Title: Study of a twisted magnetic flux tube in an eruptive event

Author_list: Romano, P., Contarino, L., Zuccarello, F.

Abstract: In the past many observations have shown sigmoidal structures
(Canfield et al., 1999) and twisted flux ropes with a helical-like
pattern of the magnetic field (Romano et al., 2003a), associated with
eruptions of filaments.
As demonstrated by Rust and Kumar (1994), a twisted flux rope will
writhe and take on the S shape if the amount of  twist in it exceeds the
limit for the kink instability, which occurs when 2pTwR/L > 0.581, with
Tw  the number of turns of the magnetic field from end to end and 2R/L
the ratio between the width and the length of the filament.
There are some controversy about whether peculiar motions of shearing or
emersion of twisted flux tubes are responsible of these unstable
configurations (see, e.g., Romano et al., 2003b).
In this context, we plan to investigate these two mechanisms, able to
reach the limit for the kink instability, analysing the eruption of a
reverse S shaped filament which preceded the onset of a flare of class
X17.2, occurred in active region NOAA 10486 on October 28, 2003.
Using H-alpha full-disc images taken at INAF-Catania Astrophysical
Observatory we intend to measure the morphological parameters of  the
filament before its eruption. Using 1 minute cadence MDI full-disc
longitudinal magnetograms taken about 24 hours before the event we plan
to measure the rate of emergence of magnetic flux and the photospheric
horizontal flows in all the active region and in areas corresponding to
filament footpoints. We will compute the velocities using the local
correlation tracking (LCT) method.
Moreover, we will compute the magnetic helicity injected by the
horizontal flows (Chae, 2001) in order to investigate a possible
correlation between the magnetic helicity accumulated along the filament
and its transformation from twist to writhe before the eruptive event.

Canfield, R.C., Hudson, H.S., and McKenzie, D.E., Geophys. Res. Lett. ,
  26, 627, 1999.
Chae, J., ApJ, 560, L95, 2001.
Romano, P., Contarino, L., and Zuccarello, F. , Sol. Phys., 214, 313,
  2003a.
Romano, P., Contarino, L., and Zuccarello, F. , Sol. Phys., 218, 137,
  2003b.
Rust, D.M., and Kumar, A., Sol. Phys., 155, 69, 1994.