flux across the photosphere based on magnetograph observation. By applying this method, we study the injection mechanism of magnetic helicity and magnetic free energy into the solar corona. In order to derive the helicity and energy fluxes, first the velocity tangential to the solar surface is constructed by applying a correlation tracking technique on the magnetic observation, and second, the velocity component normal to the photosphere is derived from the condition that the magnetic evolution must be consistent with the induction equation. Through this procedure, we can determine the helicity and energy fluxes separately for the shear motion effect and for the flux emergence effect. Based on this new method, NOAA Active Region 8100 was analyzed from 1997 November 1 to 5 using data observed by the Solar and Heliospheric Observatory Michelson Doppler Interferometer and the vector magnetograph at the National Astronomical Observatory of Japan (NAOJ) in Tokyo. The results indicate that the photospheric shear motion and the flux emergence process have equally contributed to the helicity injection and have supplied magnetic helicity of opposite signs into this active region.

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“…The role played by current helicity allows a comparison of results obtained by using the two-scale approach of mean-field theory with concepts which assume the conservation of magnetic helicity (as the Taylor relaxation concept) or consider inverse cascades of magnetic helicity as important for inverse energy cascades, respectively. Longcope, Fisher and Pevtsov (1998) discussed the flux-tube twist resulting from helical turbulence. This process, designated the Sigma-effect, operates on isolated magnetic flux tubes subjected to buffeting by turbulence with a nonvanishing kinetic helicity < u · × u >.…”

Section: Evolution Of Magnetic (Current) Helicity With Solar Cyclementioning

confidence: 99%

Helicity of solar magnetic field from observations

Zhang

2009

Proc. IAU

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Abstract. The helicity is an important quantity to present the basic topological configuration of magnetic field transferred form the solar subatmosphere into the interplanetary space. In this paper, we present the observational solar magnetic field and the relationship with the magnetic helicity.Keywords. Sun: magnetic fields, helicity, interior; turbulence, MHD Importance and definition of magnetic helicityHelicities are topologically a measure of the structural complexity of the corresponding fields. Woltjer (1958) presented the basic properties of magnetic helicity firstly. As indicated by Taylor (1986) that the topological invariants of ideal plasma so that only total magnetic helicity survives. Helicity is described in terms of the internal structure of a flux tube and the external relations between flux tubes. The helicity of open field configurations not bounded by magnetic surfaces is discussed by Berger and Field (1984), and a measure of the propagation of topological structures across open boundaries is given also. Pouquet, Frisch, and Leorat (1976) indicated that the turbulence and magnetic fields are a common feature of many celestial bodies. The study confirms the importance of helicity both in its kinetic and in its magnetic form for generation of large-scale magnetic fields by turbulence. Keinigs (1983) and Keinigs and Gerwin (1986) analyzed that in a magnetized plasma the alpha effect represents a turbulently generated emf directed along the mean magnetic field. The alpha effect is reevaluated in terms of ensemble-averaged properties of the magnetic fluctuation spectrum. The results indicate that the turbulent current helicity must be opposite in sign to the mean-field current helicity in order for the alpha effect to play a role in overcoming the resistive diffusion of large-scale magnetic fields. Kleeorin and Rogachevskii (1999) analyzed the evolution of the magnetic helicity tensor for a nonzero mean magnetic field and for large magnetic Reynolds numbers in an anisotropic turbulence.The magnetic helicity density h m =A · B, with A the vector potential for magnetic field B, measures the chirality of magnetic lines of force. The magnetic helicity is defined as

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Flux‐Tube Twist Resulting from Helical Turbulence: The Σ‐Effect

Cited by 203 publications

References 36 publications

Shear and vortex motions in a forming sunspot

Shear and vortex motions in a forming sunspot

Measurement of Magnetic Helicity Injection and Free Energy Loading into the Solar Corona

Helicity of solar magnetic field from observations

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