<p>It is just sun light refracting in small ice crystals in the air but makes it easier to believe in some "UFO observations". What makes this halo unusual, is its elevation below the observer level: it is a downhill in front. 
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Sölden, Austria, 2005.
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<a rel="nofollow" class="external text" href="https://www.flickr.com/photos/uncle-leo/1336635365/">Amazing halo effect</a>