Abstract:
A major two-ribbon X17 flare occurred on 28 October 2003, starting at 11:01 UT in active
region NOAA 10486. This flare was accompanied by the eruption of a filament and by one of the fastest
halo coronal mass ejections registered during the October – November 2003 strong activity period.
We focus on the analysis of magnetic field (SOHO/MDI), chromospheric (NainiTal observatory and
TRACE), and coronal (TRACE) data obtained before and during the 28 October event. By combining
our data analysis with a model of the coronal magnetic field, we concentrate on the study of two events
starting before the main flare. One of these events, evident in TRACE images around one hour prior
to the main flare, involves a localized magnetic reconnection process associated with the presence
of a coronal magnetic null point. This event extends as long as the major flare and we conclude that
it is independent from it. A second event, visible in Hα and TRACE images, simultaneous with the
previous one, involves a large-scale quadrupolar reconnection process that contributes to decrease
the magnetic field tension in the overlaying field configuration; this allows the filament to erupt in
a way similar to that proposed by the breakout model, but with magnetic reconnection occurring at
Quasi-Separatrix Layers (QSLs) rather than at a magnetic null point.
