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

Shangbin Yang   Submitted: 2009-10-08 02:34

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 (counter-clockwise). 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.

Authors: Yang, S.; Zhang, H.; Büchner, J.
Projects: SoHO-MDI

Publication Status: A&A (published)
Last Modified: 2009-10-08 07:40
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Magnetic Helicity Exchange Between Neighboring Active Regions  

Shangbin Yang   Submitted: 2009-10-08 02:30

Addressing the long-lasting problem of the magnetic helicity distribution in the solar corona, a proof for magnetic helicity exchange between two neighboring emerging active regions (ARs) was found: when AR 9188 emerged, it first started to accumulate positive helicity, while the later neighboring emerging AR 9192 accumulated negative helicity. At a later time, after the bright connecting loops became visible between the two ARs, AR 9188 also suddenly started to gain negative helicity. At the same time AR 9192 started to loose negative helicity. It was found that the magnetic helicity fluxes of the two ARs changed simultaneously by almost the same amount. At one instant it was even possible to determine that the connecting loop between the two ARs carried negative helicity. We excluded the possibility that magnetic flux emergence was causing the observed variation of the magnetic helicity. Hence, magnetic helicity was indeed transferred from the late emerging AR 9192 to AR 9188 via an unbalanced magnetic torque along the loop. Such kinds of helicity transfer might be a common mechanism of redistribution of magnetic helicity in the solar atmosphere, which has just not been widely observed yet.

Authors: Yang, S.; Büchner, J.; Zhang, H.
Projects: SoHO-MDI

Publication Status: ApJL (publised)
Last Modified: 2009-10-08 07:41
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Magnetic helicity accumulation and tilt angle evolution of newly emerging active regions
Magnetic Helicity Exchange Between Neighboring Active Regions

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