Sunspot group AR 11520 as it appeared on 9 July 2012. Sketches and copyright Harry Roberts ©, all rights reserved

Sighting a really big and complex group at the sun’s east limb is one of the joys of sun watching– they can appear without warning having developed unseen behind the sun. At first it seemed AR11520 was one of these. In the excitement of logging July eighth’s newcomer, I forgot to check the logs for precursors at the site.

Of mid June’s large group AR11504 I had written:

It’s noteworthy that while AR11504 had begun as a large (p) spot with complex chains of (f) spots – it ended as a large (p) spot followed by a pair of stable ones: an unusual outcome. This stability may mean that the whole structure returns at the eastern limb in early July – keep a close watch.

And it did, readers!

A comparison between sunspot groups AR11520 and its predecessor AR11504. Sketches and copyright Harry Roberts ©, all rights reserved

Overlay: Figure 2 shows the outline of 11504 over that of 11520 (July 9th) – it is clear that 504 is the ancestor of new group 520 and that, surprisingly, 504 grew much larger during its transit of the sun’s far side. The area of 504 was ~600 units, while 520 is well over 1000.

This is unusual in my experience. Plenty on groups make a second rotation of the sun’s disc but they mostly return as scattered spot clusters of less area than before – yet here is an example of a trio of stable spots morphing into a much larger spot group.

Recall that 11504 was briefly Hale Beta-Gamma class with short-lived mixed polarity in its irregular central (mainly violet) spot. This spot soon lost its few red spots and the group reverted to Beta class, with a dominant red (p) spot and two violet followers (f). And one rotation later here it is again: a very stable configuration.

In fact it seems certain that the main bulk of 11520 has grown from the blending of the (p) and (f) spots of 11504.

Longitude Length (LL): There is debate about the number of active regions that comprise the whole 11520 site. Mt Wilson’s workers suggest just one super-group well over twenty degrees in length – and say so on their daily logs.

NOAA on the other hand ‘sees’ three groups at the site, AR’s 11519, 11520 and 11521. I guess NOAA will win in the end since they write the history! Yet even NOAA has AR11520 with a LL of twenty degrees. It’s a huge group.

Fig 1 shows the group on July 9: an immense irregular spot preceded by a scatter of smaller spots too complex to map (yet), though a huge active region filament (with some surging) reached west for 14 degrees to enfold several lesser ones. All this complexity has not yet produced large flares at the site – with only a few lesser M-class – though X-class is predicted.

Indeed the whole site is an activity centre (or ‘nest’) with much detail to record and discuss as more logs become available. Sun watchers; enjoy!

Harry Roberts is a Sun and Moon observer, a regular contributor to the Sydney Observatory blog and a member of the Sydney City Skywatchers