Last week, the remainder of a large sunspot complex rounded the west limb. For more than 2 weeks, it had dominated the outlook of the Sun. Initially consisting of 6 groups, only the two largest groups survived the transit over the solar disk. The graph underneath shows the daily sunspot number from 9 till 30 June as determined from SDO/HMI continuum images (1024x1024).
As can be seen, this sunspot cluster pushed the daily sunspot number to a higher level for more than a week. On some days, it contributed as much as 75% of the total sunspot number. The 6 groups were packed together in an area measuring roughly "only" 220 times the surface of the Earth. Images underneath show the cluster on 15 June (all 6 groups) and on 21 June (most flares in one day). Most prominent were active regions NOAA 1772 (stretched region on the right) and NOAA 1775 (compact region to the left).
Despite the high sunspot numbers, flare activity from these mostly simple regions remained modest. During the visibility period of the sunspot cluster, about 80% of its 25 flares had their origin in either NOAA 1772 or NOAA 1775. These were all relatively weak C-class flares, no medium-class flares were registered. The peak in flare activity on 21 June corresponds to an upswing in sunspot area in NOAA 1772. It is noteworthy that none of the two M-class flares during this period were produced by one of the cluster groups, as can be seen e.g. in this news item on the 21 June event. NOAA 1775 really did not live up to the expectations, despite some spots with opposite magnetic polarity being close together.
The appearance of this kind of sunspot group clusters is not so unusual. Quite often, these regions do produce substantial solar flares during their transit. A nice example is a trio of large sunspot groups that was visible from 21 July till 4 August 2002 in an area that was comparable in size to the most recent outbreak (SOHO-image underneath). At the time, the 3 groups produced a total of 78 flares, including 10 M-flares and 2 strong X-class flares. A really good reason to keep an eye on those sunspot's nests!
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