Solar filaments are clouds of ionized gas above the solar surface squeezed between magnetic regions of opposite polarity. Being cooler and denser than the plasma underneath and their surroundings, these magnetic borderlines appear as dark lines when seen on the solar disk using special filters. As filaments grow longer, they are more likely to erupt, often accompanied by a coronal mass ejection (CME).
A long filament in the northeast quadrant erupted on 30 July, as can be seen in the above images from the GONG H-alpha Network. In less than 2 hours, this 60.000 km long string of ten thousand degrees hot plasma was slingshot into space, leaving the area on the solar disk remarkbly blank. Coronal dimming and post-eruptive coronal loops ("arcade") were observed, but no x-ray flare. The associated CME was mainly directed to the north, but SOHO and STEREO imagery showed some faint outlines indicating that part of the CME envelope may be directed to Earth. A glancing blow was expected by 2 August, but either missed Earth or had only some faint signature during the afternoon of 3 August with little geomagnetic effect.
Many thought that this was the end of the story, but the filament decided to write an additional chapter to it. Indeed, a small outer end (northeast) and the original filament channel (visible only in EUV) were still intact. Becoming unstable 2 days later, another filament eruption occurred very similar to the first one, but this time the associated CME was somewhat slower (about 600 km/s compared to 800 km/s) and clearly did not have an Earth-directed component despite it being so much closer to the disk center.
One might wonder why this filament erupted (twice), whereas two other filaments which were even slightly longer, did not erupt (see H-alpha image underneath on the left). One of the reasons may be because of the shape of the filament. The erupting filament had an L-shape that may have caused locally increasing stresses. The two other filaments were fairly straight or mildly curved at most. Another reason may be that the two quiet filaments were far away from any active areas such as sunspot groups, whereas the erupting filament was sitting right on top (north) of a small equatorial coronal hole and to the west of a coronal hole extension (see the SDO/AIA 211 image on the right). Interaction between the filament and the coronal holes may have caused the necessary instabilities for an eruption. Other reasons, combinations not excluded, are possible.
A movie of the two eruptions as seen in SDO/AIA 193, as well as of the associated CMEs seen by SOHO/LASCO (CACTUS) and STEREO-A (solar farside) can be found here.
Credits - Data and imagery for the movie clips were taken from the GONG H-alpha network, SDO, STEREO, and SOHO/LASCO, CACTUS and Helioviewer.
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