More New Halos

Christmas arrived early in Rovaniemi, Finland. On Sunday night Dec 7, '08 halo expert Marko Riikonen (site) imaged a spectacular skyfull of hitherto unseen halos.

Finland is halo land and its capital must now be Rovaniemi. Earlier there, at on the night of Nov 5th, Marko used a 150W halogen lamp to light high quality crystals formed downwind of ski-slope snow machines. He obtained magnificent halos (1,2). Next morning the sun banished the lamp to show a new halo. Now Marko has more new arcs.

The fisheye image is a stack of two 30s exposures. The arcs are subhorizon halos seen paradoxically against the night sky. The camera was below the lamp and pointing upwards at the zenith. This turned the lamp 'halo sky' upside down so that the subhorizon arcs were in the sky. Sun generated subhorizon halos are difficult to study because they are only visible looking downwards from aircraft, ski-slopes or as nearby diamond dust glints against dark ground.

Where do we start! Use the HaloSim ray tracing for orientation. The simulation is preliminary because Marko's Sunday evening pictures provide material for long analysis and this is just a first look.

At right is a bright subsun (sublamp!) flanked by two subparhelia. These are the below horizon halos most often seen from aircraft. The subparhelic circle bright in the antisolar (subanthelic) direction at left is also seen from the air and is actually sharper than the parhelic circle. At right a colourful 46-degree halo courses across the image.

Now for the new halos. Just on the frame's lower edge is a sub-120° parhelion, never before imaged and the subhorizon counterpart of the 120 degree parhelion.

At center we are effectively looking straight downwards towards the nadir. The coloured arc to its right is the subhorizon equivalent of our familiar circumzenithal arc - a 'circumnadiral' arc. The CNA has probably never been seen before.

But the real shock is the complete circle, brighter in parts, around the nadir. This is a sub-Kern arc. The extremely rare Kern arc around the zenith was itself only imaged a year ago (again in Finland, that time by Marko Mikkil�). Now we have its nadiral counterpart.

Another arc is the helic (or heliac), streaming away from the lamp across the subsky.

Plate crystals produced most of the display. Regular hexagonal plates could have made the CNA. Subhorizon arcs result when there is one (or more) internal reflection from the big lower faces of plates. To make the CNA, downward going rays entering the crystal top face are internally reflected back upwards before leaving a vertical side face.

The subKern, like the Kern, requires either improbably thick plates or ones tending to a triangular shape i.e. hexagons with alternate sides long and short. Many crystals formed downwind of snowguns have a quasi-triangular habit. The sub-Kern results from rays reflected off the lower face (#2) to give ray paths like 1-3-2-5 compared to the 1-3-5 for the Kern itself.

The helic and short Tricker arcs come from a low concentration of Parry oriented column crystals. The crystal populations changed constantly, earlier there were singly oriented columns, then plates and then the mixture of plates and Parry crystals here.

The subhorizon images (there are more) are only part of the night's events. There were rare arcs strewn also across the lamp's equivalent of the above-horizon sky. And the Moon, never to be outdone by a mere light-bulb, cast its own halos of rare beauty.