Initiation of CMEs: the role of magnetic twist

Recent multiwavelength observations, modelling results and theoretical deve lopments indicate the importance of twisted magnetic configurations in sola r active regions (ARs) in the initiation of coronal mass ejections (CMEs). Through multiwavelength analysis of a few representative events we make an attempt to provide constraints for CME models. The two events presented her e in detail start with the expansion of sigmoids (S- or inverse S-shaped lo ops) observed in soft X-rays. Both events (on 25 October 1994 and 14 Octobe r 1995) occurred before the launch of the SOHO spacecraft, but indirect evi dences (i.,e. signatures of an outward propagation traced up to approximate to 20 solar radii and an associated magnetic cloud) suggest that both of t hem were related to CMEs. We show evidence that sigmoids are the coronal ma nifestations of twisted magnetic flux tubes, which start expanding presumab ly due to a loss of equilibrium, It is noteworthy that the analysed CMEs oc curred in a complex (not simply bipolar) magnetic environment and in all ca ses we found evidences of the interaction (magnetic reconnection) with the surrounding fields. We propose a scenario for sigmoid expansion related CME events and suggest that twisted magnetic configurations are good candidate s for being source regions of CMEs. (C) 2000 Elsevier Science Ltd. All righ ts reserved.