46° halo

A 46° halo is a rare member of the family of ice crystal halos, appearing as a large ring centred on the Sun at roughly twice the distance as the much more common 22° halo. At solar elevations between 15–27°, the 46° halo is often confused with the less rare and more colourful supralateral and infralateral arcs, which cross the parhelic circle at about 46° to the left and right of the sun.[1]

Sun dogs with a larger and fainter 46° halo and a 22° halo with an upper tangent arc

The 46° halo is similar to, but much larger and fainter than the more common 22° halo. It forms when sunlight enters randomly oriented hexagonal ice crystals through a prism face and exits through a hexagonal base.[2] The 90° inclination between the two faces of the crystals causes the colours of the 46° halo to be more widely dispersed than those of the 22° halo. In addition, as a lot of rays are deflected at larger angles than the angle of minimum deviation, the outer edge of the halo is more diffuse.[3]

To tell the difference between a 46° halo and the infra-/supralateral arcs, one should carefully observe sun elevation and the fluctuating shapes and orientations of the arcs. The supralateral arc always touches the circumzenithal arc, while the 46° halo only achieves this when the sun is located 15–27° over the horizon, leaving a gap between the two at other elevations. In contrast, supralateral arcs cannot form when the Sun is over 32°, so a halo in the 46°-region is always a 46° halo at higher elevations. If the Sun is near the zenith, however, circumhorizontal or infralateral arcs are located 46° under the Sun and can be confused with the 46° halo. [4][5]