Insights into Filament Eruption Onset from Solar Dynamics Observatory Observations

We examine the buildup to and onset of an active region filament confined eruption of 2010 May 12, using EUV imaging data from the Solar Dynamics Observatory (SDO) Atmospheric Imaging Array and line-of-sight magnetic data from the SDO Helioseismic and Magnetic Imager. Over the hour preceding eruption the filament undergoes a slow rise averaging {approx}3 km s{sup -1}, with a step-like trajectory. Accompanying a final rise step {approx}20 minutes prior to eruption is a transient preflare brightening, occurring on loops rooted near the site where magnetic field had canceled over the previous 20 hr. Flow-type motions of the filament are relatively smooth with speeds {approx}50 km s{sup -1} prior to the preflare brightening and appear more helical, with speeds {approx}50-100 km s{sup -1}, after that brightening. After a final plateau in the filament's rise, its rapid eruption begins, and concurrently an outer shell 'cocoon' of the filament material increases in emission in hot EUV lines, consistent with heating in a newly formed magnetic flux rope. The main flare brightenings start {approx}5 minutes after eruption onset. The main flare arcade begins between the legs of an envelope-arcade loop that is nearly orthogonal to the filament, suggesting that the flare results frommore » reconnection among the legs of that loop. This progress of events is broadly consistent with flux cancellation leading to formation of a helical flux rope that subsequently erupts due to onset of a magnetic instability and/or runaway tether cutting.« less