Remember NOAA 2126? This relatively small sunspot group was visible from about 23 July till 3 August. It was flanked to the north and south by resp. NOAA 2129 and NOAA 2123, which were smaller and simpler sunspot groups. During its transit, NOAA 2126 produced only a few small C-class flares.
At the time of its rounding of the west limb on 3 August, the group seemed stable and even declining. It showed also some magnetic connection with NOAA 2121 in the northern hemisphere, as can be deduced from the faint trans-equatorial loops over the west limb as seen in extreme ultraviolet (EUV) images from SDO (1-2 August).
During the afternoon hours of 8 August, CACTUS detected an asymmetric halo coronal mass ejection (CME) first seen in images at 16:36UT. The bulk of the CME seemed to be directed to the southwest, having a plane-of-the-sky speed of about 700 km/s.
As there was nothing particularly happening on the Earth-facing side of the Sun, a quick examination of STEREO-images revealed that the source region was indeed on the backside of the Sun. Peaking at 16:15UT, a strong burst in EUV could be seen nearly on the centre of the solar disk as seen from STEREO-A. This location corresponded with that of old sunspot group NOAA 2126. EUV-imagery also showed an EIT-wave (see this news item for more info on these features) and a transient coronal hole (coronal dimming), indicative of the launch of a coronal mass ejection. Coronagraphic images revealed this was a full halo CME as seen from the STEREO-A vantage point. The transient coronal hole and post-flare coronal loops also clearly indicated an intimate connection with NOAA 2121 on the northern hemisphere.
From the EUV images, it has been estimated that the strength of the flare was comparable to a good-sized M-class or even a low-level X-class flare, so roughly between M4 and X1.2. The STEREO spacecraft have seen much worse. Based on correlation studies between the EUV (SDO, STEREO) and X-ray (GOES) fluxes, it has been estimated that one of the strongest flares so far this solar cycle occurred on 20 September 2012. Based on STEREO's EUV data, the strength of that eruption was estimated to be between X6 and X18, with X12 the most likely value. That would make it by far the strongest flare of ongoing SC24.
A movie of the 8 August 2014 event, showing EUV images from STEREO-A and coronagraphic images from SOHO and STEREO-A, can be found here. Further reading on estimating the strength of flares from STEREO EUV images is at the Solar Physics website.
Credits - Data and imagery for the movie clips were taken from SDO, STEREO, SOHO/LASCO and CACTUS.
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