SOHO/SUMER Observations of Prominence Oscillation Before Eruption

Context. Coronal mass ejections (CMEs), as a large-scale eruptive phenomenon, often reveal some precursors in the initiation phase, e.g., X-ray brightening, filament darkening, etc, which are useful for CME modeling and space weather forecast. Aims. With the SOHO/SUMER spectroscopic observations of the 2000 September 26 event, we propose another precursor for CME eruptions, namely, long-time prominence oscillations. Methods. The prominence oscillation-and-eruption event was observed by ground-based Hα telescopes and space-borne white-light, EUV imaging and spectroscopic instruments. In particular, the SUMER slit was observing the prominence in a sit-and-stare mode. Results. The observations indicate that a siphon flow was moving from the proximity of the prominence to a site at a projected distance of 270′′, which was followed by repetitive Hα surges and continual prominence oscillations. The oscillation lasted 4 hours before the prominence erupted as a blob-like CME. The analysis of the multiwavelength data indicates that the whole series of processes fits well into the emerging flux trigger mechanism for CMEs. In this mechanism, emerging magnetic flux drives a siphon flow due to increased gas pressure where the background polarity emerges. It also drives Hα surges through magnetic reconnection where the opposite polarity emerges. The magnetic reconnection triggers the prominence oscillations, as well as its loss of equilibrium, which finally leads to the eruption of the prominence. It is also found that the reconnection between the emerging flux and the pre-existing magnetic loop proceeds in an intermittent, probably quasi-periodic, way.