Solar-Cycle Variation of Magnetic Helicity in Active Regions

The hemispheric sign rule of helicity and its long-term variation were studied. The data were obtained from the Solar Flare Telescope at Mitaka and the 65-cm solar telescope at Okayama. The data covered the period of 1983-2001, from the declining phase of solar cycle 21 to the rising phase of cycle 23. Although the hemispheric sign rule of helicity generally holds, we also found significant time variations in the yearly values of helicity during the observation period. The hemispheric sign rule of helicity is satisfied in the solar maximum phase, but may not be so in the solar minimum phase.