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(meteorobs) Green Meteors



Bob Lunsford wrote:
>I believe that I have heard (and please correct me if I'm wrong) that
>the green color is produced by the doubly ionized oxygen produced when
>larger particles encounter the atmosphere.

The color of meteors and fireballs is a question that has caused a great
deal of debate over the years.  Spectrographic analysis lagged due to
relative paucity of data.  In recent years, more good spectra have been
generated through the efforts of many, including the European fireball
network and individuals such as Ed Majden.

I don't think I've ever seen a green meteor of less than -2 magnitude.
Vivid colors are more often reported by fireball observers because the
brightness is great enough to fall well within the range of human color
vision. These must be treated with some caution, however, because of
well-known effects associated with the persistence of vision. Reported
colors range across the spectrum, from red to bright blue, and (rarely)
violet. The dominant composition of a meteoroid plays an important part in
the observed colors of a fireball, with certain elements displaying
signature colors when vaporized.  The velocity of the meteor also plays an
important role, since a higher level of kinetic energy will intensify
certain colors compared to others. Among fainter objects, it seems to be
reported that slow meteors are red or orange, while fast meteors frequently
have a blue color, but for fireballs the situation seems more complex than
that, but perhaps only because of the curiousities of color vision as
mentioned above.

The difficulties of specifying meteor color arise because meteor light is
dominated by an emission, rather than a continuous, spectrum. The light of a
fireball is composed of the series of emission lines generated by chemical
elements radiating in the gaseous cloud of vaporized material surrounding
the meteor head.  95% of this cloud consists of atoms from the surrounding
atmosphere; the balance consists of atoms of vaporized elements from the
meteoroid itself. These excited particles will emit light at wavelengths
characteristic for each element. The most common emission lines observed in
the visual portion of the spectrum from ablated material in the fireball
head originate from iron (FeI), magnesium (MgI), and sodium (NaI). Silicon
(Si) may be under-represented due to incomplete dissociation of SiO2
molecules. Manganese (Mn), Chromium (Cr), Copper (Cu) have been observed in
fireball spectra, along with rarer elements. The refractory elements
Aluminum (Al), Calcium (Ca), and Titanium (Ti) tend to be incompletely
vaporized and thus also under-represented in fireball spectra.

In fast meteors the H and K lines of CaII at 3394A and 3968A are often the
brightest lines.  The MgI triplet in the green is often very bright, and FeI
contributes a complex series of lines with bright features in the blue and
yellow.  Perhaps the bright green MgI lines, and the combination of blue CaI
and FeI lines with the yellow FeI lines yield reports of green
fireballs/meteors.

The atomic oxygen OI 5577A emission (the "forbidden" line) appears primarily
in the wake of high-velocity meteors.  The Leonids, at 71 km/s certainly
qualify as high velocity, and I'm not surprised to see the number reported
as green!  Borovicka et al (1996) unexpectedly identified emission lines of
OIII at 5007A in spectra of two long-enduring Perseid trains.

References:

Baggaley, W. J. (1977). "The velocity dependence of meteoric green line
emission",
    Astronomical Institutes of Czechoslovakia, Bulletin, vol. 28, no. 5,
1977, p. 277-280
Borovicka, J. (1993). "A fireball spectrum analysis", Astron. Astrophys.
279, 627-645.
Borovicka, J. (1994a). "Line identifications in a fireball spectrum",
Astron. Astrophys.
    Suppl. Ser. 103, 83-96
Borovicka, J. (1994b) "Two components in meteor spectra",
    Planetary and Space Science, vol. 42, no. 2, p. 145-150
Borovicka, J.; Zimnikoval, P.; Skvarka, J.; Rajchl, J.; Spurny, P. (1996)
"The identification
    of nebular lines in the spectra of meteor trains", Astronomy and
Astrophysics, v.306, p.995
Ridley, H.B. (1994), "The Spectrum of a Sporadic Fireball", J. Br. Astro.
Assoc. 104, 1, 1994



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