Detailed sketches of reversed sunspot group AR 11429 on 4 & 6 March 2012 (UT). Image and copyright Harry Roberts ©, all rights reserved

Sunspots are fairly predictable when they first emerge on the sun and grow steadily bigger. Most are bipolar structures with the magnetic fields in their western (preceding) spots opposite that of the eastern (or following) spots. And the preceding (p) spots in a given solar hemisphere are also magnetically opposite to the pole of that hemisphere. The sun’s poles reverse every eleven years when sunspot cycles reach their activity peak. In this the sun is very dynamic compared to Earth: we haven’t had a pole reversal for 100,000 years or so.

Hale and Nicholson in the early 20th C discovered these polarity “laws” – and most spots are happy enough to obey them. Yet every now and again along comes a “non-conformist sunspot” with other ideas!

These are called “reversed groups” and they are pretty rare; “reversed” since their preceding spots have the same polarity as the pole of the hemisphere they inhabit – the opposite of normal groups.

Reversed groups are usually small and short-lived: consider AR 11078 the second such group for SC24 on June 10, 2010, a small bipolar group with V10 (p) and R15 (f): a reversed group since red normally precedes in the southern hemisphere (for cycle 24). The first SC24 reversed group was perhaps AR 11030 a tiny group in the sun’s north 2009 Nov 5 that lasted only a few days.

More recently on the disc in early March was AR 11423 a small pair of reversed spots in the north (at +15, 42) – it soon faded.

Big reversed sunspots are quite uncommon: I recall some from SC23 – but have yet to find them in the logs. Yet despite their rarity, there is truly large and fully reversed group on the sun right now, this is AR11429 – and it’s one for the record books!

“So strong is the Hale-Nicholson law that occasional new spot groups with the wrong polarity will die out rapidly or else undergo great activity as the p spot struggles to get from the back to the front”. (Zirin, H. “Astrophysics of the Sun” P308).

This last comment relates to delta class sunspots, where opposite polarity spots are found within one penumbra, and AR11429 is just such a group. As well it has a large area, above 1000 units – amongst the larger size spots groups. Such a large reversed group is somewhat unexpected given recent weak sunspot activity.

Image of sunspot group AR 11429 in the light of hydrogen atoms taken on 5 March 2012 (UT). Note that this image is orientated upside down to the sketches above. A flare of X-ray class M1.5 started 10 minutes after this image was taken. The sunspot group produced even larger flares such as the M8 class flare on 10 March 2012. Image and copyright Monty Leventhal OAM ©, all rights reserved

Readers are urged to closely monitor this group, particularly in H-alpha, as large flares are expected for such a delta group (Fig). Brace yourselves for fireworks!

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