Possible new arc from Moilanen crystals

I would like to share some thoughts about a possible new arc. This arc seems to be present on the two observations of Moilanen arc at high source elevation from Jari Luomanen ( 1 ) and Marko Mikkilä ( 2 ). It appears as a diffuse glow below Moilanen arc. This arc can be simulated using any crystal producing Moilanen arc like 34deg prism pointing up (which is not aerodynamically stable) or crystal plates twinned at 34deg (which does not look like a possible twinning) tumbling with large angles along their horizontal axis. As a consequence, this new arc does not help a lot on identifying what crystals cause Moilanen arc.

At lower source elevation, the new arc would mix with the usual Moilanen arc so it is difficult to make the distinction between the two arcs. The main difference is that usual Moilanen appears as a hollow “V” while new arc would be more like a solid wedge. This new arc could be responsible of some appearance of the Moilanen arc when it appears as diffuse ( 3 ) compared to more usual sharp arc ( 4 ).

To draw a parallel, the usual sharp Moilanen is a bit like the upper sunvex Parry arc while the new arc is more like the upper tangent arc. To the extremes, if the crystals take all the rotational angles, there would be a lower component like the lower tangent arc, or if the source is very high, there would be a complete circle like the circumscribed halo.

More observations are needed to confirm the existence of this arc especially at high source elevation when it is clearly separated from usual Moilanen arc, so monitoring about this possibility is encouraged!

Nicolas Lefaudeux