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Re: (meteorobs) Fireball mags...
I realize that I have been eating up more bandwidth than usual this week,
but I thought I might post an answer to the several meteor
light/magnitude/train questions that have been cropping up. Most of this
comes out of the FAQ's posted on the AMS web site, which include reference
lists. Part of my motivation for answering is so that if you have a better
answer (and a reference), please let me know.
From the AMS Meteors FAQ:
4. Where does a meteor's light and color come from? What is a meteor train?
The majority of light from a meteor radiates from a compact cloud of
gaseous atoms and molecules immediately surrounding the meteoroid or
closely trailing it. This cloud consists of a mixture of atoms and
molecules ablated from the meteoroid itself as well as from the surrounding
air. These excited particles will emit light at wavelengths characteristic
for each element/compound. The most common emission lines from meteors
originate from iron (Fe), oxygen (O), magnesium (Mg), sodium (Na), nitrogen
(N), and calcium (Ca). Less frequently seen are the emission lines of
hydrogen (H), Silicon (Si), Manganese (Mn), and Chromium (Cr).
While most meteors produce a wide blend of these emissions, giving the
meteor an overall white color, specifically colored meteors are often
reported by meteor observers. Usually, such colors are rather weak in
appearance; however, vivid colors are occasionally reported, especially
with fireballs. Reported colors range across the spectrum, from reds,
yellows, greens, and blues, to gold, orange, and (infrequently violet. The
velocity of the meteor also plays an important role, since a higher level
of kinetic energy will excite the atoms/molecules to a higher degree. Slow
meteors are often reported as red or orange, while fast meteors frequently
have a blue color. Due to the nearly identical composition and velocity of
meteors belonging to a particular shower, several showers are known for
their characteristically colored meteors.
Often, a brief glow will remain after the passage of the meteor. If this
glow persists for less than 0.5 seconds, it is called a wake. This residual
glow is caused by the same atoms which produced the original light from the
meteor, only at lower excitation energies.
If the glow from the meteor trail persists for a longer period, this is
called a meteor train. Trains are most often seen from fast, bright
meteors, in the altitude band from about 100 to 120 km (62 - 75 miles).
This type of train usually lasts about 1-2 seconds, and is primarily
generated by the green emissions of the neutral nitrogen atom. On very rare
occasions, a train may persist for several minutes, and will be observed to
change shape as the trail is blown by upper atmosphere winds. Such
persistent meteor trains provided scientists with their first data on winds
in this region.
Personal notes:
Most of the light-emitting cloud is made up of atmospheric gases (something
like 95 %), while the remainder is ablated material.
As the atoms are ionized and excited, they also generate a cloud of free
electrons which remain in the wake of the meteor as a long, thin,
cylindrical-paraboloid trail. It is this trail of free electrons which is
capable of reflecting radio waves. Immediately upon trail formation, the
negative electrons repel each other, causing the trail radius to increase
over time. This diffusion process causes most trails to rapidly dissipate,
keeping radio reflections relatively short in duration.
Note also that the long duration trails (those lasting several minutes) are
still not very well understood. Some aurora-like mechanism is still needed
in order to maintain the excitation of the oxygen and other atoms for this
extended period of time.
Steve's question also reminded me of smoke trails.
From the AMS Fireball FAQ:
The second type of trail is called a smoke trail, and is more often seen in
daylight fireballs than at night. Generally occurring below 80 km of
altitude, smoke trails are a non-luminous trail of particulate stripped
away during the ablation process. These appear similar to contrails left
behind by aircraft, and can have either a light or dark appearance.
Personal Notes:
The question of how to relate meteor magnitudes to meteoroid size and mass
is a very sticky one, and it seems that every paper or textbook has its own
approach to this fundamental problem. Estimates for the 0 magnitude meteor
range from 0.1 grams to 10 grams depending upon your source, with most
accepting 1 gram as about ball park.
Fireball experts use several measures to derive the initial mass of the
meteoroid, such as trajectory, light curve, spectrum, initial velocity, and
deceleration characteristics. Non-meteorite dropping fireballs probably
range from about 100 g in mass up to a few kg, with densities ranging from
0.3 g/ml for the lightest cometary meteoroids to 3.7 g/ml for the typical
stony asteroidal meteoroids. Thus, a 100 g meteoroid could have a
spherical diameter ranging from 3.7 cm up to 8.6 cm, encompasing a range of
ablation characteristics.
Take care, everyone,
Jim
James Richardson
Graceville, Florida
richardson@digitalexp.com
Operations Manager / Radiometeor Project Coordinator
American Meteor Society (AMS)
http://www.serve.com/meteors/
References: