Harry is disappointed by the placid demeanour of the giant sunspot group AR 11416

February 22, 2012


Three views of sunspot group AR1146. Image and copyright Harry Roberts ©, all rights reserved

AR 11416 first appeared in the sun’s SE quadrant on Feb 8 around 16:00 UT, as a small grey pore with a magnetic field of 1500G; just 24h later it was a cluster of maybe a dozen spots with two compact penumbral spots preceding (p) – an impressive rate of development. The stage was set for fireworks, it seemed.

These early stages were not logged in Sydney due to cloud – unusually persistent cloud – but they were recorded in drawings atop Mt Wilson, using the 150’ solar telescope.

My first view of AR11416 was on Feb 11 (05:30 UT, Fig, top) in WL [white light] during a brief clearing when a mid-size bipolar group was recorded – with an area about 100 units and Hale class Beta-Gamma, that is – Bipolar groups with no marked north-south inversion line (Zirin, H. “Astrophysics of the Sun”. Cambridge Uni Press. 1986. P316).

Note that both (p) and following (f) spots had fields of 2300 gauss (R23 and V23kG). This growth from a pore to a complex group in only three days was remarked upon by many (see Spaceweather.com for Feb 12), and the probability of an M class flare in the group was put at 50%, and of an X-class at 10%.

While earlier northern group AR 11410 (by now gone) had hosted flares to GOES X-ray class M1, the expected fireworks in AR 11416 did not materialise, despite the rapid growth and strong fields – its strongest flare was a meagre C1 on Feb 9th when it was just 24h old.

My second sketch shows 11416 on the 12th (Feb 11 21:00UT, only 15h after the first, Fig, below) when big changes were seen. At least 23 spots were mostly concentrated in the large (p) and (f) penumbrae, with a few scattered between. The (p) spots still had 2300G umbral fields, but the following spot’s umbrae had split into many smaller ones, and the field was commensurately weaker, V20 maximum. Despite this, there was much interesting complexity in the group– with spot chains and irregular penumbral outlines that usually denote strong flares – yet very little occurred. In H-alpha several small active region filaments were seen (arf) but little plage.

By the 15th the (f) spot had faded to scattered tiny spots with a few small penumbrae and on 16th the (p) spot still retained a large oval penumbra holding two round umbrae (not shown). It would be good to know the field strengths but MT Wilson had been under cloud since the twelfth. Their available umbral fields have been added to the sketches, but note they are not co temporal with my logs due to time zones, and the spot group had rapid changes of shape over the period.

My last view of AR 11416 was on the 17th 22:00UT (Fig, RHS) with it only 6º from the sun’s SW limb. The two round umbrae in an oval penumbra had survived it seemed, and might still have had fields of R23, if only Mt Wilson had clear skies! A patch of H-alpha plage was seen, but no activity was logged above the limb – all remained quiet.

AR11416 was notable in that it failed to develop the complexity needed for flaring- despite having flux emergence or sunspot motion due to flux emergence (Zirin P343). Plenty of new spots had emerged and the preceding spot had rotated maybe 40º clockwise in just 15h – but still no flares! The reasons for its placid demeanour must remain an interesting puzzle.

Harry Roberts is a Sun and Moon observer, a regular contributor to the Sydney Observatory blog and a member of the Sydney City Skywatchers

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