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(meteorobs) Fwd: Meteors, Wakes and Trains
A few days ago, someone reported an unusual fireball sighting involving
what sounded like a non-symmetrical meteor "head" shape. A possibility
was raised that this asymmetric feature might have been a "bow-shock",
and the natural question arose from the list, "What is a bow-shock?"
In response to that question, here's a post I sent to the list in Feb,
summarizing some of my own reading in Ernst Oepik's classic text, "The
Physics of Meteor Flight in the Atmosphere" from earlier this Century.
Enjoy, and as always, (polite) corrections and clarifications welcome!
Lew
------- Forwarded Message
To: Meteor Observing Mailing List <meteorobs@latrade.com>
Subject: (meteorobs) Meteors, Wakes and Trains (was Re: Fireball UK 22-2-99)
Date: Thu, 25 Feb 99 14:29:27 -0500
From: Lew Gramer <dedalus>
Folks, my understanding is also that the persistent train (as distinct from
the wake) is a column of gases which can be much larger than the "meteoroid"
itself (the interplanetary dust bunny that causes all this mayhem in the first
place). Nor does the train require any significant time to grow this large.
This is because - again in my limited understanding - the train is thought to
actually be formed in the air by the ionizing effect of a meteor's ultraviolet
radiation... Thus the train, when it is observable at all, may bloom tens or
hundreds of meters to either side of the meteor, at nearly the speed of light
(plus recombination time). So trains may also take many minutes to dissipate.
Note that the brightness of the train - according to this explanation - should
be directly related to the *energy* of the meteor. This in turn is dependent
on all three of the size, density, and geocentric velocity of the meteoroid,
but especially velocity. Thus a small, very fast meteor can actually produce a
brighter, longer-lasting train than a large, slower meteor... And meteors
slower than about 30 km/s geocentric or so shouldn't generate trains at all!
OTOH, the wake is a "bow shock" or "tail" that forms around and following the
meteor, and is directly caused by (rapid!) diffusion of ionized and vaporized
gases from the entering body itself... (Can y'all tell I've been reading Oepik
again? ;>) This may be the phenomenon which many of us have observed as a
"head" or "comet tail" surrounding and trailing after a very bright fireball.
Clear skies!
Lew
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