Mem. S.A.It. Vol. 74, 651 c  SAIt 2003 Memorie della P. Romano, L. Contarino and F. Zuccarello Dipartimento di Fisica e Astronomia - Sezione Astrofisica, Universit´a di Catania, Via S.Sofia 78, 95125 Catania, Italy; e-mail: prom@ct.astro.it Abstract. Using EUV images aquired by TRACE, we analysed the eruption of a prominence, occurred on July 19, 2000 in the Active Region NOAA 9077. We approximated the prominence to a cylindrical curved flux tube and estimated the behaviour of several geometrical parameters during the activation and the eruption phases. We found a decrease in the total twist of one helical thread from Φ ∼ 10π to Φ ∼ 2π during the prominence eruption, indicating a relaxation of the magnetic field towards a more stable configuration. Moreover we determined that, at the onset of the activation, the number N of turns of a magnetic field line over the whole length of the prominence was ∼ 5.0, while the value of the ratio P/r 0 between the pitch of the magnetic field lines and the prominence width was ∼ 0.45, in agreement with the kink mode instability. Key words. Active prominence – Helical-like structure 1. Introduction The evolution of prominences over their lifetime is characterized by several stages during which the initial structure gets more and more complex. When the structure be- comes too intricate, the prominence be- comes unstable and erupts. One of the most important properties of eruptive prominences is their helical mag- netic configuration. On the basis of this observational evidence, several prominence models involve helically twisted cylindri- cal magnetic field configurations (Priest (1990)). If we consider a prominence as a magnetic Send offprint requests to : P. Romano Correspondence to : Dipartimento di Fisica e Astronomia. Via S.Sofia 78, 95125 Catania, Italy. flux tube, the onset of its ascending mo- tion can be interpreted as being due to kink mode instability (Sakurai (1976)). The study of the linear instability of magnetic flux tubes, for the magnetic field configura- tion used by Dungey and Loughhead (1954) or by Gold and Hoyle (1960), showed that the kink instability sets in if P/r 0 (ratio be- tween the pitch of magnetic field lines and the prominence width) decreases and/or the number N of turns of a magnetic field line over the whole length of the promi- nence increases. These variations can be caused by photospheric motions winding up the filament at both ends or by the emer- gence of a new twisted flux. Several recent works have singled out that the prominences, typically observed in the Hα line, correspond, in EUV lines, to inten- sity depletions, usually referred to as EUV