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(meteorobs) Fw: trails, trains, etc
The following is a list of definitions regarding trails, trains, wakes,
etc provided to me by Dr. Jiri Borovicka and I am posting them with his
permission. He hopes in the near future to write a paper on this topic.
The only accepted definitions are the ones that are approved by the IAU.
This is a work in progress.
An interesting related paper has just been published in MAPS,
Meteoritics & Planetary Science 36, 1217-1224 (2001), "Heterogeneous
chemical processes as a source of persistent meteor trails", by Edmond
MURAD, Space Vehicles Directorate, Air Force Research Laboratory, Hanscom
AFB, Mass.
Ed Majden
1. METEOR WAKE
A luminosity just behind the meteor. It moves with the meteor
and forms a kind of tail. The wake is often present in bright
fireballs, which are then sometimes described as a comet-like objects
by the witnesses. In this sense, the meteor can be described as
consisting of meteor head and meteor wake. At a given position,
the wake duration is only a fraction of second.
The spectrum of meteor wake is different from the spectrum of meteor
head. The wake spectrum consists chiefly from low excitation lines.
Typical lines belong to Na I, Fe I, Mg I, Ca I, i.e. to the atoms
released from the meteoroid.
After a meteoroid fragmentation, small fragments decelerate more
rapidly and stay behind the main body. They may look like a wake of the
main body but this is not a true head+wake, rather a multiple meteor
with similar spectra in all parts.
2. SHORT-DURATION METEOR TRAINS (OR TRAILS)
Luminous trains left behind the meteor for up to about 3 seconds. They are
often observed visually and by video techniques in fast
meteors like Perseids. They are present also in faint meteors, of
magnitude +4 or so. In fact the ratio of the train/meteor brightness is
larger in faint meteors than in bright meteors. The train is not
connected with the meteor. In fact, it forms at a given position
with some delay after the meteor passage. The the train is also
considerably shifted to higher altitudes than the meteor which produced
it.
The short-duration trains are formed by only one spectral line,
the green auroral lines of neutral atomic oxygen at 5577 A. This
is a forbidden line. The luminosity is produced (very probably) by
the atmospheric oxygen.
3. PERSISTENT LONG-DURATION METEOR TRAINS
Luminous trains left behind the meteor for from 3 seconds up to
more than an hour in rare cases. The trains are self-luminous, i.e.
the luminosity is not produced by reflected sunlight or other external
source. Persistent trains are produced much more easily by fast
meteors. Leonids are very favorable. The typical altitude for train
formation is 90 km. Persistent trains are affected by high altitude
winds and change the shape. Some trains show a hollow structure.
Persistent trains are not well understood objects. Several spectra
have been taken in the recent years, which, surprisingly, looks
differently from case to case. The spectra show both continuous
or quasi-continous radiation and atomic lines. The most important
and most persistent line, common for all spectra, is the sodium line
at 5893 A. This suggests that the long-living luminosity is due
to similar mechanism which produces the sodium airglow. The recent
Leonid train spectrum is similar during the first few seconds to a
meteor wake spectrum. The train formed at the position of meteor flare.
The meteor ablation products therefore played an
important role. Nevertheless forbidden lines of neutral and ionized
oxygen were also reported in some train spectra.
4. DAY TIME DUST TRAINS (OR TRAILS) OF METEORS
Trains observed during the day time or twilight after a passage
of a very bright fireball. They may be visible for more than an hour
and change the shape. These trains are visible in the sunlight reflected
(or absorbed) on the dust debris of the meteoroid. During
the night, they may be visible for a short time due the thermal
radiation of the dust or other mechanisms. Dust trains may be formed
at any altitude, depending on the dust deposition by the fireball.
Of course, more dust is produced by massive objects and they often
explode at an altitude around 30 km.
5. METEOR IONIZATION TRAILS (OR TRAINS)
Trails detected using a reflection of radio waves on a column of free
electrons produced by a passage of a meteor. The relation to the
trains detected by optical methods is not clear.
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