Harry dissects the anatomy of the giant sunspot group AR11520

July 25, 2012

AR1520 on 12 July_SDO_HMI

The Sun as it appeared on 12 July 2012. Sunspot group AR11520 is the large group to the left of centre. Compare its appearance with the three sketches below. Courtesy SDO/HMI

Sunspot cycle SC24 continues to fascinate. For the moment northern activity has been halted in its tracks – while a burst of southern activity is reshaping the statistics of this cycle.

One paradox of solar activity is that spots cover only a tiny fraction of the sun’s visible area: AR11520, is just 0.14% of the disc– yet it rightly attracted attention as the biggest southern group for SC24, so far, and second biggest overall. If you have ‘eclipse glasses’ you could see it without a ‘scope.

AR11520 was the return of AR11504 that had crossed the disc in June. And given its great size in July we can expect AR11520 to make a third transit of the sun in mid-August!

One Group or Three? From the start it was a confusing group – and some deemed it to be one vast spot group stretching across thirty degrees of solar longitude, while others (including NOAA) held it to be several smaller and overlapping groups, with tiny spots scattered throughout.

However, magnetograms showed a complex interweaving of fields that suggested many groups rather than just one; highlighting the challenges of interpreting solar activity. The confusion did not stop there.


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

Let’s examine the best-defined part of AR11520 – the part mostly within one big elongated penumbra (Fig1). Excluding its many ‘satellite’ spots, this one large penumbra had an area of ~1000 units with two huge preceding (p) spots of ‘like’ polarity (‘red’ ~2500 G) and it stretched across ten degrees of longitude (ln).

From the start this penumbra held many smaller spots of ‘opposite’ polarity (i.e. violet) in its eastern (or following) parts – making the group magnetic class Delta, i.e. ‘mixed’ polarities in one penumbra, the most complex class. (Again, not all agreed on this classification). When gaps divided the large penumbra from time to time, the magnetic class changed accordingly.

The major evolution of this big group was the slow change from a long single penumbra into a more round and compact one, (Figs 1 to 3).


Sunspot group AR 11520 as it appeared on 11 July 2012. Sketches and copyright Harry Roberts ©, all rights reserved

The Delta classification depends upon the accurate measure of polarity in often tiny (3000km wide) spots – and Fig 2 shows some that Mt Wilson logged as ‘violet’ polarity one day and ‘red’ on the next! Did they really change polarity or was the Zeeman ‘splitting’ of the spectral line ambiguous? It’s probably the latter. Also some logs show distinct penumbral gaps when maybe faint links persisted: yet more confusion.

Flaring: Class Delta predicts strong flares – and AR11520 had maybe six M-class flares and one X1.4 flare on July 12 (16:49UT). A strong performance but no ‘gold medal’ – many SC24 groups have had more. Sadly, the writer logged no flares stronger than C-class.


Sunspot group AR 11520 as it appeared on 13 July 2012. Sketches and copyright Harry Roberts ©, all rights reserved

Surrounded! The two strong (p) spots became increasingly surrounded by violet polarity (Figs) and a strong filament developed at the inversion line separating the unlike fields. This was at first some way from the main spots but as violet ‘pressed’ it grew darker and closer to the red pair: a strange sight in the eyepiece with the (p) spots ‘cocooned’ by the filament and a bright ring of plage (Fig 3) This was the site of many flares.

Fig 3 shows a remote spot (pp) sited at longitude 95º giving the group a longitude length of 20º – but this is rather speculative.

Verdict: AR11520 was a giant spot, second biggest for this cycle – but the weak delta classification meant only modest flaring: the single X1.4 is surpassed by the X6.9 of July 2011, and three earlier groups have had twin X-class flares so far. No flaring records ‘fell’.

Still, this was a major SC24 group and a sure sign that southern activity is at last well established: soon more and stronger flares can be expected – in both the south and the north. When it passed the western limb AR11520 was to unleash some beautiful coronal transients that some persistent sun watchers would see: more in a final ‘instalment’ on this southern giant (Editor willing).

Keep the solar ‘scopes at the ready!

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


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