I am trying to think of the last time I saw one out here and nothing immediately somes to mind. Growing up out west, I saw them a few times though. They appear directly below the sun at equal distance as the sun is high. They are most favorable a few hours after sunrise or before sunset and must be below your field of plane (ie, summits, chairlifts, etc) but if in a airplane, they can occur at any time of the day. They are formed by a reflection off small plate shaped ice crystals that are "falling" parrallel to each other to create a mirror like effect. The more parallel they are, the sharper the image. They are most commonly found in freezing fog, arctic cold (like the temperatures over the northeast right now) as the air is so cold, small ice crystals form, or in high, thin cirrus clouds. It is similar to the process that forms halos and sundogs.
Staff Meteorologist/Night Observer, KMWN (Mt Washington Obs., NH)