Toward Understanding the 3D Structure and Evolution of Magnetic Flux Ropes in an Extremely Long Duration Eruptive Flare

In this work, we analyze the initial eruptive process of an extremely long duration C7.7-class flare that occurred on 2011 June 21. The flare had a 2 hr long rise time in soft X-ray emission, which is much longer than the rise time of most solar flares, including both impulsive and gradual ones. Combining the facts that the flare occurred near the disk center as seen by the Solar Dynamic Observatory (SDO) but near the limb as seen by two Solar Terrestrial Relations Observatory (STEREO) spacecraft, we are able to track the evolution of the eruption in 3D in a rare slow-motion manner. The time sequence of the observed large-scale EUV hot channel structure in the Atmospheric Imaging Assembly (AIA) high-temperature passbands of 94 and 131 angstrom clearly shows the process of how the sigmoid structure prior to the eruption was transformed into a near-potential post-eruption loop arcade. We believe that the observed sigmoid represents the structure of a twisted magnetic flux rope (MFR), which has reached a height of about 60 Mm at the onset of the eruption. We argue that the onset of the flare precursor phase is likely triggered by the loss of the magnetohydrodynamic equilibrium of a preexisting MFR, which leads to the slow rise of the flux rope. The rising motion of the flux rope leads to the formation of a vertical current sheet underneath, triggering the fast magnetic reconnection that in turn leads to the main phase of the flare and fast acceleration of the flux rope.