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Slow-Rise and Fast-Rise Phases of an Erupting Solar Filament, and Flare Emission Onset

We observe the eruption of an active-region solar filament on 1998 July 11 using high time cadence and high spatial resolution EUV observations from the TRACE satellite, along with soft X-ray images from the soft X-ray telescope (SXT) on the Yohkoh satellite, hard X-ray fluxes from the BATSE instrument on the CGRO satellite and from the hard X-ray telescope (HXT) on Yohkoh, and ground-based magnetograms. We concentrate on the initiation of the eruption in an effort to understand the eruption mechanism. Prior to eruption the filament undergoes a slow upward movement in a slow-rise phase with an approximately constant velocity of ≈15 km s-1 that lasts about 10 minutes. It then erupts in a fast-rise phase, accelerating to a velocity of ≈200 km s-1 in about 5 minutes and then decelerating to ≈150 km s-1 over the next 5 minutes. EUV brightenings begin about concurrently with the start of the filament's slow rise and remain immediately beneath the rising filament during the slow rise; initial soft X-ray brightenings occur at about the same time and location. Strong hard X-ray emission begins after the onset of the fast rise and does not peak until the filament has traveled to a substantial altitude (to a height about equal to the initial length of the erupting filament) beyond its initial location. Our observations are consistent with the slow-rise phase of the eruption resulting from the onset of "tether cutting" reconnection between magnetic fields beneath the filament, and the fast rise resulting from an explosive increase in the reconnection rate or by catastrophic destabilization of the overlying filament-carrying fields. About 2 days prior to the event, new flux emerged near the location of the initial brightenings, and this recently emerged flux could have been a catalyst for initiating the tether-cutting reconnection. With the exception of the sudden transition from the slow-rise phase to the fast-rise phase in our event, our filament's height-time profile is qualitatively similar to the plot of the erupting flux rope height as a function of time recently computed by Chen and Shibata for a model in which the eruption is triggered by reconnection between an emerging field and another field under the flux rope.