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[Phys-L] green meteor / forbidden oxygen transition



Hi Folks --

A green meteor went overhead last night, pretty much from
horizon to horizon. I was a couple seconds too late to see
it, but people have been asking me about it.

AFAICT the green color comes from a "forbidden" transition
in atomic oxygen at 557.735 nm.
http://articles.adsabs.harvard.edu/cgi-bin/nph-iarticle_query?bibcode=1981A%26A...103..154F&db_key=AST&page_ind=1

This is the same transition that gives a green color to some
auroras. It goes from a singlet S state to a singlet D state
involving the 2p electrons. That violates both the ΔL and
ΔJ selection rules that apply to ordinary electric-dipole
optical transitions.

The forbidden transition has a lifetime on the order of 1 second.
(This stands in contrast to a typical optical allowed transition
which has a lifetime on the order of nanoseconds.) The long
lifetime means you perceive the meteor as a streak, not just a
moving dot. The streak cannot be explained by persistence of
vision, nor by slowly burning particles (as you may have seen
in artificial fireworks).

The line is correspondingly weak. The fact that you can see it
at all means you are looking at a *lot* of oxygen atoms. There
is of course oxygen in the air, but keep in mind that stony-iron
and stony meteors contain silicate minerals, so they provide
their own oxygen atoms.

Note: You might have guessed based on high-school chemistry
flame tests that the green color came from copper ions, but
that is almost never the right explanation for green meteors.
Yeah, there "could" be some copper involved, but that doesn't
explain the lifetime. And it's the wrong shade of green anyway.
https://www.nationalgeographic.com/news/2010/4/100415-meteor-in-wisconsin-fireball-sky/

I hate to admit it, but as of yesterday I probably would have
guessed wrong on almost every facet of this story. I think
what I've said here is right, but I wouldn't be shocked if
further corrections were necessary.



there is always a well-known solution to every human problem — neat,
plausible, and wrong
-- H.L. Mencken