Abstract
Using a nonlinear force‐free field (NLFFF) method to extrapolate photospheric magnetic fields, we compute the force‐free field parameter (α), which is a measure of torsion of the magnetic field lines. We calculated the α parameter for 62 active regions that produced major eruptive events in solar cycle 24. We verified statistically how the 62 studied active regions obey the hemispheric rule of helicity. In agreement with previous studies, we found that 67% of active regions located in the northern hemisphere have a negative sign for the global alpha parameter αg, and 69% of the active regions situated in the southern hemisphere have a positive sign. We have identified and analyzed five cases of evolution of αg at times when eruptive phenomena occurred in the selected active regions. We find that 52% of cases show an increase of torsion during flares, while only 22% show a decrease. This shows that increasing torsion plays an important role in flare ignition. For the other cases, a definitive conclusion cannot be reached because of multiple variations.
Published Version
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