New Halo by Marko Riikonen

Last updated on September 16, 2023

New Halo by Marko Riikonen: A Fascinating Atmospheric Optics Phenomenon

Atmospheric optics never ceases to amaze us with its stunning displays of light and color. One such remarkable event was captured by Marko Riikonen on November 6, '08, in Rovaniemi, Finland. Marko's observant eye spotted a new halo during a marathon halo observing session, adding to the already spectacular displays he had witnessed the previous night (1,2).

The daytime display commenced at sunrise and continued throughout the morning, but it was around 11:30 am that Marko noticed the new halo. These halos, like the ones he witnessed during the night, were generated by ice crystals nucleated by ski-slope snow machines. However, the crystal formation during the day was peculiar. While Rovaniemi was engulfed in fog, the supercooled water fog near the snow guns, located about 1-2 km away, transformed into ice crystals due to the small ice particles emitted by the guns (3).

Marko's main camera encountered some technical issues, but he managed to capture this extraordinary event using a backup pocket camera. The images reveal the dominance of gull-wing-shaped upper tangent arcs in the scene, along with a faint sunvex Parry arc positioned above them. However, the real surprise was the sharp V-shaped arc, with its lower tip just 7-7.5° above the sun. This arc bore a resemblance to the enigmatic Moilanen arc, which had occasionally been observed in the past (4).

The appearance of this new arc sparked considerable excitement and discussion among halo experts. Walter Tape provided an explanation for the phenomenon, suggesting that it was a "reflected" lower sunvex Parry arc. According to this theory, a ray enters the lower sloping face of a Parry-oriented column crystal and undergoes internal reflection from the upper horizontal face before exiting through the opposite upper sloping face (5-3-8 ray path) (5).

However, the explanation did not end there. Marko proposed that the weakness of the "normal" upper sunvex Parry arc and the absence of the upper suncave Parry arc could be attributed to the presence of Parry-oriented columns with a flattened habit. These columns possess sloping side faces that are significantly shorter than the horizontal upper and lower faces. Although this configuration is unusual, it is physically realistic and can account for the observed phenomena (6).

As the sun descends lower on the horizon, the arc climbs higher. When the solar elevation reaches approximately 5°, the new halo aligns with the position of the Moilanen arc. To gain further insights into these arcs, it is crucial to conduct more observations during low sun conditions. Marko's experience also emphasizes the importance of carrying not just one, but two cameras, to ensure that such extraordinary events are not missed (7).

The sighting of this new halo by Marko Riikonen highlights the ever-evolving nature of atmospheric optics. Through his observations and those of other halo enthusiasts, we continue to deepen our understanding of these fascinating optical phenomena. The exploration of ice crystals, their unique formations, and their interaction with light continues to unravel the mysteries hidden within our atmosphere (8).

So keep your eyes peeled and your cameras ready, as you never know when the sky will gift us with another breathtaking display of atmospheric optics like the new halo captured by Marko Riikonen in Rovaniemi, Finland (9).

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New Halo seen by Marko Riikonen (site) on November 6,'08 at Rovaniemi, Finland. Images ©Marko Riikonen, some images were published previously in Tähdet ja avaruus magazine, Finland.

Marko sighted the new halo in the daytime part of a marathon halo observing session. During the previous night he had lit diamond dust crystals with a halogen lamp to generate spectacular and rare displays (1,2). The next morning the sun took over from the lamp.

"The daytime display had been going on from sunrise (it was just a continuation of the night crystal storm), but it was only at 11.30 am that I noticed the new halo." Like the night's display, the halos were generated by ice crystals nucleated by ski-slope snow machines. "The .crystal. formation was peculiar in that it was actually foggy all over Rovaniemi, but around the snow guns at about 1-2 km distance the supercooled water fog was nucleated to ice crystals by the small ice particles that the guns generate."

Marko's main camera had problems and these images were taken with a back-up pocket camera. The upper image is unsharp masked to strengthen the halos. The near noon sun was 7.5� high. The gull-wings of an upper tangent arc dominate the scene. Above it there is a faint sunvex Parry arc. But the surprise was the sharp V-shaped arc whose lower tip was just 7-7.5� above the sun. The arc resembled the unexplained Moilanen arc which indeed was also sometimes visible as in the lower images.

The new arc generated considerable excitement and discussion among halo experts. Passage of rays through an ice wedge of 23� compared with that of 34� for the Moilanen arc generates an arc in the correct position. Pyramidal crystals with very unphysical angles might have the required dihedral but simulations showed that in any case they produced an arc that was too wide.

The halo was explained by Walter Tape as a 'reflected' lower sunvex Parry arc. A ray enters the lower sloping face of a Parry oriented column crystal. It is then internally reflected from the upper horizontal face (just once or for a total odd number of internal reflections) before leaving via the opposite upper sloping face (ray path 5-3-8).

The weakness of the 'normal' upper sunvex Parry arc and the non visibility of the upper suncave Parry was explained by Marko by invoking Parry oriented columns of flattened habit. The interfacial angles remain the same and, although unusual, they are physically fully realistic. Those used at left in the HaloSim ray tracing simulation had sloping side faces less than10% of the length of the horizontal upper and lower faces.

As the sun gets lower the arc climbs higher. At solar elevations ~5° it is in the same position as the Moilanen arc. More low sun observations on both arcs are desirable together with the advice 'always carry not just one but two cameras'!

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