The Magnetic Helicity Budget of Solar Active Regions and Coronal Mass Ejections

Nindos, A.; Zhang, J.; Zhang, H.

doi:10.1086/377126

Cited by 139 publications

(154 citation statements)

References 46 publications

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“…Its sum over the emergence phase, normalized by the maximum magnetic flux to the second power, provides an estimation of the number of turns (of an equivalent uniformly twisted flux-tube). These estimations range from a few times 0.01 to a maximum value just above 0.1 (Nindos, Zhang, and Zhang, 2003;Liu and Zhang, 2006;Jeong and Chae, 2007;LaBonte, Georgoulis, and Rust, 2007;Tian and Alexander, 2008;Yang, Zhang, and Büchner, 2009). The magnetic helicity contained at a given time in the coronal field can also be estimated from magnetic-field extrapolations of the photospheric magnetograms, using the force-free model that best fits the observed coronal loops.…”

Section: Resultsmentioning

confidence: 95%

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: 95%

Evidence of Twisted Flux-Tube Emergence in Active Regions

et al. 2014

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“…They compared their results with those of other related studies, and they discussed, in particular, whether the kink instability is relevant to explain the peculiar evolution of some active regions. Nindos and Zhang (2002) and Nindos, Zhang and Zhang (2003) investigated whether the bulk of magnetic helicity carried away from the Sun by CMEs comes from helicity injected to the corona by such motions or by emerging magnetic flux. Green et al (2002) studied the magnetic helicity evolution in an active region (NOAA 8100) in which the main photospheric polarities rotate around each other during five Carrington rotations.…”

Section: H Zhangmentioning

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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“…Nindos et al (2003) computed the magnetic helicity injected into the corona by the photospheric horizontal motions in six solar active regions (ARs) associated with halo CMEs by using the method described by Chae et al (2001). Nindos et al (2010) computed the velocities using the local correlation tracking (LCT) method (November & Simon 1988) and found a broad consistency between the helicity injected into the corona and the helicity carried away by the CMEs with the magnetic cloud (MC) helicity computations as proxies to the CME helicity.…”

Section: Introductionmentioning

confidence: 99%

“…The helicity flux density maps resulting from Chae's method (the so called G A maps) are contaminated by spurious bipolar signals that may mask the real spatial distribution of helicity injection. Pariat et al (2006) applied a better proxy to the helicity flux density calculation (the so-called G θ , proposed by Pariat et al 2005) to the same set of data studied by Nindos et al (2003). Although they still used the LCT method, they obtained a much more homogeneous pattern for all ARs in the maps of helicity injection because they were able to partially remove the fake polarities.…”

Section: Introductionmentioning

confidence: 99%

“…Therefore, we believe that the investigation of fluxes of different sign of magnetic helicity may be more useful than the total helicty flux in the estimation of an AR complexity and in its instability. For this reason we considered the same dataset in this work as the one studied by Nindos et al (2003) and by Pariat et al (2006) to measure the horizontal velocity fields by the differential affine velocity estimator (DAVE) method (Schuck 2005) and to estimate the magnetic helicity flux distribution by the Pariat et al (2005) method. Then we determined how fragmented the maps of helicity flux are to single out the number of magnetic features that are characterized by different signs of helicity flux and investigated its correlation with the flare and CME occurrence.…”

Section: Introductionmentioning

confidence: 99%

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Flare occurrence and the spatial distribution of the magnetic helicity flux

Romano

Zuccarello

2011

A&A

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Context. The accumulation of magnetic helicity via emergence of new magnetic flux and/or shearing photospheric motions is considered an important tool for understanding the processes that lead to eruptive phenomena. Aims. We highlight a specific aspect of the magnetic helicity accumulation, providing new observational evidence of the role played by the interaction of magnetic field systems that are characterized by opposite signs of the magnetic helicity flux in triggering solar eruptions. Methods. The amount of magnetic helicity injected into the corona through the photosphere in a sample of active regions (ARs) during their passage across the solar disk was measured by inferring the apparent motion of photospheric footpoints of magnetic field lines from a time series of MDI full-disk line-of-sight magnetograms. The temporal variation of the maps of magnetic helicity flux was analysed by measuring the fragmentation of the patches that are characterized by the flux of magnetic helicity. The temporal correlation between the number of these patches and the flare and coronal mass ejection (CME) occurrence has also been studied. Results. The fragmentation of the patches singled out in the maps of the magnetic helicity flux provides a useful indication of the evolution of the AR complexity. The more fragmented the maps of the magnetic helicity flux are, the higher is the flare and CME frequency. Moreover, most of the events occur for low values (∼3 ÷ 17) of the difference of the number of patches with opposite signs of magnetic helicity flux. Conclusions. These results indicate that not only the accumulation of magnetic helicity in the corona, but also its positive and negative fragmentation and distribution should be taken into account to provide a more confident indication of AR complexity and flare/CME productivity. In particular, the interaction of magnetic systems characterized by opposite sign of magnetic helicity flux may be responsible for many observed eruptions.

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The Magnetic Helicity Budget of Solar Active Regions and Coronal Mass Ejections

Cited by 139 publications

References 46 publications

Evidence of Twisted Flux-Tube Emergence in Active Regions

Evidence of Twisted Flux-Tube Emergence in Active Regions

Helicity of solar magnetic field from observations

Flare occurrence and the spatial distribution of the magnetic helicity flux

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