Drawings of the shape-changing sunspot group AR12104 over four days. Sketch and copyright Harry Roberts ©, all rights reserved
In telescopes sunspots look substantial enough: yet they are sculpted by magnetic fields and can change shape abruptly as new flux arises. Most spots are round or some simple ovoid shape, but there are exceptions – and they make sun watching most interesting; one such was AR12104.
This group appeared at the eastern limb on June 29th, already in a complex shape, when “Helio” freeware sited it at –12,270 (preceding) and –12,267(following). The records showed that the newcomer was perhaps the return of AR12080 from the previous rotation (CR151) that had stretched between –10,263(p) and –13,254(f), a big spot group of Hale Beta-Gamma-Delta class, the most complex magnetic type: it had hosted at least one GOES M-class flare during its transit.
As Zirin says, “sunspots like to be round”, but complex magnetic developments during earlier rotations can cause irregular shapes – and new flux emergence can magnify that irregularity; this may explain the shape of new group 2104. Whatever the cause, the new group was not yet done with ‘shape-changing’ and over the next few days it would evolve into even more complex shapes (Figs1-4).
Fig1. (June 29). The two main spots look like a big single that has been cut in half –the (p) half being red polarity 1500G(R15), the (f) half violet 1500(V15). The latter has some small polarities in the same penumbra: I suspect they are ‘red’ polarity but Mt Wilson made no record. The single spot above the two is likely also a ‘red’ spot. We note that the line-of-sight was nearly oblique to the spot at this time and that polarity values would increase as the ‘los’ angle changed. Several faint active region filaments (arf) were nearby.
Fig2. June 30 saw a sharp rise in the number of new spots and in the complexity of the penumbrae: small chains of spots and elongated spots were logged – signs of spot motion and strong linear fields. The (p) spot held many umbrae and the (f) spot was growing northward. Mt Wilson now had an R16 spot within the (p) spot’s complex penumbra – giving the group a Hale Delta classification, i.e. mixed polarities in a single penumbra – a predictor of strong flaring.
Yet it had no big flares, though AR2106 in the north hosted an M1. Meanwhile, just 7° to the south, AR12107 grew to a large penumbral group – the return of AR12085 maybe.
Fig3. July 1 showed dramatic changes of shape in the (f) spot, due to yet more flux emergence, while the (p) spot was hardly changed! The group was now ~40° east of the central meridian (CM) and the fields recorded are R24 in (p) and V23 in the main (f) spot. Mixed fields are seen within the (f) penumbra – but still no major flares?
Fig4. July 2 saw a rise in area of the group’s penumbra to ~500 units, mostly in the (f) component and the red polarities seemed to now cluster along the west edge of the (f) penumbra as several linear umbrae and small spot chains. The (p) umbrae attained 2300G fields and the large (f) 2400G – strong fields for SC24: a cycle with unusually weak umbral fields. A presumed Ellerman Bomb (EB) arose as shown.
Sketching group 12104 reminded the writer of some major SC23 groups – but they were mostly much larger. July 2 seemed to mark the end of flux emergence – and Mt Wilson showed declining umbral fields over following days, despite the now favourable ‘los’. AR12104’s tenure was ending – and despite the persistent Delta configuration no significant flares had occurred: though the many exotic changes of shape proved challenging to record.
Magnetograms showed the AR12104 and 107 regions of the Sun comprised a ‘nest’ of spot activity – and it may well survive to make perhaps its third transit of the disc in August: what interesting new or returned groups will then appear?
Harry Roberts is a Sun and Moon observer, a regular contributor to the Sydney Observatory blog and a member of the Sydney City Skywatchers.