South Pole Halos - Zenith View

South Pole 11th January 1999.

All sky view with the camera pointing to the zenith by Marko Riikonen.

The sky is completely ringed by a parhelic circle. Plate, singly oriented column and Parry column crystals all help light it.

Plate crystals form 120° parhelia.

Singly oriented columns produce a bright and colourful supralateral arc and even rarer halos. Mouse over the "Halo key" to identify them.

Touching the upper tangent arc and extending back to the anthelic point is a Wegener arc. Similar ray paths to those generating the tangent arc form it except that they are additionally internally reflected from an end (basal) face.

Other rare halos are subhelic arcs curving upwards from the horizon and touching the Tricker arc above the anthelic point. Subhelic arcs result from ray entrance and exit through prism end faces with two intermediate internal reflections. Crystal perfection is needed!.

Parry oriented columns form, among many others, the Hastings arc (of the upper suncave Parry arc). This has the same ray path as the Wegener arc (but from Parry crystals) and is very difficult to distinguish from it. The enhanced image below helps to show it.

Hastings arc

The right hand view is the display image in b/w negative form after contrast enhancement and unsharp masking. Negative images often show halos better. At left is a 30 million ray HaloSim simulation.

The exceedingly rare Hastings arc, red arrowed, is just visible. The heliac, antisolar and Wegener arcs show up strongly in the enhanced image.

Another view.

	Wegener arc Formed by singly oriented columns. Similar ray paths to upper tangent arc (to which it is itself tangential) except that there is an internal reflection from a prism end face. Rays enter a side face, reflect off an end face and then leave through another side face inclined at 60º to the first. The arc wraps around the circumscribed halo at high sun.

	Subhelic arc Formed by singly oriented and Parry columns. Rays enter an end face, internally reflect off two side faces and leave the opposite end of the crystal. The net deviation as viewed from the end of the crystal is always 120º like rays forming the 120º parhelia.