Sunday, 25 March 2012

AR11429 An Island Delta group

Island Delta is a term used by Zirin, Liggett and others to describe the most active class of sunspots - and AR11429 was the first example for SC24, I think. The post below shows the group when it contained mixed polarities within a single penumbra (Mar 5 and 6) - and those polarities are reversed.
    In a northern hemisphere sunspot like this one, violet polarity normally precedes in a spot group. In this group we see the large preceding spot is red rather than violet. And the following spot (at the other end of the group) is reversed too; its violet, when it should be red. This a fully reversed group! Such groups have the "greatest" flares. But AR11429 disappointed in this respect, its strongest flare was modest X5.4, the second strongest of SC24. The strongest of this cycle was a GOES X6.7 in 2011 August. This group is perhaps the most interesting from a polarity stand-point for SC24. Solar rotation is to the right in all my logs. This Fig shows the evolution of the group in detail after the single large penumbra split into many smaller ones. The reversed arrangement persisted. Zirin suggests the "true" preceding spot, the smaller spot with violet polarity just behind the large red one, should have forced its way into leading position - but this did not happen (weakness of the Hale-Nicholson force perhaps?. For an example where it did, take a look at AR 10930 in 2006 - try to find the Swedish telescope animation - its amazing)!
     In such a reversed group the members may attach to other distant polarities rather than its own group members. The SDO image (far right) shows AR11429 attaches to AR11431 way across the sun's equator in the south (white dotted line). This was the largest reversed group for SC24, and I think the largest reversed group I've seen in 13 years! It may return at the end of 2012 March - despite the "rule" that they are normally short-lived groups.

Monday, 19 March 2012

Island Delta spot group

The term "Island Delta" was coined by Zirin and Liggett in the mid-eighties to describe this rare class of spot group. Magnetic complexity and reversed polaritiy are the chief characteristics of "island delta" class.  When it was first seen at the sun's east limb AR11429 showed a mix of (p) violet and (f) red polarities, all in the one penumbra - by definition a Hale Class delta group. But the polarities were "reversed" - that is, normal northern groups have violet polarity preceding (p) but this group had RED preceding. As well, it had violet following - when it should have been red. It was, and remained, a fully reversed spot group. The invisible barrier between opposite polarities, the inversion line, is dotted in blue (speculative. Also rotation is to the right in Fig. The preceding spot has polarity R23 at longitude 303.)
          "Island deltas produce the strongest flares"(Zirin). But this one did not act as expected - it did have strong flares, up to GOES X5.4 - but the largest flare of SC24 remains an X6.9 in a normal Beta-Gamma-Delta group 2011 Aug 9.  AR11429 was the first "island delta" for SC24, and I dont recall any from SC23 - but they must have been there! I checked AR486 that had the largest recorded flare (http://users.telenet.be/j.janssens/Flares/Powerflare.html)- but it seemed a normal Beta-Gamma-Delta. Comments anyone? (Polarities (c)Regents of University of California, Mt Wilson).