Small sunspot, big flare

Last week, the Sun produced a few M-class (medium) flares. However, the strongest flare -an M5.9 peaking at 12:17UT on 24 August- originated from NOAA 2151, a relatively small and magnetically simple sunspot group near the southeast solar limb. It was the strongest flare since the M6.5 flare on 8 July and the X-class flares on 10-11 June. Images underneath show the affected area on the Sun in white light and gradually overlaid by extreme ultraviolet imagery (SDO/AIA 304). This indicates that the position of the flare and the erupted material was actually a bit behind the sunspot group.

This movie shows the eruption in successively higher temperatures in the solar atmosphere (AIA 304, 171 and 131). The combination movie shows that the flare is actually related to the eruption of a small, dense filament, which might have become unstable from previous nearby solar activity. A series of post-flare coronal loops (“arcade”) can be seen, but there’s no obvious sign of coronal dimming or an EIT wave (PROBA2/SWAP difference images; see also here for more information). A type II burst was recorded by the Humain Solar radio Observatory, indicating that a shock travelled through the solar atmosphere with a speed of 593km/s as estimated by the San Vito station. The coronal mass ejection (CME) was seen propagating mostly in the eastern direction, with a plane-of-the-sky speed of 473km/s (CACTus estimate).

The interesting thing about this CME is that SOHO/Lasco imagery indicates that part of the erupted core filament is not ejected, but reaches a height of about 4 solar radii before raining back along the magnetic field lines onto the solar surface. This is not so uncommon, with the 7 June 2011 eruption a notorious example (see this news item).

Credits - Data and imagery were taken from SDO/AIA, PROBA2/SWAP, SOHO/LASCO, Humain Solar Radio Observatory, CACTus, Solar Demon, (J)Helioviewer, and daily reports from NOAA/SWPC and SIDC/RWC.

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