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When an active region takes on an "S" shape, it's known as a "sigmoid". Solar observers are very keen to keep an eye on such regions because they're particularly likely to erupt, spewing charged particles in a potentially earthward direction. The "S" shape (or in this case, inverted "S" shape) is thought to indicate the presence of a "flux rope", which is a bundle of magnetic field lines twisted around a central axis. As the flux rope becomes increasingly twisted, the "S" shape develops and eventually erupts when the configuration becomes unstable. This is likely what we see in the animation above. Click the image for a 2-day long movie, over which you'll see the region become increasingly S-shaped before erupting. After the eruption, we see the fuzzy-looking "post-flare loops", which arc over the space previously occupied by the flux rope. You might notice that there are two gaps in the data near the eruption. The first one (just at the onset) is due to the satellite passing through the South Atlantic Anomaly, during which time we typically don't take data. The second gap, however, was due to XRT's twice-daily full-disk observations. Click here to see a full-sun image taken right in the midst of this eruption.
Keywords: Sigmoid, Flare, CME, AR Tracking