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).

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