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This comment mentions the amazing double-spiral event seen in the night sky from Norway that Wikipedia calls the 2009 Norwegian spiral anomaly. The Gizmodo article Giant Mysterious Spiral Takes Over the Skies of Norway includes the image shown below, and the Gizmodo article This Is How the Mysterious Giant Spiral Happened includes a link to a YouTube video explaining some basic physics behind the mechanism (shown below). Loosely paraphrasing, one is mostly axial, the other more tangential.

So far I have not figured out or find an explanation for the reason for the strong, uniform blue coloration of the (presumably) reflected sunlight from one of the two spirals, while the other appears to be white.

Question: Why is one of these two concurrent fuel-dump spirals blue?

Looking for authoritative, sourced answer if possible. I know that bulk liquid LOX has a slight blue color, but I don't think that LOX dumps are generally blue.

enter image description here

above: From here

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    $\begingroup$ This is from a submarine-launched missile, so it's highly unlikely the liquid-fueled 3rd stage uses cryo propellants. Probably storable ones like hydrazine and nitrogen tetroxide. en.wikipedia.org/wiki/RSM-56_Bulava $\endgroup$ – Organic Marble Jan 12 '18 at 12:46
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    $\begingroup$ @OrganicMarble Thanks for sorting that out. I just wanted to avoid hypothesizing answers or comments, e.g. "Maybe it's LOX, it's kinda blue" and keep the focus on the phenomenon. $\endgroup$ – uhoh Jan 12 '18 at 12:52
  • $\begingroup$ I remember this event. I notice that the axial trail looks much less blue in some photos (nsd.se/nyheter/markligt-ljussken-over-kiruna-5054543.aspx), and in other photos the tangential trail looks equally blue (csmonitor.com/World/Global-News/2009/1210/…). $\endgroup$ – Tom May 10 '18 at 9:26
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    $\begingroup$ @Tom and yet reading your second link, it mentions that only one is blue; "As to the tighter, bluish covered spiral..." $\endgroup$ – uhoh May 10 '18 at 11:06
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    $\begingroup$ Looks like a combination of neutral and ionized gas to me. This is just before sunrise, so the altitude where it all is visible is in sunlight. That lights up the neutrals from scattered light, and the solar UV flux ionizes some of it.The neutral stuff spreads outward like any other mass would, but the ions are confined to spiral along magnetic field lines. When you get the occasional recombination (de-ionization) the cascade down the energy levels emits a blue photon, maybe a few green and others as well. $\endgroup$ – Tom Spilker May 17 '18 at 0:48
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I think this shows two types of materials emanating from the errant Russian rocket: neutral gases, and ionized gases. The neutral gases (and any entrained condensed particles, liquid or solid) make up the whitish spiral, and the ions produce the blue-ish "trail". (Note: nothing is coming out of the rocket ionized; that happens a bit later) This can be compared to the two tails often seen from a comet, the "dust tail" and the "ion tail". That comparison is what led me to the conclusion that with the spiral we're seeing a neutrals-and-ions phenomenon.

At LEO altitudes above the realm of spy satellites there's not much atmosphere to impede the outrush of gases from a venting, spinning rocket. As parcels of gas and entrained particulates are vented they continue moving away from the source at a roughly constant speed, generating the symmetric spiral. In addition to their radial velocities the gas molecules also have random thermal velocities, so the initially well-defined arms of the spiral slowly merge and lose contrast as they get farther from the source. Also as they get farther from the source the gases are expanding, so their density decreases, and so the tenuous atmosphere they encounter can indeed slow their outrush a bit.

The light seen from the neutral cloud is sunlight scattered from the entrained particulates, and maybe a tiny bit from the gas itself. This occurred ~07:45 in the morning, when the people on the ground were still in darkness, but at the altitude of the rocket there was full sunlight.

That sunlight is important for the blue trail. UV photons in the unfiltered (by the ozone layer) sunlight can quickly ionize gases. Once ionized in Earth's magnetic field, the VxB forces confine them to spiral tightly along magnetic field lines, so they're not free to expand unimpeded. Any component of velocity downward, along magnetic field lines, would be maintained, but any component perpendicular to those lines would result in circular motion with an axis along the field line direction. The combination of the circular motion perpendicular to the lines, plus constant-velocity motion along the lines, produces a helix. This is pretty much what we see in the blue trail.

As those ions are propagating downward they occasionally grab a free electron and neutralize, only to be quickly re-ionized by the solar UV flux. During this neutralization process the electron can cascade down the parent atom's (or molecule's) energy levels, emitting photons as it goes. For an electron cascading down from a UV energy level to much lower energy levels, blue is a common photon wavelength to emit; green would also be fairly common, and you might get some farther down the spectrum. But most would be blue or green, hence the color of this trail: you're seeing photons emitted by the temporary neutralization of ions streaming down magnetic field lines.

None of the news sources, or the images they showed or linked to, said anything about which direction the observers were looking. But I'd be willing to bet that the blue trail was headed off to the northwest of the source. Earth's magnetic pole is in northern Canada. Magnetic field lines will be headed toward that pole, plunging downward somewhat, due to the largely dipole nature of the field. If you get out your globe (you have one, right?) and draw a line from northern Norway to the location of the magnetic pole, it departs Norway headed roughly northwest. Magnetic field lines converge toward the pole, so in addition to some foreshortening of the ion trail due to its recession into the distance, it would also contract a bit, making it appear to get smaller as it recedes from the source.

The Russians aren't about to give us any details about that launch, the rocket, or anything else associated, but I think I can draw a conclusion or two based on the blue spiral's appearance. The mostly-blue appearance is an indication that there is something other than just nitrogen and oxygen in this trail. Otherwise, you'd get colors like the natural aurora, and though one source compared the color to that of the aurora, the images and other eyewitness accounts indicate it was more blue. Things with lots of carbon in them fluoresce decidedly blue, things with C-C bonds, C-H bonds, etc., like plastics and hydrocarbons (shine a UV source on some motor oil sometime and see what you get!). If the rocket's propellants were just hydrazine (N2H4) and NTO (N2O4) there wouldn't be any carbon. But if the hydrazine were one of the variants like MMH (monomethyl hydrazine, hydrazine with a CH3 replacing one of the hydrogens) or UDMH (unsymmetrical dimethyl hydrazine, hydrazine with two methyls), you'd get plenty of carbon, and that would produce a blue trail. So I suspect they were using either MMH or UDMH in that 3rd stage, along with NTO.

Thanks to @uhoh for steering me to good news reports!

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  • $\begingroup$ Very, very, very nice! I'll be able to give this a read later today, thanks! $\endgroup$ – uhoh May 18 '18 at 4:09

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