  We report observation of a large-amplitude filament oscillation followed by
an eruption. This is used to probe the pre-eruption condition and the trigger
mechanism of solar eruptions. We used the EUV images from the
Extreme-Ultraviolet Imaging Telescope on board SOHO satellite and the H-alpha
images from the Flare Monitoring Telescope at Hida Observatory. The observed
event is a polar crown filament that erupted on 15 Oct. 2002. The filament
clearly exhibited oscillatory motion in the slow-rising, pre-eruption phase.
The amplitude of the oscillation was larger than 20 km/s, and the motion was
predominantly horizontal. The period was about 2 hours and seemed to increase
during the oscillation, indicating weakening of restoring force. These results
strongly indicate that, even in the slow-rise phase before the eruption, the
filament retained equilibrium and behaved as an oscillator, and the equilibrium
is stable to nonlinear perturbation. Moreover, the transition from such
nonlinear stability to either instabilities or a loss of equilibrium that leads
to the eruption occurred in the Alfven time scale. This suggests that the onset
of the eruption was triggered by a fast magnetic reconnection that stabilized
the pre-eruption magnetic configuration, rather than by the slow shearing
motion at the photosphere.
