Two views of sunspot group AR11504 on 14 & 16 June 2012. Sketches and copyright Harry Roberts ©, all rights reserved

Mid-2012 solar activity is faltering – or so it seems. Although quiet periods are not uncommon in the lead-up to a Solar Maximum – are we having too many in SC24?

Another interesting consideration is that southern activity lags well behind northern, as it did during the past three solar cycles. If we look at the size of sunspots we find the biggest so far were all in the sun’s northern hemisphere, with four >1000 units, while only one in the south has reached just 750 units – this is June’s large group AR11504.

Constant cloud hid its early stages and when first viewed on June 14th a large complex group was seen (Fig1 lhs) already with an area >600 units, making it the largest southern group of SC24 thus far.

Even at this stage there was contention: Mt Wilson’s researchers held it to be a simple group, magnetic class beta, with a well defined separation of preceding (p) and following (f) polarities. NOAA, by contrast, had data that showed it to be the most complex of Hale classes: beta-gamma-delta, i.e. with mixed polarities in a single penumbra. It was not clear who was right.

Fig 1 lhs shows a N-S chain of small spots where, perhaps, the unlike fields were present (triangle arrows) – but Mt Wilson’s logs did not detect the mixture.

By the 16th however there were clear signs of mixed umbral fields in the big central spot – the polarities are underlined for clarity (Fig 1 rhs). The spot group’s area peaked at 750 units about this time.

Two further views of sunspot group AR11504 as it approached the edge of the Sun on 18 & 20 June 2012. Sketches and copyright Harry Roberts ©, all rights reserved

As we might expect from the magnetic class, two long-lived ~M2 flares occurred at this time too. However, the ‘explosive’ mix of fields was gone when the group was viewed again on the 18th (Fig 2 lhs, arrowed). The red field that had invaded the violet central spot now had apparently rejoined the red (p) spot – and the loss of complexity led to a fall in flaring. Low-level GOES C-class flares were the best the group could then manage as it rotated towards the sun’s western limb.

Fields in the big (p) spot were strong at 2500G (R25) with the main (f) spots at ~2300G (V23): giving the three main spots large penumbrae.

Barely five degrees of latitude north of 11504 was small group AR11505 – so close that viewers of the past might call it part of 11504 – but magnetograms showed it was a separate bipolar entity.

Delta groups may decline rapidly, leaving just the (p) spot, yet AR11504 stayed at peak size as it neared the west limb – confirmed by Helio freeware. The small spots in the group faded but the big three remained in a ‘tight formation’ that was just visible to the suitably protected naked eye (Fig 2 rhs) and looked spectacular in the ‘scope.

No flares were logged in AR11504 but some faint active region filaments were seen at times. It’s noteworthy that while the group 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.

AR11504 was the largest southern group yet of Cycle 24 and as it went behind the west limb it left the sun almost devoid of spots in either hemisphere (!) – a sight we have not seen since the deep minimum of 2008/2009. What does this portend?

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