A pure breed uppervex Hastings

In snow gun diamond dust displays Parry orientation is often strongly emphasized in relation to column orientation. There may be no signs of column orientation at all, except for perhaps a slight tanget arc brightening on top of 22° halo.

Such displays have made observers to ask themselves whether the uppervex Hastings arc  the Hastings arc component that is touching the uppervex Parry arc  could be sometimes be obseved in addition to the usual Wegener, or even without it. So far displays where light source elevation allows separation of uppervex Hastings and Wegener have not resulted in any candidates.

However, in spotlight displays where lamp is at or below the horizon, we have managed to photograph during the last and this winter a several of cases that are suggestive of an uppervex Hastings. And not even suggestive, but outright assertive.

The uppervex Hastings intensifies with lowering light source elevation, only complication is that it then starts to overlap with Wegener and identification must be done on the basis of other halos in the display. If Parry orientation halos are prominent and column orientation halos such as 46° lateral arcs, diffuse arcs and subhelic and Tricker arc are very weak or absent, then we might say that the “Hastgener” in the display is indeed Hastings arc.

As an example of a display that leaves little doubt of its Hastings nature, here is shown a one that Marko Mikkilä observed this year, on the 2nd January in Sievi. The lamp was resting on the ground on a rather level field and is according to Mikkilä about 3 degrees below the camera.

We see no evidence of column orientation no 46° lateral arcs, no Tricker, no subhelic arc. Instead, in the image there is an overwhelming helic arc and Tape arcs, which both are solely Parry orientation born. Thus the faint “Hastgener” must be an uppervex Hastings arc, rather than Wegener.

47437_303cabc3bd2f77532c75240f35c029a6Above is a attempt at simulating with HaloPoint. It supports the Parry scenario untarnished by columns, even though the details may not be quite right. Perhaps the most striking mismatch is with the subanthelic arc, which is bright in simulation but seems to be missing from the photo. However, there is a darker area which looks as if shaped by subanthelic arc. This seems to be a real effect, we have seen similar looking dark voids inside anthelic arcs before and in an upcoming post I will be showing a good example of such darkness associated with this halo. Perhaps the more intense parts of subanthelic arc are outside left outside of Mikkilä’s photo, although in that case it should have been possible to make a matching simulation. I actually did find an option which was better in that respect, but other parts got too wrong to take it seriously.

An earlier display in the same location by Mikkilä seems to also contain a pure uppervex Hastings.


Parameters for the simulation

2 thoughts on “A pure breed uppervex Hastings

  1. I have seen this brightening (which is very common in spotlight displays) taking V-shape when the display simply gets brighter. But this is not an answer to the simulating problem. If I give some Lowitz tilts for the Parry crystals in simulation, then there appears a similar looking brightening. Helic arc and subanthelic arc are well defined with this solution, but Hastings is not. It spreads apart, nothing like what is seen the photos, so that pretty much demolishes the Lowitz solution.

    Another way to make brightening and not V is using very thin plates (like h/d 0.02). These give brightening also in column orientation.

    As for the lower Hastings arcs, getting positive lamp elevations is always a problem. And even if that is possible, then there may be the problem with the lowervex component of distinquishing it as it shows up against the ground.

    Of course one could argue that uppervex Hastings becomes lowervex Hastings once you put the lamp below horizon. After all, in my way of treating the spotlight displays subparhelia & co. switch above the horizon once the lamp goes below and then from the same logic would follow that uppervex Hastings or Parry should become lowervex. But unlike plates, Parry crystals don’t necessarily have horizontal symmetry. They tend to have (semi)triangular habit at least in snow gun displays, so you can’t really make the switch like you can with plates. This principle also avoids the appearance of a lower Moilanen arc once you put the lamp below horizon.

  2. Nice work once again, leaving no doubt about the uppervex Hastings component in this case. Do you have some idea on why Parry arc appears so poorly-defined in the photo, as compared with the sharp V-shape in the simulation?

    As for the Hastings arc, are we aware of observations on lower components as yet? I assume you’d need to place the lamp at higher elevation to get those.

Leave a Reply