Tuesday, 29 August 2017

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

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