NUMERICAL MODEL PREDICTIONS FOR 1998 LEONIDS

1998 Leonid Model Prediction

Using a total of 12 different models for the ejection of meteoroids from
comet Tempel-Tuttle, a preliminary "best" guestimate for the location of
the strongest peak in activity and its associated ZHR for the 1998
Leonids has been found.

The 12 model approach involves using three major variations in meteoroid
density (0.1, 0.8 and 4.0 g/cm^3 for bulk density of the
meteoroid). For each of these three densities, four different variations
in the initial ejection velocities are also employed - one follows
the distributed production model of Crifo which produces broad
distributions in initial ejection velocity which
has a mean velocity lower than the classical Whipple/Jones ejection
model. In addition to Crifos distributed production model, a
Whipple/Jones ejection velocity model is used, as well as a second
variant of the same with a heliocentric velocity dependance of
r^-0.5 in place of the usual r^-1. The fourth model is again a variant
on the Jones/Whipple model in which the ejection velocity at a
given heliocentric distance is not single-valued in the monte carlo
generating routine, but rather has a parabolic distribution of
probable velocities about the average Jones/Whipple velocity for the
chosen heliocentric distance. See Brown and Jones (1998), Icarus, v.
133, pp. 36 - 68 for more details.


The results of the modelling for the Leonids, using ejections at all
perhelion passages of the comet back to 1499 AD (ie 15
revolutions of the comet prior to the current epoch). A simple summation
of the meteoroids which are then visible at Earth at the
present time from this ensemble and which would produce visually
observable meteors (mass > 1 mg) was then computed from all
ejecta. A meteoroid is defined as being Earth-intersecting if its nodal
radius is within 0.005 AU of Earth at the longitude of its
descending node. All models suggested a steep increase in activity
beginning in December, 1997/early 1998 accompanying the
passage of Tempel-Tuttle. The resolution of the modelling is of order 2
months and thus all models suggest that this November will
show significantly increased activity relative to 1997 (when the peak
ZHR reached just short of 100), and likely activity approaching meteor
storm levels (ZHRs of order 1000). Using 1997 as a baseline and taking
the peak ZHR to have been 96 +/- 13 at 235.22 +/- 0.02 (J2000) in 1997
we have extrapolated the relative model difference between the activity
strength predicted by the model in 1997 to that observed and that
predicted for 1998. Using a mean of all  models, produces a predicted
location for the peak in 1998 of 235.26 +/- 0.04 (J2000) with a peak ZHR
of 1200 +/- 280. This solar longitude corresponds to Nov 17 at 19:20 UT
with a 1-sigma uncertainty of 60 minutes. We emphasize that due to the
model results sensitive dependance on density of the meteoroids, the
range of possible ZHRs extends from slightly lower than the bound given
above to nearly 10 000 (the higher values associated with the models
using the least dense meteoroids and lowest ejection velocities).

The use of relative modelling difference between 1997 and 1998 implies
that the veracity of the prediction in 1998 relies entirely on the
accuracy of the magniude of the ZHR reported in 1997 under full moon
conditions. As well as the above, the models suggest that broad
activity, persisting for of order a full day centred about this peak
should be noticeably above normal Leonid background levels and should be
rich in larger meteoroids in 1998 most notably after the time of the
peak. The model suggests ZHRs of order 100 or greater in the 3-4 hour
window prior to the peak and ZHRs of order 100-200 persisting for many
hours after the peak.
The mass index near the time of the peak over the visual magnitude range
will be near 1.6 +/- 0.1. It is worth noting that a significant decrease
in the mass index from 1.8 +- 0.1 several hours prior to the peak to
this lower value and then upward again after the peak is visible in most
models.


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Peter Brown
Meteor Physics Lab
Department of Physics and Astronomy
University of Western Ontario
London, Ontario
N6A 3K7
Canada

Voice:1-519-679-2111 x6458
Fax:1-519-661-2033
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