Sketches of the adjacent sunspot groups AR11671 and AR11671 in white light and in hydrogen alpha – the red light of hydrogen atoms. Sketch and copyright Harry Roberts ©, all rights reserved
On Feb 10th a mid-sized sunspot came around the sun’s northeastern limb; dubbed AR11671, it was a single round spot sited at +14,75. It rotated across the sun, pretty much ‘minding its own business’, for a week or so. Now and then a tiny following (f) spot appeared briefly, a few degrees behind it – but they didn’t last.
This ‘placid’ scene changed abruptly on Feb 18, when a new sunspot group (or dipole) burst into view close by and grew rapidly bigger – it was centred at ~+9,70. Dubbed AR11678 it showed some odd behaviour. As the new spots emerged, looking much like a normal bipolar group, two new spots also emerged to the NW of old group 1671. SDO continuum images showed these new spots emerged simultaneously with AR11678 – as if part of it – in effect, blending the new group with the old: unusual sunspot behaviour. The new spots (Fig, at top, arrows) had the same polarity as the old 1671 spot.
This behaviour seemed to be a variant on the ‘standard model’ of sunspot growth, discussed previously, whereby a sunspot dipole may grow to a maximum of ~50Mm, and any further growth is the result of a new dipole emerging nearby and joining ‘end to end’ with the earlier group (Zirin “Astrophysics of the Sun” Cambridge Uni Press (1986) P. 320). Rearrangement of sunspot fields is the usual result, and this causes flares, ejections, surges etc.
In the 1671/1678 example the new dipole was far enough away to be termed a new group, albeit very close. Older systems of sunspot classification treat close groups as separate if they are more than 5º of latitude or 10º longitude away from each other – a useful ‘rule of thumb’, but not always correct. SDO logs show sunspots freely interconnect their fields even to groups many times further away.
Old and new. The blending of old and new groups in our case did produce a strong rise in activity – much of it centred between the two groups – and flare ‘stats’ attributed events to one or other of the groups accordingly. None were ‘great’ flares – but many GOES C class and B class erupted, almost hourly. The Figure shows a very pretty C1.7 flare (visual class 1N, Feb 20, 21:58UT) close to the AR11786 (p) spot, attended by a mass of brief surges and some active region filaments. This one example was typical of the many low-amplitude short-lived flares and dark surges that, in my case, were mostly hidden by cloud. An M1.9 on Feb 17 was in an unrelated spot group, AR11675.
Umbral fields. Nothing seemed abnormal in the umbral fields of AR11678 – in fact Mt Wilson assigned Hale-class beta (low complexity) throughout – but NOAA (using other data it seems) quickly assigned Hale class beta-gamma-delta to the group: the most active class. There are hints in the Mt Wilson data of ‘mixed’ (or delta) polarity in the two lesser spots sited at +9, 66 (Fig, below).
Possible return. The possible return of blended groups 1671 and 1678 around March 10 may shed more light – even as they went behind the NW limb the presumed delta spot had gone – as delta spots do.
While there’s many a small flare, where are the great ones?
Harry Roberts is a Sun and Moon observer, a regular contributor to the Sydney Observatory blog and a member of the Sydney City Skywatchers.