A subhelic arc is a rare halo, formed by internal reflection through ice crystals, that curves upwards from the horizon and touches the tricker arc above the anthelic point. Subhelic arcs are a result of ray entrance and exit through prism end faces with two intermediate internal reflections.
Formation
A subhelic arc is formed when sun rays enter one end face of an ice crystal in singly oriented columns and Parry columns, reflect off two of the crystals side faces, and exits the crystal through the opposite end face. The ray leave the crystal in the exact opposite angle, resulting in a net deviation angle of 120°, the angle for the formation of 120º parhelia.[Cowley, "South Pole Halos - Zenith View"] ]The subhelic arc touches the top of the tricker arc, an indication the two have closely related ray paths.[Cowley, "South Pole Halos - Anthelic View"] ]The subhelic arc crosses the parhelic circle in an acute angle, and at a sun elevation of 27° it passes exactly through the 120° parhelion.[Vornhusen, "First Report on the Danzig Halo Display"] ] See also
* Infralateral arc
* Wegener arc
Notes
References
* cite web
url = http://www.atoptics.co.uk/halo/spzenith.htm#arcs
title = South Pole Halos - Zenith View
first = Les | last = Cowley | authorlink = Les Cowley
publisher = Atmospheric Optics
accessdate = 2008-10-10
* cite web
url = http://www.atoptics.co.uk/halo/spanti.htm
title = South Pole Halos - Anthelic View
first = Les | last = Cowley | authorlink = Les Cowley
publisher = Atmospheric Optics
accessdate = 2008-10-10
* cite web
url = http://www.meteoros.de/halres/hevele.htm
title = First Report on the Danzig Halo Display
first = Mark | last = Vornhusen
publisher = Arbeitskreis Meteore e.V.
accessdate = 2008-10-10
External links
* cite web
url = http://www.ursa.fi/ursa/jaostot/halot/eng/intro.html
title = A short introduction into halos
first = Jarmo | last = Moilanen
publisher = FHON - Finnish Halo Observing Network
accessdate = 2008-10-10 (Including a drawing from a halo observation in Oulunsalo, Finland, in April 1996.)
* cite web
url = http://www.ursa.fi/blogit/ice_crystal_halos/index.php?title=subhelic_arc_in_finland&more=1&c=1&tb=1&pb=1
title = Subhelic arc in Finland
first = Jarmo | last = Moilanen
publisher = Ice Crystal Halos
accessdate = 2008-10-10 (A halo display including a subhelic arc, observed in Oulu, Finland, May 2008.)
