Fascinating work Trevor - that spot shows up quite well after looking at it for a while and it also appears to be associated with other structure in that zone. has anything similar shown up in past images from Cassini or Hubble? I had a bit of a look around - there are some small spots in this image, but not on the same scale as you show.
http://storiesbywilliams.files.wordp...th_auroras.jpg