Tuesday, 6 January 2009
Reflected Lowitz arcs and 46° contact arcs in Hyvinkää
Reflected Lowitz arcs were seen for a short period of time in Hyvinkää, Finland, on the very first day of the year. I used the spotlight as a light source. In the beginning of the night fierce pillars formed above each and every lamp there was. This esthetically impressive (but rather poor from halo viewpoint) stage started to transform into something better when superparhelia appeared. I had no time to waste and started to look for a suitable place to take photographs with the spotlight. The crystals did not spread very far, so I had to settle for a concise and light-polluted parking lot. There the reflected Lowitz arcs revealed themselves.
The lamp as seen with the camera eye is 1.5° below the horizon. In the upper part of the image above the three reflected Lowitz components are illustrated. The red (raypath 3-2-5) and yellow (4-2-6) components can be seen in the photos, but the third (cyan, 5-2-7) is missing. We have ideas, but no definite explanation for the relative intensities of the three components. Modifying the crystal into more tabular shape helps a bit to wipe out the cyan component from the simulations, but at the same time can cause undesired effects for the remaining two components. A crystal sample, as usual, would have been nice to have but the whole situation ended in 15 minutes during which time I did not manage to take samples.
If you click the image above a larger version appears. In that there is a simulation showing the 46° contact arcs. These arcs are faint in the photos, but especially in the upper left corner photo the familiar shape of the 46° contact arcs can be seen. In the lower left corner photo their shape is more distorted, but the "46° halo" still suggests the presence of the 46° contact arcs.
After several trials it became apparent that distorting the shape of the 46° contact arcs in the simulations is pretty difficult. This was confirmed in discussions with Marko Riikonen, who has a lot of research experience with Lowitz- and 46° contact arcs. So, once again a crystal sample could have shed some light on the issue.
Typically, if there are randomly oriented crystals between the light and observer (in diamond dust) the 22° and 46° halos are complete and their intensities are homogeneous. In this display, howevever, the incompleteness of the halo in vicinity of 46° rules out the 46° halo. Instead, with the reflected Lowitz arcs in the display, one easily concludes that the faint halos 46° from the light source are indeed 46° contact arcs.
More images from the night of 1st - 2nd January are here.