Harry is disappointed that recent sunspot group AR11598 was only a modest achiever


Two views of sunspot group AR11598. Sketch and copyright Harry Roberts ©, all rights reserved

As solar cycle number 24 (SC24) unfolds, patterns of development are unclear. An early burst of northern hemisphere activity (2010 – 2011) saw some big spot groups – sun watchers were relieved, as the sunspot minimum had been so deep.

In 2011 –2012 the southern hemisphere finally awoke too– with some large groups and major flares: while the north became quiet.

However, in late 2012 the north is having few spots, mostly small ones, while the south too is not maintaining its strong performance. “We are back in minimum again” is the refrain!

October’s new group AR11598, however, did its ‘bit’ for southern activity. It flagged its approach to the eastern limb with fine prominence ejections above the limb well before spots were seen. There is quite a log of ‘pre-appearance’ phenomena for this spot group.

Flares The GOES satellite logged a series of strong flares at ~10º south on the eastern limb from the 19th onwards, the strongest a GOES M9 on the 20th at 17:40UT.

Ejections were seen above the site on Oct 19th (UT). The Figure shows three stages of a very bright ‘explosive’ event on Oct 19 from 21:05 to 21:25.

The earliest stages were missed (during WL [white light] observations) though a small bright H-alpha ejection between 20:23 and 20:36UT was logged. This earlier rapid event was sited at the north edge of the big event 25min later. They were perhaps linked, though not shown here for clarity.


A prominence ejection from AR11598 on 19 October 2012. Sketch and copyright Harry Roberts ©, all rights reserved

The large event (Fig) was fast, with motion clearly seen, and was hard to sketch. It was unusually bright at ~0.8 (80%) of chromosphere brightness. Its southern part fitted Zirin’s description of a ‘spray’ – “bits flying out in all directions: the overlying fields disrupted”.

The northern part seemed more collimated – with curved shapes perhaps following field lines above the unseen spots. Just six minutes later the brightness had fallen to between 0.4 and 0.1 of the chromosphere – rapid cooling was underway. (Note: a quiet prominence is ~ 0.1 of the chromosphere brightness).

The height reached was just under 100Mm, though earlier ejecta may have been higher.

By 21:25 there was little left – some faint ejecta was confined to arch shape fields above the unseen spots.

Next day on the 20th UT tall plumes of material were seen near the site–but it seems not in motion.

Clearly the event of the 19th was associated with a sunspot group – compared to the slow ejection of a quiet filament. The high velocity and ‘spray-like’ ejecta indicate this – and a strong flare would have been involved: yet the GOES showed nothing at the far-side site (though a C3.6 erupted in AR11589 in the sun’s NW at the same time). The spot group hosting the ejection still lay ten degrees behind the limb. A coronal mass ejection most likely erupted at the time (in fact they slightly precede such events).

AR11598 When at last it appeared the spot group was of modest size: two views are shown (Top Fig).

We see a small complex group with large (p) spot and a light bridge (LB) dividing its umbrae. Following this is an irregular scatter of smaller spots with some penumbrae. Mt Wilson saw no signs of mixed polarities in any penumbrae (ie Hale Delta class) – and made it Hale class Beta. NOAA, however, made it Beta-Delta – a very active class.

X-class Flare Though small, it had a big flare -an X1.8 on Oct 23 at 03:17UT. Such a big flare ensures AR11598 a place in the SC24 record book.

In the Fig note the rotation of the light-bridge over two days: it seems the (p) spot rotated ~80º counter clockwise in 24 hours’ suggesting the sudden rearrangement of fields in the group.

On both days Helio sited the (p) spot at -11,115, while the scattered following (f) spots moved eastward by ~3 degrees and the group’s overall length (LL) reached 9 degrees.

Mt Wilson umbral fields in the (p) spot were red 2200 and 2300G, fairly strong fields and cause of the regular penumbra. The scattered (f) spots all had fields well below 2000G.

NOAA’s Beta-Delta class suggests mixed polarities in one penumbra – perhaps in the scattered (f) spots – the X1.8 flare perhaps an outcome of this complexity; Mt Wilson was fog bound much of the time.

By Oct 26 (not shown) much of the (f) ‘tail’ had gone but a single penumbra now enclosed mixed fields – yet so far with no effect in the GOES flux.

Conclusion A small but active group, it had some great flares and ejections– yet overall it could not change the spreading ‘malaise’ of weak-fields – now both hemispheres seem to be affected. What next? Have we in fact reached solar maximum? (I hope not!)

Maybe the sun will surprise us with a salvo of ‘pent-up’ energy – but it must do so very soon.

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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