The magnetic topology of a sigmoid

Recent surveys of solar features have linked the “sigmoid-to-arcade” scenario observed in the soft X-ray corona to coronal mass ejection (CME) onset (Geophys. Res. Lett. 26 (1999) 627, Geophys. Res. Lett. 14 (1998) 2481). Further to these observations, incorporation of extreme-ultraviolet, white light and H-alpha data into such a survey (Geophys. Res. Lett. 27 (2000) 2161) has illustrated the need for a quantitative definition of the term “sigmoid” and further understanding of such features if they are to be used as a means by which to predict CME onset. We analyse two sample active regions in detail, each appearing both sigmoidal and eruptive in Yohkoh soft X-ray telescope (SXT) full-disk data. Both regions were observed during October 1997 and each produced a flare displaying eruptive characteristics. In each case, formation of a flare-arcade was observed by both SXT and the extreme ultraviolet imaging telescope (EIT) following the event. EUV dimming and coronal EIT waves were also observed in each case. We have studied each active region both before and after eruption using soft X-ray, EUV and H-alpha data. A linear force-free field extrapolation has also been applied as a means by which to determine the active region field deviation from potential in each case. Each active region was observed to erupt by means of a different mechanism and while both events show signatures of eruption and consequently, mass ejection, only one produced a CME large enough to be observed by the SoHO large angle spectroscopic coronagraph. The implications of these observations in terms of CME prediction are discussed.