While sub-Kern was strong in the 7/8 December display, the Kern arc was less so. For one thing, I did not see it visually. In the two photos showing it, it can hardly be distinguished from the overlapping heliac arc. The other one of these photos is shown here. On the right is a heavily enhanced version zoomed at the zenith region.
Strange how that the regular Kern is faint and hard to see but the sub-kern was strong and easy to see. Theres something about those subhorizon halo raypaths that makes arcs like sub-Kern brighter. I just don’t get it.
A light ray does not travel easily 1-3-5 raypath that makes Kern. Before it reaches face 5, it is likely to meet another face and thus contribute to another halo. But with sub-Kern raypath 1-3-2-5 reaching face 5 is easier because of the basal face reflection before that. This is why sub-Kern is brighter.
Solution for more intense regular Kern is to make plates thicker (even column-like) or make them triangularish.
In this photo, contrary what I wrote, actually the Kern might not be that much overlapping with helic arc, but rather continuing it (imperfectly). Compare to previous posting photos.