A Check for Consistency between Different Magnetic Helicity Measurements Based on the Helicity Conservation Principle

Lim, Eun-Kyung; Jeong, Hyeon; Chae, Jongchul; Moon, Yong‐Jae

doi:10.1086/510575

Cited by 32 publications

(37 citation statements)

References 21 publications

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“…This provides an estimation independent of the helicity injection computed above. The equivalent number of turns typically ranges from 0.1 to 0.2 Green et al, 2002;Mandrini et al, 2005;Lim et al, 2007).…”

Section: Resultsmentioning

confidence: 99%

Evidence of Twisted Flux-Tube Emergence in Active Regions

et al. 2014

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Elongated magnetic polarities are observed during the emergence phase of bipolar active regions (ARs). These extended features, called magnetic "tongues", are interpreted as a consequence of the azimuthal component of the magnetic flux in the toroidal flux-tubes that form ARs. We develop a new systematic and user-independent method to identify AR tongues. Our method is based on the determination and analysis of the evolution of the AR main polarity inversion line (PIL). The effect of the tongues is quantified by measuring the acute angle [τ ] between the orientation of the PIL and the direction orthogonal to the AR main bipolar axis. We apply a simple model to simulate the emergence of a bipolar AR. This model lets us interpret the effect of magnetic tongues on parameters that characterize ARs (e.g. the PIL inclination, and the tilt angles and their evolution). In this, idealized kinematic emergence model, τ is a monotonically increasing function of the twist and has the same sign as the magnetic helicity. We systematically apply our procedure to a set of bipolar ARs (41 ARs) that were observed emerging in line-of-sight magnetograms over eight years. For the majority of the cases studied, the presence of tongues has a small influence on the AR tilt angle since tongues have much lower magnetic flux than the more concentrated main polarities. From the observed evolution of τ , corrected by the temporal evolution of the tilt angle and its final value when the AR is fully emerged, we estimate the average number of turns present in the subphotospheric emerging flux-rope. These values for the 41 observed ARs, except one, are below unity. This indicates that sub-photospheric flux-ropes typically have a low amount of twist, i.e. highly twisted flux-tubes are rare. Our results demonstrate that the evolution of the PIL is a robust indicator of the presence of tongues and constrains the amount of twist present in emerging flux-tubes.

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Section: Resultsmentioning

confidence: 99%

Evidence of Twisted Flux-Tube Emergence in Active Regions

et al. 2014

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“…For our purpose, we only need to know the integral of the helicity flux densities, while the pattern of the helicity flux is not important for this. Note that Lim et al (2007) compared the consistency of the use of the LCT method to measure magnetic helicity injection through the photosphere and that of the LFFF method to determine helicity. Their results support the reliability of previous method determining the injected helicity.…”

Section: Helicitymentioning

confidence: 99%

Magnetic helicity accumulation and tilt angle evolution of newly emerging active regions

Yang

Zhang²,

Büchner

2009

A&A

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Context. It has been known for years that there is a general dominance of negative (positive) helicity of active regions (ARs) in the northern (southern) solar hemisphere. For a better understanding of the role of helicity in the evolution of active regions, it is necessary to know more about the accumulation of helicity during the emergence of active regions. In particular, different conclusions were drawn in the past about the relationship between the accumulated helicity and the writhe of active regions. Aims. We investigate the accumulation of helicity in newly emerging simple bipolar solar active regions. We also investigate the relation between the accumulated helicity and writhe. Methods. We obtain helicity accumulation by applying Fast Fourier Transforms (FFT) and local correlation tracking (LCT) to MDI data. We deduce the writhe of the active regions according to the evolution of the tilt angle between the connecting line of the weighting centers of opposite polarities in the ARs. Results. It is found that the accumulated helicity is proportional to the exponent of magnetic flux (|H| ∝ Φ 1.85 ) in the 58 selected newly emerged simple ARs. 74% of ARs have a negative (positive) helicity when the above defined tilt angle rotates clockwise (counterclockwise). This means that the accumulated helicity and writhe have the same sign for most of the investigated ARs according to the tilt angle evolution of ARs. We also found that 56% (57.6%) of these ARs in the northern (southern) photosphere provide negative (positive) helicity to the corona in the course of the emergence of magnetic flux.

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“…These features seem to be invariant with respect to solar cycle, as would be predicted by helical dynamos, even when the field itself reverses sign. Note also that Hα filaments seem to exhibit dextral (right handed) twist in North and sinstral (left handed) in south (Martin and McAllister 1997) BUT: right handed Hα filaments may be supported by left handed fields and vice versa (Rust 1999) There have been efforts to measure the rate of injection of magnetic helicity (Chae 2004;Schuck 2005;Lim et al 2007), particularly, its gauge invariant cousin: the "relative magnetic helicity" (Berger & Field 1984; the difference between actual magnetic helicity and that of a potential field) by tracking footpoint motions. It can be shown that the footprint motions provide a direct measure of this input rate (Démoulin & Berger 2003).…”

Section: Helicity Fluxes In Galactic and Stellar Contextsmentioning

confidence: 99%

Magnetic Helicity and Large Scale Magnetic Fields: A Primer

Blackman¹

2016

Space Sciences Series of ISSI

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Magnetic fields of laboratory, planetary, stellar, and galactic plasmas commonly exhibit significant order on large temporal or spatial scales compared to the otherwise random motions within the hosting system. Such ordered fields can be measured in the case of planets, stars, and galaxies, or inferred indirectly by the action of their dynamical influence, such as jets. Whether large scale fields are amplified in situ or a remnant from previous stages of an object's history is often debated for objects without a definitive magnetic activity cycle. Magnetic helicity, a measure of twist and linkage of magnetic field lines, is a unifying tool for understanding large scale field evolution for both mechanisms of origin. Its importance stems from its two basic properties: (1) magnetic helicity is typically better conserved than magnetic energy; and (2) the magnetic energy associated with a fixed amount of magnetic helicity is minimized when the system relaxes this helical structure to the largest scale available. Here I discuss how magnetic helicity has come to help us understand the saturation of and sustenance of large scale dynamos, the need for either local or global helicity fluxes to avoid dynamo quenching, and the associated observational consequences. I also discuss how magnetic helicity acts as a hindrance to turbulent diffusion of large scale fields, and thus a helper for fossil remnant large scale field origin models in some contexts. I briefly discuss the connection between large scale fields and accretion disk theory as well. The goal here is to provide a conceptual primer to help the reader efficiently penetrate the literature.

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A Check for Consistency between Different Magnetic Helicity Measurements Based on the Helicity Conservation Principle

Cited by 32 publications

References 21 publications

Evidence of Twisted Flux-Tube Emergence in Active Regions

Evidence of Twisted Flux-Tube Emergence in Active Regions

Magnetic helicity accumulation and tilt angle evolution of newly emerging active regions

Magnetic Helicity and Large Scale Magnetic Fields: A Primer

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