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On August 1st, XRT observed this lovely eruption from an unnumbered region just north of disk-center, which appears to be a decayed active region from the previous rotation. The region exhibited the characteristic S-shape (Ƨ-shaped in this case) of so-called "sigmoids," which are composed of highly sheared magnetic field lines that form along boundaries between positive and negative surface magnetic flux (polarity inversion lines). Sigmoids grow unstable as they become increasingly twisted and S-shaped, leading to eruptions like the one seen here. Overlying loops expand as the structure rises, and post-eruption loops form perpendicular to the Ƨ after the eruption. The escaping material (along with cool filament material that is invisible to XRT) forms a Coronal Mass Ejection (CME) that is detected about an hour later by the LASCO/C2 instrument. No GOES flare is attributed to this region, but a C-flare occurs during the eruption in an active region to the southeast (12130), which also produced an M-flare a few hours later.
Keywords: Sigmoid, CME, Loop Expansion