www.elsevier.nl/locate/jastp Initiation of CMEs: the role of magnetic twist

Recent multiwavelength observations, modelling results and theoretical developments indicate the importance of twisted magnetic congurations in solar active regions (ARs) in the initiation of coronal mass ejections (CMEs). Through multiwave-length analysis of a few representative events we make an attempt to provide constraints for CME models. The two events presented here in detail start with the expansion of sigmoids (S- or inverse S-shaped loops) observed in soft X-rays. Both events (on 25 October 1994 and 14 October 1995) occurred before the launch of the SOHO spacecraft, but indirect evidences (i.e. signatures of an outward propagation traced up to 20 solar radii and an associated magnetic cloud) suggest that both of them were related to CMEs. We show evidence that sigmoids are the coronal manifestations of twisted magnetic ux tubes, which start expanding presumably due to a loss of equilibrium. It is noteworthy that the analysed CMEs occurred in a complex (not simply bipolar) magnetic environment and in all cases we found evidences of the interaction (magnetic reconnection) with the surrounding elds. We propose a scenario for sigmoid expansion related CME events and suggest that twisted magneti