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Re: (meteorobs) Leonid annual and outburst components
joe wrote
>The 1966 display occurred 19 months after the nodal crosing of the
>parent comet. The upcoming 1999 display comes 21 months after nodal
>crossing. Hence I would suspect the same type of magnitude spread this
>year that was observed in 1966 . . . there likely will be a fair number
>of fireballs and bolides, though not on the same proportional levels as
>to what was observed in 1965 and 1998.
David Asher and I discuss this in our article "Leonid dust trails and
meteor storms" WGN 27:2 pp85-102 (1999). Whilst the time from the nodal
passage of the comet is involved, it is more complicated that Joe
suggests. If we use as an example the year 2001 when the Earth is
predicted to encounter two trails some 8 hours apart, the above reasoning
would imply we would witness the same magnitude distribution from each
trail. However, the velocity of ejection for the 4-revolution old trail
at the section encountered is 14 metres/sec and that of the 7-revolution
old trail is 8 metres/sec. (We confirm these results from the 1997 paper
by Kondrat'eva, Murav'eva and Reznikov). The mass distribution
encountered depends upon this ejection velocity with lower ejection
velocities favouring heavier particles and thus brighter meteors. When
deriving rates from these trails, another factor is the overall mass
distribution of the ejected particles. Rather than attempt to use some
theoretical formulation for the mass distribution of all the ejected
particles, we simply fitted the parameters of the dust trails to the
observed ZHRs for historical Leonid storms as derived by Peter Brown.
This allows the ZHR to be derived for any trail encounter without any
specific knowledge of the mass distribution at either ejection or
encounter. The reverse is possible however. From observation we can
derive the the mass distribution at encounter, and from the dust trail
parameters can derive the mass distribution of the overall ejected
particles.
The situation for 1966 and 1999 is interesting, as it is the very same
trail that is being encountered. In 1966 the section of the trail
encountered was ejected from the comet at 17 metres/sec in 1899 and for
the 1999 encounter, at 14 metre/sec in the same year. Although very similar,
the consequence of this is for a greater proportion of bright meteors in
1999 than in 1966. Sadly, the 1999 encounter is through the edge of the
dust trail and not just a few Earth radii from its center as in 1966.
Overall, the rates in 1999 will probably only be around 1% of 1966 :(
Cheers, Rob
Robert H. McNaught
rmn@aaocbn.aaodot gov.au
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