  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.
  With the SOHO/SUMER spectroscopic observations of the 2000 September 26
event, we propose another precursor for CME eruptions, namely, long-time
prominence oscillations.
  The prominence oscillation-and-eruption event was observed by ground-based
H$\alpha$ 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.
  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 H$\alpha$ 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 H$\alpha$ 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.
