The Sources of Magnetic Field Twist in Solar Active Regions

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Observations have revealed that a hemispheric preference of magnetic chirality (handedness) exists throughout the solar atmosphere. For example, the current helicity of active regions is predominantly negative (left-handed twist) in the northern hemisphere and positive (right-handed twist) in the southern. The explanation of this hemispheric tendency is still open to question. In this paper we first review several possible mechanisms and clarify some misunderstandings. In our views, in the photosphere, the differential rotation acting on already emerged sunspot magnetic fields will lead to negative current helicity in the northern hemisphere and positive in the southern, but the same effect caused by the Coriolis force is opposite in sign. In the turbulent convection zone, the Coriolis force acting on the rising magnetic flux tubes will result in negative/positive helicity in the northern/southern hemisphere, but the corresponding action by the differential rotation will give rise to a reversed result. Moreover, in this region the α-effect will produce the wrong sign to explain the observed sense of magnetic twist. It should be noteworthy that the two current helicities generated by the α-effect, that in the mean field and that in the fluctuations, have opposite signs, and the former is positive/negative in the northern/southern hemisphere while the latter is negative/positive in the northern/southern hemisphere. In the overshoot region at the base of the convection zone, the current helicity created by the α-effect has the sign needed. Finally, we suggest that some surface flows (e.g., converging flows that can lead to cancellation of opposite-polarity flux in the photosphere) and magnetic reconnection are also important to the redistribution (or regeneration) of magnetic twist in active regions.