Abstract
Magnetic helicity provides a theoretical tool for characterizing the solar dynamo and the evolution of the coronal field. The magnetic helicity may be inferred from several types of observation including vector magnetograms of the photospheric magnetic fields. The helicity of an active region reflects, to some degree, the twist in the magnetic field below it. Photospheric observations reveal a tendency for left-handed chirality in the Northern hemisphere, although one-quarter to one-third of the active regions twist in the opposite sense. This means that coronal magnetic field has negative helicity in the North. Sub-photospheric fields will have left-handed twist in the North, although the net helicity also depends on the writhe of the flux tube axes. We show that buffeting by turbulence; the so-called Σ-effect, can explain the handedness and level of intrinsic variation of observed twist. This mechanism does not generate helicity, rather it produces twist and writhe of opposite signs. In this scenario, helicity of one sign propagates into the corona, while opposing helicity propagates downward in the form of torsional Alfvén waves.
Published Version
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