Plates 1 and 2 for the Bravais book

The internet copy of the Auguste Bravais book "Memoire sur Les Halos..." has incompletely scanned figures. So I went to the Kuopio repository library to have them copied from the original. Earlier, before this only Finnish copy of the Bravais was moved to repository library, I had already copied plates 3 and 4. Given below are now full reproductions also of plates 1 and 2, that a worker at the library kindly scanned.

There is a re-print for sale of the Bravais book, but I have heard it too has incomplete figure plates. Possibly it is made from the same scan that is available online.

Pseudo-anthelion

On the evening of 9th February I left my apartment to hunt diamond dust halos, but was also supposed make it to Lappland Chamber Orchestra concert. So with the proper clothing for such an high society occasion underneath the overall and several other layers (it was close to -30° C) I half hoped it would be total crap to allow me to slip to the concert to hear live the Shostakovitch chamber symphony op. 110.

It indeed was quite crappy, or at least I thought so, and after taking some lunar and spotlight shots I was already driving towards the city - only to turn around and come back to do one last check. That settled it: there was an all sky display developing and DSCH was no more on agenda.

Here is highlighted only one set of photos of that display. The three images show an anthelion that moves below the parhelic circle as lunar elevation rises from 30.1 to 30.9 degrees (according to USNO calculator). The images from left to right are stacks of 4, 3 and 4 successive frames with 30s exposures and the values in the upper corner indicate lunar elevation.

In my attempt to simulate the effect with Halopoint there is only one population of column oriented crystals considered with orientations of the crystals restricted to 12 degrees rotation about the c-axis (parameters are shown at end of this post - the last one of the three tables). This, together with plate shape and slightly triangular habit indeed reproduced something that looks like the pseudo-anthelion in the photos.

A look at the raypaths revealed that the effect is an intensified apex of the Tricker arc. This explains the movement as Tricker separates from parhelic circle at light source elevations higher than 30 degrees. To show it in its true form, below left is a filtered simulation that has only the Tricker rays responsible for the effect and for comparison next to it a full simulation with all rays. Further below is depicted the beautiful raypath. This type of variation of Tricker arc raypath which enters and exists through basal face is also drawn in Robert Greenler's "Rainbows, Halos and Glories" on page 85 and Tape's "Atmospheric Halos" on page 26.

The set of simulations below compares two scenarios of traditional diffuse arc anthelion with the Tricker arc pseudo-anthelion. Actually, in the middle simulation both effects seem to be present. Parameters for these simulations are given further below. Left to right order for simulations corresponds to top-bottom order for parameters.

So accepting that this pseudo-anthelion is an anomalously brightened Tricker arc top, I am however less certain about the correctness of the suggested mechanism behind. The simulation does not come out that convincing when you look at the other parts of the display. It seems to me there is going on something that I have no idea of (or then I just didn't simulate enough, like has happened in the past).  

One curious thing is the lack of subhelic arc in the display. Only if I used plates that were h/d 0.2 or thinner could I get rid of the subhelic arc. But that made the simulation in other respects even further from the reality. Such h/d values were also too small for the pseudo-anthelion which seemed to thrive in quite delicate balance around 0.3 value. For reference, see an earlier case where simulation with thin plates in restricted column orientation (or poor Parry orientation) produced a good overall match with spotlight display. I guess next winter I must shape up on crystal sampling to see if displays like this really don't have columns in the dish.

The display itself was rather faint. No hope of seeing, for example, that pseudo-anthelion even though I saw from the camera display that it was there (and thinking it is just a normal anthelion). I'll be posting on some later occasion more photos from this night, including spotlight stuff.

Thin plates in rotating Parry orientation as an explanation for thedisplay on the night of 8/9 November 2016, in Rovaniemi

In an earlier post I told simulation attempts were not successful for this display. Well, I really did not put that much effort into it. Now I have given it a fresh look and managed to get some success.

The problem was the subhelic arc and anthelic arcs that could not be get rid of. In new simulations made with HaloPoint the subhelic arc issue is pretty much resolved and the anthelic arcs also play it low key.

As usual, the simulations do not stand scrutiny of details, but that does not matter regarding main message: that it was necessary to use thin plates in rotating Parry orientation to keep the subhelic arc and anthelic arc stuff in check.

Two simulations are shown above together with the photo. They are identical except that in the other the thin Parry crystals are rotating 15 degrees and in the other 5 degrees. The subhelic arc is actually in there, but it is masked by background noise from the random population. Well, maybe its curve can be detected in the 5 degree simulation, but it is shadowy. It becomes clearer with more burn and finer dot.

The simulation with 5 degree rotation replicates also the diffuse area of light seen above the Wegener arc in the image. It is a spread out helic arc. So maybe we could regard the Wegener rather as an intermediate form between Wegener and Hastings, the “Wegstings”.

If thin plates indeed are the culprit, how can they fall with their basal faces vertical? If we maintain that singular plates can not knife through the air in such orientation, then perhaps there were copious 90 degree crosssed plates in the air, built so that one was glued to the center of the other one’s basal face to provide the right balance for the required orientation. I did not take crystal samples, with one camera it is too much a hassle, particularly as we have again and again come to learn that samples rarely give answers.

Parameters for the other simulation in HaloPoint software. Shown is the thin plate in rotating Parry orientation.

 

 

 

 Finally, a technical factor could have influenced the subhelic arc visibility in the photo. I had placed the camera slightly outside the center of the beam sideways to enhance the sides of the display and this may have had a dampening effect on the visiblity of the higher located subhelic arc. The effect is probably not significant, the camera offset was just a little, but in the future this practise must be dropped in order to get best comparability with simulations (of course, as the spikes at the anthelic region attest, my beam is not regular anyway, so to some extent this issue is there aways). Off the record, I don’t really think that this could have made subhelic arc disappear, rather the opposite, because when the camera is right in the center of the beam, you have more masking bright glitter than when the camera is offset and any threshold intensity halos should be then lost easier. But it is good to let this issue have known.