Harry finds that sunspot AR11628 provides excitement on a quiet Sun


A sketch of the arch prominence above sunspot AR11628. Sketch and copyright Harry Roberts ©, all rights reserved

The Sydney Harbour Bridge spans half a kilometre, one of the largest single span bridges, and made out of very dense steel. December 5 showed a solar arch that ‘spanned’ 150Mm (150,000km), made of very low-density plasma.

The sketch (Fig) shows the scene in the C8 with the 1.5Å Lumicon filter: an impressive sight. The scale bar is 200Mm long with 50Mm divisions – and the top of the arch was about 40Mm high. Note that this sketch has north to the right – all other figures have north to the top.

Measured 1½h later the height was less – due to rotation and showing the ‘footpoints’ were just on the visible side of the sun. Rotation carried the ‘structure’ eastwards – and by the 6th UT the great arch was no longer visible. The dark filament was then on the disc, unseen in cloudy Sydney.

While photographers like making prominences brighter than the disc – they never are. Typical prominences are just 10% of chromospheric brightness; in places (Fig lhs) the ‘big arch’ was 40% of the disc – rather bright.

Helio freeware (© Peter Meadows) sited the two ‘footpoints’ at +6,223 and +25,222, a separation of about 230Mm: almost eighteen Earth diameters!


The large sunspot AR11611 in early November. Sketch and copyright Harry Roberts ©, all rights reserved

AR11628: hardly visible on the disc below the arch, was a tiny sunspot, sited at +11,245. A couple of days later this was tagged AR11628, though it barely lasted 24 hours. The logs to the site one rotation earlier showed the tiny spot had a much larger ancestor: AR11611 (Fig). This group had two large spots 8º apart. How do we know this was a precursor to 11628? The (p) spot was located at +12, 244 – the exact location – and 11611 was last seen closing with the sun’s NW limb on Nov 19. Presumably the Dec 5 tiny spot was the sole survivor of the far-side transit.

Zirin and others stress the longevity of active region filaments (ARF) and on the 5th a large one was seen stretching N-S beside the tiny spot, associated with very bright plage: presumably the inversion line of the earlier group, AR11611.

Flare history: during its first transit of the disc AR11611 had some moderate flares: strongest was an M1.7 on Nov 8th; but there was nothing unusual to suggest the region’s longevity.

In case we missed it, new AR11628 flagged its presence with a spectacular ‘Spotless Flare’ on Dec 6th ~18:00UT, local nighttime.


The flare from AR11628 on 6 December 2012. Courtesy NSO/GONG

The flare was logged, however, by the global NSO-GONG network (BBSO) and we see (Fig) it was bright and covered a wide area, along the N-S line of the ARF mentioned –in fact where the plage had been logged 22h earlier. Although bright and extensive the flare was not strong at GOES class B6.4.

The few ‘spotless flares’ I’ve logged all had a strong ARF at their core and were spread out along that filament – due no doubt to the simplicity of the spotless fields: the presence of spots causes magnetic complexity.

The message? While spots may be ephemeral, filaments can have a long life. Regrettably this may also highlight the current lack of emergent fields (new flux) that in a stronger cycle would soon modify old regions. It will be interesting to see if the AR11628 site preserves the filament much longer.

The big arch is now another dark filament currently crossing the disc some twenty degrees behind (east) of the relic sunspot – it too is liable to eject from the sun spectacularly – since it lies within current sunspot latitudes: we should watch closely for this. (PS: it ejected about Dec 9 00:00UT).

Rumour has it that loads of solar ‘scopes may soon become ‘land-fill’ – due to the lack of solar activity: a bit rash perhaps! But the sun IS very quiet: way too quiet compared to recent 20th C sunspot cycles. We now see that solar activity in the second part of the 20th century perhaps reached a four-century peak.

Yet the sun, as we have seen, is still capable of spectacular activity – though there is now little doubt that SC24 will be historically weak: and if the NSO teams are right, “solar cycle 25 may not occur at all”.

Around 2020 will be a better time for retailers to hurl themselves from the Harbour Bridge! Meanwhile, keep the ‘scopes ‘at the ready’, there is much to see.

Harry Roberts is a Sun and Moon observer, a regular contributor to the Sydney Observatory blog and a member of the Sydney City Skywatchers. He is the recipient of the Astronomical Society of New South Wales’ McNiven Medal for 2012, which is the highest award from the society.

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