Wednesday, October 03, 2007

46° contact arcs

About a year ago in Muonio, Finland, a diamond dust display produced a new halo, the 46° contact arcs. The display was shortly reported in the blog, but no simulation was shown. So here is a simulation, together with a composite of the photos that were taken by Päivi Linnansaari. The 46° contact arcs, which arise from Lowitz-oriented crystals, appear as three arcs below the circumzenith arc.

The Lowitz crystals used in the simulation are regular plate-like hexagons, with aspect ratio of 0.4, tilt about the Lowitz axis 28° and Lowitz axis rotation 1°. Sun elevation is 9°. The Lowitz arcs themselves are fainly visible at 10 and 2 o'clock positions, separating from the 22° halo and reaching towards the upper sunvex Parry arc. This is the circular component of the Lowitz arcs, also known as the c-component (after Greenler).

Occasionally, in high cloud displays there is seen a short patch of 46° halo under the circumzenith arc, as shown here in the photo by Stepanka Kosova, taken on 20 August in Prague. It has been sometimes suspected that these might be indications of 46° contact arcs. Whether that's the case, it may be confirmed if a series of photos are taken for stacking.

The simulation is made with HaloSim by Les Cowley and Michael Schroeder.


Blogger Claudia Hinz said...

Congratulation! I've never seen this in nature to now!

10 October, 2007  

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