The weather was cold with a light breeze from the west, so I headed east to Jokkavaara gravel pits to check whether the reach of the diamond dust extended that far. The three previous times I have had odd radius sub-plate arcs were at these pits, all in winter 2016/17, and I was thinking maybe now it is this winter's turn for the Jokkavaara magic.
There was diamond dust at Jokkavaara, and as the image above shows, it was indeed of the pyramid crystal kind: inside the subparhelia we see pretty decent sub-18° plate arcs. This is an average stack of the eight best photos showing the halo. They are included in collage below which is a larger set of successive 30 s exposures showing the changes in the display. I made also a video of these photos (one or two last photos are excluded in it).
As these photos show, the sub-18° arcs were fickle in appearance, alternating with subparhelia in successive frames. No wonder I was uncertain about them visually. At one point a broad effect was visible. I take this to be the time when in photos both sub-18° arcs and subparhelia appear. Thus my visual appears to tell that they were then in the sky simultaneously, not separately during the 30 seconds exposure.
And so on to some simulations. For starters, let's mention that I was unable to simulate the vertically greatly elongated shape of the subparhelia and sub-18° arcs seen in many photos. Increasing crystal tilts didn't lengthen them.
But in three successive photos, starting from the middle image on the third last row in the above collage, the sub-18° arcs are actually quite compact. Because the pyramid crystal orientations have likely been least unstable at this stage, a stack of these three photos is what I decided to compare the simulations with.
The single pyramid makes only sub-18° arcs and a weak sub-23° arc, which would seem to explain the photo pretty nice. But the spot on the right side of the display (marked with an arrow in b-r) is not reproduced by this crystal. If we use instead a double pyramid then we get a matching spot which, with the pictured crystal, is contributed equally by overlapping 35° arc and sub-35° arc. A different double pyramid shape could also have been chosen to make either one.
The double pyramid makes also sub-20° arc, 18° arc and 23° arc, and while there may be a suggestion of the last one in the photo, the first two are not there. Some adjustment on pyramid proportions may get rid of the sub-20° arc, but that will just make 20° arc emerge, which is not seen in the photo either. And the 18° arcs stick there always with the double pyramid. So this kind of crystal neither satisfyingly explains the display.
Concerning the 35° spots, it is good to read also what has been written on them concerning another display on 4/5 January 2017 (under the collage image with simulation parameters). There seems to be a possibility even for an impostor.
An earlier stage of the display, as shown below, seems to contain the 35° spots as well. These are the first eight photos I took. In addition, I would say a there a pretty certain 23° arc and the b-r appears to show 18° arc on the left. The sub-18° arcs are also there. While setting up the tripod I thought of seeing them and was nervous whether they would still be present when I was ready.