Instability analysis of an active prominence

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 Phi ~ 10 pi to Phi ~ 2 pi 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/r0 between the pitch of the magnetic field lines and the prominence width was ~ 0.45, in agreement with the kink mode instability....