Astronomical Society of Coonabarabran

AR 10960: The end of Cycle 23 postponed?

by Harry Roberts

I have not systematically observed a previous solar Minimum and watching Sun activity decline is fascinating. Predictions that the Sun would be in Minimum by now have been proven wrong.

Dr. David Hathaway's thesis (RASNZ Solar Newsletter 2006 May 19) that the southern sunspot conveyor has ceased moving towards the solar equator seems to be borne out by current observations. At the time of writing sunspots both large and small keep emerging in the Sun's southern hemisphere mostly between 10º to 15º below the equator. While some of these active regions are small and short-lived, they are often joined by truly large and active groups; like AR 960.

AR 960 started life, apparently, as AR 955. I first saw this group near the centre of the Sun on 2007 May 11. It looked very active with a dense crescent of small spots in a single penumbra preceding a cluster of penumbral spots, spread over about 7º of longitude totalling 15 spots. It had patches of bright H_ plage and active surges. Yet over the next few days AR 955's spot count dropped until there were only three spots on the fifteenth and none on the sixteenth, just a large area of plage and some active region filaments (ARF).

While AR 955 was (apparently) fading behind the west limb, a small pair of spots emerged near the eastern limb, AR 956. This was a northern group only 2º from the equator, and northern groups have become very rare. AR 956 grew quickly to become quite active with over twenty umbrae in a single penumbra. But it faded completely by the 24^th leaving a patch of bright faculae at the west limb, an active spot group that lasted only half a solar rotation, or about ten days.

Meanwhile, behind the Sun AR 955 was "bulking up", and burst back onto the stage on June 1^st. And what a change: the old group had been rummaging in "Sly" Stallone's travel bag! It now stretched (June 4^th) across sixteen degrees of longitude making it a type Fkc group, the biggest class of all. Also the spots were joined in long curving chains (see Fig), a sure sign of powerful magnetic shears and predicting big flares.

Sure enough, between June 2^nd and the 4^th AR 960 produced four M-class flares and many C-class; truly unexpected fireworks! As Monty Leventhal commented, AR960 looked more like a big group from around solar maximum (2000 or 2002). With two other small southern groups for company, the sunspot count (R) for June 4^th was over 60. AR 960 had an area of 540 units and a magnetic class of Beta-Gamma-Delta.

My sketch shows the large and complex group. A big spot leading had 8 umbrae within one penumbra, from where a dark surge emerged at 03:52. Centrally there were two large spots joined by an unusual curving chain of smaller spots indicating strong magnetic shears and sunspot motion. Trailing the group was a wide scattering of small curved penumbrae dotted with tiny spots.

Excitement mounted as a flare ribbon arose near the central pair of spots and grew rapidly northwards. This flare peaked at GOES C1 and was accompanied by a dark and blue-shifted surge at 04:55. At this time the Sun went behind local buildings, and I did not see the flare rise further to M6 over the next 40 minutes.

Exciting times still for the Sun.