The last few weeks, a significant increase in the number of sunspot groups was observed. Ten to fifteen active regions on the solar disk at the same time were no exception. Though most of them were small, a few were relatively large and complex. NOAA 4168 (SIDC sunspot group 588) was such a well-sized mixed-up active region. It started to develop on 2 August, just south of decaying NOAA 4165 (SIDC sunspot group 581), reaching a maximum sunspot area of 350 MH (millionths of a solar hemisphere). That doesn't sound like much, but it is the equivalent of about twice the total surface area of the Earth. The SDO imagery underneath shows the evolution of NOAA 4168 from 2 till 10 August in white light (left) and magnetically (right). In the magnetogram, a red color indicates a positive magnetic polarity (magnetic field lines leaving the solar surface), a blue color denotes a negative magnetic polarity (field lines returning to the solar surface).
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Due to its complex magnetic structure, NOAA 4168 quickly became a very flare-active region, producing 17 M-class ("medium") flares during its transit. The strongest two, an M4.4 flare on 5 August and a long-duration M3.9 event two days later, are shown underneath. The extreme ultraviolet (EUV) imagery (SDO) combines AIA 193 images (brownish; 1.25 million degrees) with AIA 094 (pink; multi-million degrees) and show well the post-flare coronal loops and especially the coronal dimming (pre- and post-flare images), indicating material was ejected. The coronal mass ejection associated with the M4 flare may have affected the earth environment on 8 August, but its effect is difficult to disentangle from that from a high-speed stream that arrived around the same time.
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Probably the nicest M-class eruption from NOAA 4168 took place on 10 August. At that time, the active region was already close to the Sun's west limb, and so the eruption could be nicely seen in profile. The M2.2 flare peaked at 15:07UTC. The post-flare coronal loops reached a height of nearly 200.000 km, that's half the Earth-Moon distance. Also very nice supra-arcade downflows (SADs - see this STCE newsitem for more info and examples) could be seen in filters showing the multi-million degree corona (the Sun's outer atmosphere). The coronal loops are best visible in AIA 171 (700.000 degrees; greenish), the SADs -the ragged, tadpole-like structures- in AIA 094 (pinkish-blue). In this still, the images are about 7 hours apart.
