Thursday, 21 September 2017

Halos on the night of 14/15 December 2016 in Rovaniemi, part II

Here is a collection of images showing an arc that looks like a downwards curving Hastings or Wegener. It's nothing new: the arc was first observed in Rovaniemi on 23 November 2015 and again nine days later in the evening of 2 December. I don't know how it is made, but it seems to occur when the display is suboptimal as compared to its best development. A halo that thrives on (relatively) crappy crystals?

In the display on the evening of 14 December 2016 this anomalous Hastgener was captured on several stacks, one of which is shown above and which also has normal Hastgener. Below are two more photos of which at least the left side one shows the effect. It was true to form, as these stages did not represent the peak of the display.

There was an additional oddity: the Hastgener / anomalous Hastgener has a kind of parhelia near the tangent arc. The feature occurred intermittently and was well visible to the eye. Below are two images showing this "parhelia", the one on the left is a single frame. The brightening seems to be also present in the main image above.

Saturday, 16 September 2017

Finnish Display 2nd September 2017

Kimmo Laitinen captured a nice display with Wegener arcs and a really strong lower tangent arc on the 2nd September 2017. I'll let Kimmo describe his observation in greater detail in his own words. 

He says, "we drove from Siilinjarvi to our summer house in Leppavirta via Kuopio. In Siilinjarvi the weather was sunny but before Kuopio a thin uniform layer of high clouds covered the sun and we spotted the first halos as we passed the city centre of Kuopio about 11.45.... At this point the 22 deg lower tangent arc was not so impressive and only a short section of the parhelic circle could be seen. Then we went on southward to the Puutossalmi cable ferry.... In Puutossalmi the 22 deg lower tangent arc was the most impressive and reflected nicely from the lake. After the ferry the 22 deg lower tangent arc disappeared but the parhelic circle became all the time longer and a very faint Wegener arc was seen by the naked eye. After driving a few km more southward the low clouds disappeared from the northern sky and the parhelic circle became complete about 12.30 in the village of Lansi-Saamainen in Leppävirta. The 120 deg parhelia were clearly visible and the intersection of the Wegener arcs with the parhelic circle was revealed from the images. After this the display got dimmer but still lasted for a few hours.

All images used by permission. © Kimmo Laitinen. 

Monday, 11 September 2017

The taxonomy of halos - towards a new system of halo classification and nomenclature

The comments in a recent post brought very sharply into focus the problem of halo classification and nomenclature; what do we call a particular halo form, why do we call it that, who gets to decide on that name and where does it fit into the overall halo classification.

The study of halos is, generally speaking, a relatively new science and discipline. Serious research into halos has accelerated during the latter half of the twentieth century up to the present and by and large it has been overwhelmingly led by committed amateurs with only the occasional dalliance of “professionals”. One result of this has been a certain lack of clarity in the classification and naming of halos. We can all probably think of at least one halo that has two or more names or halos that have had a change of name over the years as the understanding of its nature has deepened.

In other branches of science there are various organisations and protocols which govern the naming and ordering of things. However, in the halo community we are basically on our own. No-one is going to step in and wave a magic wand and it is up to us as a community to come to some sort of consensus and develop a meaningful and workable taxonomy.

In recent years we have sometimes witnessed very heated debates amongst those who are most active in serious halo research. This has led to a situation where because of a certain amount of stubbornness and entrenched views, individuals and the wider community are no longer addressing and engaging with the problem. The aim of this short post is therefore to try to open up the conversation once more and solicit new ideas and suggestions on how we should proceed. The need for this discussion is becoming increasingly urgent. There are so many new halos being discovered all the time that we need some way of describing them and being able to relate them to some overarching system of classification.

What I would like you to do is to think about taxonomy in general, and halo classification and nomenclature in particular. It may be that no one system is “correct” and that two or more may adequately describe and account for what we see. I think that whatever system is finally decided upon it should be fairly open and flexible, so that it is able to handle the new and the unexpected.
I appreciate that this may seem to be a very dry subject when many people these days are content to see endless photos of increasingly rare and spectacular displays. However, I think it is of the utmost importance to our discipline. We need to get it right now so as to prevent serious problems in the future.

Over to you, the comments are open!

Tuesday, 29 August 2017

Halos on the night of 14/15 December 2016 in Rovaniemi

Last mid-December, Rovaniemi was shrouded in fog for five days in a row. With temperatures hovering in -5 to -10 C range, the conditions were as good as one can hope for. Just add icy nuclei from snow guns in the air and fog is guaranteed to freeze into a violent display.

The five day diamond dust feast did not materialize. The guns were shut down after a couple of hours after the dark of the second night, and then it was just fog. But I was there on those two first nights to take photos, a selection of which I am going to show in this and coming posts.

I start by two photos from the first night of 14/15 December. Both have, inside the Tricker arc, a faint colored arc, best visible in the blue-minus-red versions. Upon first becoming aware of this feature, I thought, with some excitement, that it might be an exotic halo. This state of mind did not last long, however, because soon the arc turned up in a simulation.

The halo is born from raypaths 3162 and 3152 in column oriented crystals. By its appearance it is a vertically mirrored copy of the more commonly photographed 361/351 arc, which is also seen in the photo (and which has been treated in an article by Walt Tape). At the upper right of the above image is simulation where these arcs are marked, respectively, by left and right pointing arrows. Two column oriented populations were used. The 3152/3162 arc is made by the population that rotates 20 degrees, not by the fully rotating population (see parameter table below). At the lower right of the above image is another simulation, where the the 3152/3162 arc from the limited rotation population is shown without other halos.

Tuesday, 22 August 2017

Blue subsun finally about right

closeup of the photo below

The year is gearing towards winter, the first glitter in northern Finland expected in less than two months. While waiting for that, I think I should continue clearing the backlog of last winter's results from Rovaniemi before new stuff starts piling up on top and risking buried-in-drawer fate. There is still plenty to show – even what might be called as major displays.

To not inflate it right away, lets start with a more moderate, but nevertheless interesting display. On the night of 16/17 January 2017 a display with full set of kaleidoscopic arcs appeared, and towards the end of the action, when it turned plate dominated, I tried to get the blue subsun photographed.

Earlier past winter and the one before, on a couple of attempts at blue subsun that I had myself or with friends, the aim was to get the lamp elevation right so that the blue would be exactly at the center of the subsun (otherwise you might feel a tinge of uncomfort calling it a blue subsun). It is a precision job: the optimum is at right about 58.5 degree light source elevation, with the tolerance of around half a degree, after which you may not be able to sell it to a demanding customer.

We did't get it quite right then, or the display was otherwise uninspiring, but this time there is a clear blue color passing pretty much through a middle of a well defined subsun. Connoisseurs might still see manoeuvre for one or two tenth of degree improvement, but I am good enough with this to not pay effort to go after it no more. However, there is some other stuff that warrants visiting the extremely low lamp haloversum (like sub-cha and its extension), so blue subsuns are certainly going to come along with the ride.

Wednesday, 2 August 2017

Halos from deep space

Now don't get too excited, it's not the news of first exoplanet halo observation. Instead, a space craft has observed halos on Earth - from deep space, 1,5 million kilometers from Earth.
NOAA’s Deep Space Climate Observatory, (DSCOVR) is located at first Lagrangian point. It's 1,5 million km from Earth, between Earth and the Sun. It takes hourly (or every two hours, depending on time of year) a picture of sunlit Earth with its Earth Polychromatic Imaging Camera (EPIC). Some of these pictures show a bright glint, which was initially thought to be solar reflection from still ocean surface but soon discovered appearing on land areas too. Size of the glint was too big to be caused by lakes. An idea of horizontally oriented ice crystals in upper atmosphere as a reflecting surface was introduced and challenged with series of tests. It proved to be a winning theory. One of the tests was to study whether the angle between the Sun and Earth is the same as the angle between the spacecraft and Earth on the location of the glint. And it was a match for every glint recorded. So, the reflection from ice crystals can be regarded as a halo, namely a subsun. With a very maximum Sun elevation.

One might think that 1,5 million kilometers is a record breaking distance to observe halo's, but while investigating the glints NASA scientists found out that similar glints were noticed in pictures taken by Galileo spacecraft back in 1993, when it was on its way to study Jupiter and its moons. Galileo took the pictures at 2,08 million km distance from Earth. Originally glints recorded by Galileo were reported to be seen only over oceans, not over land, but that was a mistake. When inspected again, Galileo footage clearly shows glints also over land areas.

Not only being a curiosity for halo enthusiasts, detecting similar glints could be used to study exoplanets. If exo can produce halos it could be a sign of water in its atmosphere and that can give a hint about planets habitability. Hubble Space telescope successor, James Webb Space Telescope (JWST) is planned to be launched in October 2018. It will be stationed at second Lagrangian point, so observing subsun on Earth is impossible for JWST, but maybe it is capable of spotting extra Terrestrial halos? Or maybe we need to wait for the next generation telescope for that. But perhaps, some day this articles header can be re-used, having a slightly more exciting meaning.

The original article from American Geophysical Union (AGU): Ice particles in Earth’s atmosphere create bright flashes seen from space 

Friday, 21 July 2017

Halo simulation using a single ice crystal population

Halo simulation aims at finding those ice crystal shape and orientation parameters that best reproduce observed characteristics in a given display. Publicly available simulation software provide a great deal of flexibility by allowing users to include several separate crystal populations and by giving them free hands to set population-specific crystal shape and orientation parameters as they see fit. For example, simulating the 22nd September 2012 display (shown above) would require including at least five crystal populations. These are necessary as the observed halos associate with all five principal types of crystal orientation (random, plate, column, Parry, and Lowitz orientations), and each population accommodates only one orientation type. As each population needs its own shape and orientation parameters, the total number of input parameters is almost impractical.

I have recently entertained myself with the idea of introducing a mutual dependence between the orientation and shape parameters. This could reduce some of the complexity currently associated with the simulation. In such framework, even complicated-looking displays can be reasonably well simulated using only a handful of user-specified input parameters. Furthermore, the idea of only one crystal population in one halo-making cloud is conceptually appealing.

To illustrate the approach I've been thinking of, the figure above shows a possible orientation-shape dependency. Let's consider, for now, the aspect ratio as the only measure of crystal shape. The y-axis representing the angle of crystal's axis with local vertical, perfect plate and column orientations correspond to values 0° and 90°, respectively. We assume a Gaussian distribution such that 95% of crystals (at each aspect ratio) go within the shaded region. In the regime of equidimensional crystals in the middle, the orientation angle takes all values between 0° and 90°. Otherwise, it is constrained around either 0° or 90°, depending on whether the shape is more like a plate or rather like a column. The constraint is particularly strict in the extremes at far left and far right, indicating that perfect orientations are most probable to occur with the thinnest plates and longest columns.

I have produced the all-sky simulations below by running a home-made simulation program and assuming the orientation-shape dependency exactly as laid out above. Each simulation uses only one crystal population, and the six simulations differ in only two input parameters that control the distribution of crystal aspect ratio. At the top, the distribution is set narrow such that the crystals are either (a) all plates, (b) all equidimensionals, or (c) all columns. In panels (d)-(f), the distribution is made gradually wider around the equidimensional mean. Clearly, changing just two input parameters is enough to allow reproducing a variety of displays.

To simulate occurrences of Parry and Lowitz arcs, we need to introduce another dependency to constrain crystal's rotation about its axis. In my trials, I have assumed that the rotation averages to that required in the Parry orientation, but the spread around the mean rotation depends on crystal base shape (see the figure below for illustration). Only crystals with a considerable tendency towards either triangular or tabular habit are assigned with spread small enough to allow Parry arcs to form. For conventional hexagons and other possible base shapes, we set the spread large to make the distribution of rotation angles essentially uniform.

Now, using the two dependences as laid out above and running my home-made program, panels (a)-(c) below illustrate the effect of varying the distribution of column crystal base shapes alone. Four input parameters are used to control these variations. In panel (a), all crystals are nearly regular hexagons in their bases. Halos attributed to the column orientation are well reproduced, and no signs of Parry arcs are seen. In panel (c), crystals are constrained to the tabular base shape. This scenario produces only halos associated with the Parry orientation. Panel (b) is for the case where the base shape varies considerably, such that halos associated with both column and Parry orientations are represented.

To get not just Parry but also Lowitz arcs, one needs to take benefit from the two types of dependence at once. This is attempted in panels (d)-(f) for three different solar elevations. Although the all-sky projection makes direct comparison slightly complicated, I'd say the similarity of panels (d) and (e) with the appearance of the 22nd September display is remarkable.

While assuming the same dependencies to hold for the behaviour of orientation and rotation angles as a function of crystal shape, all variations in the simulations shown here were produced by changing just seven input parameters. One goes for the solar elevation, and the remaining six were used to characterize crystal shapes.