|Lunar Elliptical Halo ~ Guillaume Poulin took this perhaps the finest image of an elliptical halo at Mont-Megantic National Park, Québec, Canada.
Guillaume had been taking astro photographs in a temperature of -15 Celsius. "On our way back home we noticed that the water vapor contained in the air began to form tiny ice crystals that were falling like snow flakes and at the same time a halo was forming around the moon. A few minutes later, another halo, twice the size of the first one was also visible." The 38° high moon was just past first quarter. It had to be overexposed to capture the halos but its shape remains apparent.
All images ©Guillaume Poulin, shown with permission.
|Elliptical halos are rare, they are ephemeral and they are enigmas. They are likely related to the even more rare Bottlinger’s Rings superbly imaged in the previous OPOD.
Ellipticals are small halos just a few degrees across. They can have two or three oval rings. They are mostly seen in altocumulus cloud although here crystals in ice fog might be the source. The rings are of frustratingly varied angular size and likely depend on the solar or lunar altitude. The paucity of good observations and crystal samples has considerably hindered analysis.
Guillaume took several outstanding images. They show three rings in some detail. The innermost ring is perhaps bluish and the inside of the second ring has a definite reddish hue indicating that refraction has played at least some role in its formation. Other cues for a better understanding are the variation in brightness around the rings and the way that they are offset from the moon.
We are fortunate that some of his images show faint but identifiable stars allowing the ring dimensions to be measured very precisely. On a line through the moon the second ring is 5.6° across.
Do we understand how ellipticals are formed? Their small dimensions could imply refraction between crystal faces only slightly inclined to one another. That is in considerable contrast to the 60° responsible for the common 22° halo.
One possibility is that very flat pyramidal crystals produce the rings. The crystals are assumed to drift nearly horizontal in the same manner as ordinary hexagonal plates. The same crystals were invoked to model Bottlinger’s rings although other routes might also form the latter.
At right we have a ray tracing simulation made by HaloSim in an attempt to match the ring sizes and intensity variations. The simulation used crystals with upper and lower pyramidal faces inclined 3.5° from horizontal. This corresponds to face Miller Indices like 1,0,-1,35 which are crystallographically preposterous. Faces more usually follow planes of atoms or ions within the crystal lattices and yield Miller Indices with low integer values. One let out is to invoke snow-like crystals with ice wedges arranged dendritically..
The simple crystal yields a halo of the right size. There are three different ray paths through its faces which generate three rings not dissimilar to those observed. (The numbers are those of the faces traversed.)
The similarity is encouraging and the fit is OK - But not as good as that obtained when modeling other halo displays. It is tempting to think that a few more iterations - separately altering the upper and lower face angles, inserting flat caps at top and bottom and changing the wobble from horizontal will improve it. It doesn't - or not very much anyway. That’s the challenge. We have a theory demanding rather unusual crystals and predictions that are not quite right.
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