Large amplitude oscillation of a polar crown filament in the pre-eruption phase

Aims. 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. Methods. 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 HidaObservatory. The observed event is a polar crown filament that erupted on 15 Oct. 2002. Results. The filament clearly exhibited oscillatory motion in the slow-rising, pre-eruption phase. The amplitude of the oscillation was larger than 20 km s(-1), and the motion was predominantly horizontal. The period was about 2 h and seemed to increase during the oscillation, indicating ease weakening of restoring force. Conclusions. 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. The transition from such nonlinear stability to neither instabilities or a loss of equilibrium that leads to the eruption occurred in the Alfvvn time scale (similar to 1 h). This suggests that the onset of the eruption was triggered by a fast magnetic reconnection that destabilized the pre-eruption magnetic configuration, rather than by the slow shearing motion at the photosphere.