|Two Rare Lunar Halos imaged by Timo Veijalainen (photography, AV-Lappi) on November 29, '09 at Levi, Kittilä, Finland. ©Timo Veijalainen / AV-Lappi. Reproduction or use of this image is forbidden without the copyright holder's permission.|
|The waxing and near-full moon has produced an exceptional display containing two rare halos. Timo Veijalainen captured it in a 27 second exposure at f/4 ISO 200.
Bright and colourful moon dogs (paraselenae) flank the moon and they are linked by a white paraselenic circle. The 22° circular halo is topped by an extensive upper tangent arc.
The first rare halo is the suncave Parry arc at top above the upper tangent. Seen perhaps once a year above the sun, a lunar Parry arc is much rarer,
The even more rare halos are the two variants of Lowitz arcs near each moon dog. Faintly visible in the unprocessed image, they are more apparent in the two enhanced versions at left.
Rays passing between 60° inclined side faces of Lowitz oriented crystals can produce up to three arcs. Here we have a lower arc extending from the 22° halo to the paraselene plus a more rare middle arc crossing the moon dog.
Lowitz arcs are still not straightforward. The traditional model of a regular hexagonal plate shaped crystal taking all orientations about the Lowitz axis has a hard job to reproduce this display. So far, HaloSim ray tracings indicate that the relative intensities of the lower and middle arcs might be better reproduced by non regular hexagons like that at left.
Further, their orientations might be 'plate-Lowitz', i.e. rotational positions about the Lowitz axis are restricted with those towards the large hexagonal face being horizontal more favoured.
These type of interpretations are currently rather controversial among halo experts. All sorts of other effects can play a role. There remains a great deal about Lowitz arcs that we do not understand. More high quality observations can only help.