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Subsun seen by Andrew Kirk from about 30,000ft over the US Midwest, October '07. ©Andrew Kirk, shown with permission.

Subsuns, sometimes blindingly bright, are formed by flat plate ice crystals in thin cloud at lower altitudes. The sun is reflected from their upper (and lower) near horizontal surfaces. When the plates have very small wobbles the subsun is almost a perfect reflection of the solar disk, otherwise it is elongated.
Lakes also reflect the sun. Could this have been a reflection from water? "Not likely.. ..I could actually use the subsun - visible off and on for about an hour - as a "viewfinder". Lakes would pop into view, reflecting
the sun, as the subsun 'approached' them and then they would disappear as the
subsun moved on."