15 May 2013
Correction: Hat tip to Daniel Linek who spotted my mistake. This is a CIRCUMSCRIBED Halo. Not a Circumzenithal Arc . See here: http://www.atoptics.co.uk/halo/circum.htm
The title to this post has also been corrected.
You can read more about this on the great Atmospheric Optics site: http://www.atoptics.co.uk/halo/cza.htm I saw one in Greenland (and at the South Pole for a few seconds). The bottom band I believe is a circumhorizon arc.