(meteorobs) Green Fireballs....Answer

[Larry]

Both Chris & Ed are correct.Reading (ALL) stressed..(ALL)...of this from 
AMS.... gives a properbalance to the discussion of fireball color & fireball 
composition. Frequently Asked Questions (FAQ)
About Fireballs and Meteorite Dropping Fireballs -
Question List:

5. Can fireballs appear in different colors?

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 can
play an important part in the observed colors of a
fireball, with certain elements displaying signature
colors when vaporized. For example, sodium produces a
bright yellow color, nickel shows as green, and
magnesium as blue-white. 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 majority of
light from a fireball radiates from a compact cloud of
material immediately surrounding the meteoroid or
closely trailing it. 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 (Fe), magnesium
(Mg), and sodium (Na). 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.


----- Original Message ----- 
From: "Ed Majden" <epmajden at shaw.ca>
To: "Chris Peterson" <clp at alumni.caltech.edu>
Cc: "Global Meteor Observing Forum" <meteorobs at meteorobs.org>
Sent: 2009/11/19 15:54
Subject: (meteorobs) Green Fireballs


> Hi Chris:
>
> Your quote on meteorobs:
>
>
>
> "The green seen in fireballs comes from atmospheric oxygen.
> Composition can
> be seen spectroscopically in emission lines, but the visual color is
> pretty
> much useless for assessing composition. All the different emission lines
> push the apparent color towards visual white, and no single emission
> line
> can be strong enough to strongly influence that."
>
> Chris
>
> I don't think you can make a blank statement that the green in
> fireballs comes from atmospheric oxygen.  Oxygen is found in
> fireballs with velocities greater than 40 km/sec.  This is especially
> true of the persistent radiation of the so called forbidden line of
> [ 0 ] at 557.7 nm first identified by Ian Halliday in 1958.  This
> line is not present in slower fireballs like the types originating
> from asteroids and likely to drop a meteorite.  I have discussed this
> with Jiri Borovicka and he says that slow fireballs do not have
> bright emissions in the near IR, (in contrast to fast fireballs like
> the Perseids, which have O I, NI, Ca II emissions.  I would think
> that Mg I and Mg II would be contributing to the green rather than
> Oxygen as you suggest. I agree that you cannot determine the fireball
> type from its apparent colour.  A lot still has to be learned from
> meteor spectra.  We need to record the spectrum of a fireball that
> has dropped a meteorite which has been recovered.  It would be
> interesting to compare the elements found in the actual meteorite
> with the spectrum of the fireball in the atmosphere.  This would
> clear up may questions that have not been answered yet.
>
> Ed Majden
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