Some diamond dust odd radii in Rovaniemi

These photos were taken on the night of 9/10 February. In the image above we see odd radius stuff in the beam: circular halos and some bulges on them indicative of poorly plate oriented pyramids. The arc straight above the lamp on the broad 22° halo should be Parry / upper tangent arc rather than upper 20° plate arc. Visually I could not see the odd radii stuff but the appearance of the glitter made me think for its possibility.

One additional reason to suspect odd radii was the lunar display, which did not seem like your basic 22° halo stuff. The shot below was taken soon after I turned off the spotlight and it indeed has some not so clear odd radii, partly because of the artefacts. I photographed lunar also before the spotlight, and then the stuff was even poorer (this photo is not shown). So the spotlight in between gave the best display, which is of course what one expects

The enhanced version of this photo really brings out the artefacts. Br fouled it so completely that I am not showing it. Up until now I had had this nuisance entering the scene only in raw sun shine, but now it is has expanded its range to moon lit nights.

        

On the brighter side the artefacts became less disturbing when the milky background started clearing as shown by the photo below. Yet even then the br gives a rather ugly result.

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.

Fine Lowitz arcs in lunar diamond dust display

Vesa Korhonen photographed this lunar display in Hämeenkyrö in the evening of 10 February at 2038 local time. The circular Lowitz arcs are splendid. The upper and lower components are expected to accompany, and seem to be there, but are rather overwhelmed by the intense 22° halo.

The "leftovers" from the 23 November 2015 display in Rovaniemi

By Jarmo Moilanen, Marko Mikkilä and Marko Riikonen

In three previous posts we have dealt with different stages of the 23 November display that had some interesting features, like an anomalous Hastings/Wegener arc and a possible 4th Tape arc. Here we show the rest of the material we got that night.

After the lunar display was gone, wind direction changed and we followed the diamond dust to a new location. As usual, there is only one place to choose from when temperatures are not very low and the new field we found ourselves in was less optimal than the earlier one, having a rather confined space and yellow streetlights nearby. But you photograph where you can.

The image above and two below show that it was overcast again, moon glowing weakly through the cloud. In the beam, the display was not nearly as good as it had been before, yet it was by no means poor – it is only because we had been just a bit earlier spoiled by a monstrous halo complex we didn’t feel that inspired anymore. Add to that the ever more demanding calls of the land of slumber (we had been up most of the previous night and not sleeping in the daytime either because halos wouldn’t let us), we packed it all up and headed for the caressing warmth of the indoors even though the display was still kicking on.

                                                     Lamp elevation is around 0 degrees in the three photos above.

Who knows what there would have been on the offing after we gave up, there was still plenty of night left. As were about to doze off, the stratus clouds started breaking and moon shining bright again. Any change in conditions means also change in halos, and this one may have been for the better, judging from earlier breaking of clouds which produced a major spectacle.

In the image gallery at the bottom there are also photos from the first action of the night, soon after it was dark. Crystals were collected and there were plenty of plates with internal structures.

Lunar all-sky diamond dust display on 23 November 2015 in Rovaniemi

By Jarmo Moilanen, Marko Mikkilä and Marko Riikonen

This night conditions were overcast and we were watching halos in spotlight beam. But there was a moment when clouds all turned into ice crystals, allowing moon shine bright and make a passable display. We switched off the spotlight and managed to take some photos before it got cloudy again. Camera lenses were frosted, which caused some blurred areas and bright dots in the images.