Re: (meteorobs) Leonid Storm?

  • Subject: Re: (meteorobs) Leonid Storm?
  • From: Rob McNaught
  • Date: Fri, 27 Aug 1999 10:31:17 +1000 (EST)
Luis Gonzalez wrote:
> What's a Leonid Storm and Cloud. Also what is the coorect Punctuation for
> Leonid this how i say them (leo-noids).

A meteor storm is when the rate of meteors is exceptionally high.
An average rate of around 1 meteor per second would be a storm.  During
the greatest storms, witnesses describe meteors falling like snow flakes!
Storms are always from a particular meteor stream and are usually caused
by a "dense" trail of dust ejected from the parent comet recently (few
revolutions old).  In the case of a Leonid storm, the Earth encounters a
young thick trail of dust from comet 55P/Tempel-Tuttle.  The designation
55P refers to this comet being number 55 in the catalogue of well observed
periodic comets.

The term "Leonid cloud" is a misnomer.  A cloud implies a broad object
like a cumulus cloud.  However any broad object like that will have
dramatically different orbits for the particles at different sides of the
cloud, so it would not survive as a cloud for very long.  In fact, a
spherically expanding "cloud" of dust ejected from a comet immediately 
starts to spread out (basically along the comet's orbit) into a very thin
flattened trail.  Each time the comet passes the Sun, a new trail forms
and unless the comet passes close to a planet, all these trails remain
"connected" to the comet, both in front and behind, but become more
separated from each other the further one goes from the comet.  The
youngest trail is shortest and densest and the older ones are longer
and less dense.  The overall cross-section of the trails is
surprisingly unchanged over many revolutions.  The effect of sunlight is
to make most particles of the size that cause visible meteors, to lag
behind the comet, so storms tend to occur after the comet has passed
perihelion, its closest point to the Sun.

To have a storm, the Earth must
a) pass through the region of these young dust trails
b) pass through at least one of these trails ("a" might be satisfied but
   not "b", i.e. the Earth crosses a highway but misses all the vehicles).
   This explains the fickle nature and previous unpredictability of storms
c) the encountered trail must have a sufficient density of meteoroids of
   the size that produce visual meteors.

The exception to this general rule is the existence of old dense
trails of particles that are "trapped" near the comet due to the
gravitational interaction with Jupiter or some other planet.  This can
explain the Leonid fireball outburst of 1998.  It is not likely for such
a "resonant trail" encounter to occur for the Leonids more than one year
away from the passage of comet 55P/Tempel-Tuttle.

Pronunciation is LeonIDS.  I've heard it mispronounced Leonoids quite
frequently and can't imagine where it comes from (other than terms like
meteoroids and asteroids).

Cheers, Rob