2010. április 9., péntek

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

2 megjegyzés:

  1. This is crazy more new unknown arcs. It only keeps getting better.

    VálaszTörlés
  2. nicolas lefaudeux2010. április 11. 11:56

    I agree with you Marko, Resolute Bay is not a good example. I have spent a lot of time to find other occurences but could not find any where it shows up clearly.

    at low source elevation, it is almost impossible to positively identify it because of usual Moilanen or sun pillar at the same location. a third image from Jaako Tähti possibly shows this effect (http://www.ursa.fi/blogit/media/blogs/haloreports/riikonen/Jaakko_Tahti_2301_panoraama.jpg) as a diffuse glow "filling" usual Moilanen. but as you pointed out to me, it could also 'just' be an excess of brightness of the sun pillar.

    at high source elevation the arc would be absent or extremely faint. This is simulated easily using limited tumbling angles (which is consistent with the absence of the lower component) which causes the arc to vanish with source elevation (http://img88.imageshack.us/img88/4592/newarc.jpg). this is absolutely not surprising, the presence at high source elevation requires very large tumbling angles, this is why i said that "if the crystals take all the rotational angles".

    the strenght of this arc at high source elevation also seems to depend on the crystal chosen to simulate the effect (this is why I prefer using my own piece of software for the simulations, even if it does not give photo quality simulations, as I can control everything and use really complex crystals).

    anyway, we still only have two observations which is enough to say this might be a new arc, but we will only be able to draw a general behaviour when more observations will be reported.

    VálaszTörlés