Sundogs of this brilliance are usually made by ice crystals in the nearby air, diamond dust, although they can be formed in high cirrus too.
   
    
Hexagonal, not necessarily regular, plate crystals floating nearly horizontal are responsible.

Sunlight enters a nearly vertical side face. It reflects up and down as it traverses the thin crystal, and then leaves via another side face inclined 60° to the first. In effect, the sun's rays shine through a 60° ice prism and are dispersed into spectral colours. Sundogs are red towards the sun and grade through yellows, greens and sometimes blue. Colours other than red overlap and are thus of muddier hue.
   
  
When sundogs are as intense as these be sure to look twice as far from the sun. The exceeding rare 44° sundogs from rays passing through two separate crystals might be visible.