EIT and SXT observations of a quiet-region filament ejection: First eruption, then reconnection

We observe a slow-onset quiet-region filament eruption with the EUV Imaging Telescope (EIT) on the Solar Heliospheric Observatory (SOHO) and the soft X-ray telescope (SXT) on Yohkoh. This event occurred on 1999 April 18 and w as likely the origin of a coronal mass ejection detected by SOHO at 08:30 U T on that day. In the EIT observation, one-half of the filament shows two s tages of evolution: stage 1 is a slow, roughly constant upward movement at approximate to1 km s(-1) lasting approximate to6.5 hr, and stage 2 is a rap id upward eruption at approximate to 16 km s(-1) occurring just before the filament disappears into interplanetary space. The other half of the filame nt shows little motion along the line of sight during the time of stage 1 b ut erupts along with the rest of the filament during stage 2. There is no o bvious emission from the filament in the SXT observation until stage 2; at that time, an arcade of EUV and soft X-ray loops forms first at the central location of the filament and then expands outward along the length of the filament channel. A plot of EUV intensity versus time of the central portio n of the filament (where the postflare loops initially form) shows a flat p rofile during stage 1 and a rapid upturn after the start of stage 2. This l ight curve is delayed from what would be expected if "tether-cutting" recon nection in the core of the erupting region were responsible for the initiat ion of the eruption. Rather, these observations suggest that a loss of stab ility of the magnetic field holding the filament initiates the eruption, wi th reconnection in the core region occurring only as a by-product.