A "Split Aura" Orb

Photo date: October 2, 2014, Photographer: Mark Mahin.

This is a type of air orb I have photographed many times. There
are three common characteristics observed over and over again:
(1) The orb is perfectly circular, and often has an outer ring or "crust" about 5% of its width.
(2) The orb is bright white, with a slightly bumpy appearance,
looking a little like what a pizza maker might have on a pizza
pan just before he poured on the tomato sauce.
(3) The orb has a "split aura" around its outer perimeter, an
aura that is half purplish and half yellow-green or
yellow-orange.

In this case the measured air quality was very good, which pretty much rules out dust as a possible explanation. I have used the IrfanView "Auto-adjust Colors" menu command to make the aura a little more visible.

Dust level: 50/6, very good. Nearby reflective surfaces: none. No orbs shown in photos taken at same location from same angle soon before and soon after this photo was taken, which argues against any dust explanation.