On Formation of the Sigmoidal Structure in Solar Active Region NOAA 8100

Using soft X-ray images taken by the Soft X-ray Telescope on board Yohkoh, line-of-sight magnetograms taken by SOHO/MDI and vector magnetograms taken at Beijing Astronomical Observatory, we have studied the formation of the sigmoidal structure in active region NOAA 8100 on 3–4 November 1997. The sigmoidal structure appeared after the occurrences of a series of flares accompanied by new magnetic flux emergence. This implies that reconnection may play a role in formation of this sigmoid structure. We calculated the self-helicity (twist) and mutual helicity of the active region before and after the formation of the sigmoidal structure and found that the mutual helicity decreased. The twist of the sigmoidal structure was higher than the twist of the emerging magnetic flux and exceeded the critical twist for kink in stability. This result suggests that the reconnection increased the twist of magnetic flux tubes by converting mutual helicity to self-helicity, supporting the previous studies by Berger (1998, 1999).