NOAA 1967 has rotated behind the west limb. It was a little bit of a disappointment that the largest sunspot group so far this solar cycle (according to NOAA statistics), only produced numerous M-flares. The approximate positions of 15 of these M-flares, produced between 30 January and 7 February, are indicated on a 2 February SDO-image of NOAA 1967.
So, what does it take to get an "eXtreme" flare? First of all, you need a compact sunspot group with a big, asymmetric main spot. Indeed, flare and sunspot statistics show that about two thirds of all X-class flares are produced by such groups. Also, about half of the X-flares are produced by groups that have several smaller but still mature sunspots between the main spots. So far so good, because NOAA 1967 had both!
A second ingredient is that the active region should contain sunspots of opposite magnetic polarity close together, usually within the same penumbra. These are the so-called "delta's". Often, the bigger the spots, the stronger the flare (if any is produced...). Again, NOAA 1967 contained delta structures for most of its transit over the solar disk. There were 2 big spots of opposite polarity in the region’s trailing section, and both had -within their penumbra- smaller spots of opposite polarity, the whole resembling an "X"-structure. An example of a big sunspot group with no delta’s is NOAA 1654 from about a year ago (image above). Indeed, the big asymmetric main spot consisted entirely of spots of the same magnetic polarity: No X-flares, and only two M-flares!
So, which key-element was NOAA 1967 lacking to produce X-class flares à volonté? This question brings us to the frontline of scientific research, and it’s already an indication there's currently no final answer to it...
One of the (many) reasons could be that the main spots of NOAA 1967 were rather "rigid", contrary to some other X-class producing groups which showed rotation in their main spots. An example is e.g. NOAA 1158 which produced the first X-class flare of solar cycle 24 on 15 February 2011. The clockwise and counter-clockwise movements are indicated on the image below and can be seen in this movie (SDO/HMI). It is suspected that this helicity transfers additional energy into the system that may just be enough to produce major flares. With NOAA 1967, aside some shape changing, very little rotational movement can be seen in the main spots.
But maybe it was all related to how the sunspots were build up in the solar interior. With techniques that monitor the up-and-down movements of the solar surface, one can probe the structure of sunspots underneath the solar surface ("helioseismology"). As results will become published in the next few months, it may reveal a new clue on the atypical flaring behavior of NOAA 1967.
In the end, NOAA 1967 produced 26 M-class flares, making this region after NOAA 1515 (30) the one with the most medium class flares so far this solar cycle. That tasted really well, but the soup could have used some more spice!...
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