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   Kern Arc  










The Kern arc is closely related to the circumzenithal arc. Like the latter, it is formed by rays entering the top faces of horizontal plate crystals and leaving through a near vertical side face. However, Kern arc rays undergo an internal reflection off a side face inclined at 60 degrees to the exit face. The contorted ray path is not possible in ordinary near regular hexagonal crystals unless they are improbably thick ~ vertical columns almost. Near "triangular" hexagonal crystals with three alternate side faces much shorter than the others allow Kern rays more readily. Marko Mikkilä sampled crystals during the diamond dust display and many had the required shape.

The HaloSim ray tracing simulation for the corresponding solar elevation of 6.2° is in two parts. The left hand side was filtered so that only rays entering a top face and leaving without any internal reflection registered. This shows the conventional circumzenithal arc. The right hand pane shows only rays undergoing at least one internal reflection from a side face. The resulting Kern arc is not uniformly bright at all angles around the zenith as indeed Mikkilä's images also show.
"Triangular" plates sampled by Marko Mikkilä during the display.
 


Single frame image showing the Kern arc.

The left hand is unenhanced, the right hand side is unsharp masked.









All images
©Marko Mikkilä,
shown with
permission.


 
    First ever images of a Kern arc captured by Marko Mikkilä at Sotkamo, Finland on 17th November 2007.   The extremely rare Kern arc is the faint upper circle. The circle's lower and brighter part is the familiar circumzenithal arc. Mouse over the thumbnail for a labeled and unsharp masked version. The wide angle fisheye view is a stack of 16 individual images. Stacking brings out faint arcs whilst smoothing ambiguous variations in sky background. The Kern arc is also visible in individual images as in the example below.

The Kern arc completely encircles the zenith whereas the more familiar circumzenithal arc only partially does so. Visual sightings have been reported on a very few occasions including during the famous Saskatoon display of 1970. However, it has never before been photographed and Marko Mikkilä's images are a triumph.