The Moilanen arc is named after its discoverer, halo expert Jarmo
Moilanen of Finland.
The rare “V” shaped arc, varying in
position with solar altitude, has been observed during natural
halo displays but is seen more often in the diamond
dust formed downwind of ski-slope snow machines. The Oslo display
(above) was seen and photographed over a wide area but the Moilanen
arc only shows in Steinar Midtskogen's images. A ski-slope with
operating machines was 1.5 km away and while the main halo display
was natural, the M-arc could have originated from machine crystals**.
The arc is easily simulated using prisms
of apex angle 34° aligned
as shown and taking all rotational positions about a vertical
axis. Identifying the ice crystals that actually produce it is
quite another matter. An ice interfacial angle of 34° is crystallographically
improbable as would be the orientation of an isolated prism. Furthermore,
no other associated halos, for example a lower M-arc or ones from
other interfacial angles present, have been observed
which further limits the crystal possibilities. Despite the hard
work of Finnish observers in photographing diamond dust displays
and sampling crystals from them, the crystal source of the arc
remains unknown. It is one of the unexplained halos.