Changes in sunspot group AR12109 plotted on four separate days. Sketch and copyright Harry Roberts ©, all rights reserved
The first week of 2014 July saw the Sun’s south-eastern hemisphere ‘peppered’ with large and complex sunspots – some reached (suitably protected) ‘naked-eye’ size! A wondrous sight in white light (WL): they were somewhat disappointing in H-alpha, with few dark filaments and modest plages. Let’s consider one of the biggest: NOAA AR12109.
AR12109 rounded the east limb on July 2nd (Fig1), looking like a typical large single spot (McI: Hsx) but for some faint patches of following (f) penumbra that may have held small attendant spots. A day earlier there had been strong signs of a large group approaching, with a spiky ‘crown’ of small but bright surges sited at –10,219. As surges occur only around sunspot penumbrae, a cluster of ‘spikes’ at the limb suggested a big spot just behind it. When first seen on the 2nd the (p) spot was already 16º from the limb, i.e. in good ‘seeing’ it might have been detectable on the 1st.
By the 3rd (Fig2) it was clear that 2109 was a complex spot group with the large (p) spot and its conspicuous light bridge attended by dozens of small spots, many in chains, suggesting complex magnetic mixes. The (p) spot was sited at –8,222 and the most remote follower (f) at –6,214: it was a new sunspot group. Mt. Wilson polarities showed red 2200G (R22) in the (p) spot and violet 1300G (V13) in the follower (f). Helio freeware © Peter Meadows gave the group an area of 650 units: a large group given this weak solar cycle.
The 5th saw the group larger than 700 units (Fig3) with the previously ‘roundish’ (p) spot now changing shape while the followers developed larger irregular penumbrae. There were now some conspicuous surges or filaments emanating from the (p) spot with one looping out of the (p) and returning to the middle of the group after an excursion some 7º northwards.
Flare. A flare erupted along what was likely the polarity boundary between red and violet spots, beginning as a sub-flare at 00:21 and reaching GOES C4 at 00:25: a minor but ‘pretty’ event.
The preceding spot now had an umbral field of R24 while the (f) spots remained at low values. Mt Wilson workers who detected mixed fields in it assigned Hale DELTA configuration to the group on the 8th. I had no detailed log of it on that day – but on the 10th (Fig4) it was also Beta-Gamma-Delta with R2600G in the (p) spot. The tiny V13 (arrowed) was deemed a DELTA mix by Mt. Wilson, while I saw that spot as separated from its red siblings by a small penumbral gap. Much earlier NOAA had deemed the group Delta class based, it seems, on disc magnetograms rather than the ‘original’ Hale definition of ‘mixed fields within the one penumbra’.
This was a most impressive sunspot group – Earth is shown dotted in Fig4; large in area with complex Delta class fields, yet its flare activity never exceed GOES C-class during its disc transit. What was missing?
One key ingredient of flaring is helicity: the degree of ‘twist’ in umbral fields. Perhaps the solar dynamo is generating fields with less helicity and that, combined with low power fields, results in lesser flares? It is noted that the vast coronal canopies formed above active regions by the sunspot fields (SDO daily logs), currently seem rather modest when compared to earlier records (e.g. year 2011). Is this effect real?
Perhaps time and more research will tell. Meanwhile, AR12109 was a great sunspot to follow as it rapidly evolved during its stately transit of the solar disc; will it reappear around August 1st? Keep a watch for clusters of small bright surges at the southeastern limb.
Harry Roberts is a Sun and Moon observer, a regular contributor to the Sydney Observatory blog and a member of the Sydney City Skywatchers.