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(meteorobs) Leonids 2000
Dear all,
David Asher and I have written a piece for the next issue of WGN that
summarises our expectations for 2000. We have discussed/critiqued some
other approaches to ZHR predictions and made clear the limitations of
our method. This note is being sent now to meteorobs for two reasons.
Firstly Sky and Telescope 2000 Nov., p 111 makes two statements about
our work that are erroneous, and I want to correct these before anyone
gives up on the 2000 Leonids. The second is that there are two
wonderful coincidences in this year's Leonids that will allow the
derivation of valuable data about dust trail structure from visual
observations. WE NEED YOUR OBSERVATIONS! (well, via the IMO!)
"... David Asher and Robert McNaught, forsee just a 'normal' display of
up to 100 meteors per hour ..." S&T 2000 Nov. p 111
Whilst our nominal predictions is of ~100, we do not discount, and never
have discounted, the possibility of storm activity. Our analysis relies
heavily on historical ZHR measures *for dust trail encounters*, and none
exist for the geometry being encountered in 2000 November (4 and 8-rev
trails). I suspect activity will be "low", but could still be the best
meteor shower many observers will have seen.
"This year will truly put the Asher-McNaught meteor-trail theory to the
test ..." S&T 2000 Nov. p 111
From the comments above, it is clear that we make no strong prediction,
so unless substantial activity occurs well away from a dust trail
encounter, there will be no test of our theory this year. Only Ferrin,
amongst those who have made predictions, has done so without consideration
of the existence of dust trails.
I've seen various values for our ZHR predictions floating about. Our
latest predictions are those that appeared in Sky and Telescope 2000 June,
p 32. The "?" appearing after the predicted ZHRs of 100 for this year are
there for a very good reason, as stated above. Hopefully we'll have another
look at representing the lower activity extremes of dust trails in the next
couple of weeks, but with such limited data available for dust trail
encounters, such extrapolations will always be unreliable. However,
this year will provide much needed data in this regard, so even if
predictions are questionable, the observed ZHRs from dust trail
encounters this year *will be extremely valuable*.
The following dust trail encounters are within 0.0050 AU of the Earth
distance ZHR
2000 Nov. 17 07:53 UT 2-rev -0.0012 AU ?
08:22 1-rev +0.0031 0
18 03:44 8-rev +0.0008 100?
05:51 6-rev +0.0030 0
06:44 5-rev +0.0028 0
07:51 4-rev +0.0008 100?
For miss distances of between 0.0000 AU to -0.0007 AU, the time of
prediction appears to be accurate to around 5 minutes. This indicates that
the dust trails are basically flat sheets, and this is in fact the first
observational evidence to that effect (see McNaught and Asher, Meteorit.
and Planet. Sci 2000, pp **-**) The timing for more distant trails
may be less precise, but as we argued in the above paper, there is no
observational evidence for distant dust trail signatures in the 1965 or
1998 observations (nor does the IMO analysis indicate this for 1999).
We do not expect any young dust trail encounters to give a broad maximum
(FWHM >~1 hour), but this year will be a good opportunity to gather
suitable data.
In the above table, only the miss distances are given for several dust
trail encounters. Other relevant parameters are the minimum ejection
velocities (represented by da0 in our original paper) and the dilution
of the trail density with age (fM in our original paper). We feel confident
that the 1, 5 and 6-rev trails will produce activity lower than the
likely background activity, but 3 trails are worthy of special effort.
Several authors have previously published predictions of detectable
activity from the 2-rev trail in 2000. We certainly consider this as
a possibility, although believe such activity will be much lower than
the 4 and 8-rev trails. If the 2-rev trail is significant in relation
to these other two trails, this will indicate a substantial assymetry in
the dust trail profile in the sunward/anti-sunward direction and/or
a notable aging effect additional to trail stretching (the only aging
factor we belive is important). [In the upcoming note in WGN, David and
I comment on four errors in the analysis of Jenniskens et al from the June
2000 WGN, as mentioned in the June 2000 S&T p. 32, which had suggested
there was an error in our assumed position for the core of the dust trails.]
The two coincidences in this years trail encounters are
1) the 4-rev and 8-rev trails are encountered at the same geometry of
+0.0008 AU. This would mean that differences in the observed ZHR are
caused by aging factors alone. Or at least they will be if the disruption
of the 8-rev trail due to perturbations is not major! Analysis of this
may be difficult, but the European longitude observations of the 8-rev
trail and the 4-rev from the Americas four hours later will prove
interesting.
2) the 2-rev trail and the 4-rev trail encounters occur exactly 1 day
apart (well, within 2 minutes sidereal time!). This means that observers
using the same location, observing in the same direction and, hopefully,
in the same conditions (Moon will make a minor difference) will get
directly comparable data on these two trails. Both the relative intensity
and the magnitude index will be important results.
So my advice is to get out and observe, not that readers of meteorobs need
any such encouragement! Also, if you are not at European or American
longitudes your observations are just as valuable. Any outburst must be
related to the background activity and who knows, something unusual
might happen. Don't forget the occasional Leonid (or Taurid!) fireball
and those wonderful long duration trains. Perhaps 2000 won't amount to
much, but if such is the case, you should feel well satisfied that your
observations will go towards refining the various theories of the
structure of the Leonids.
Good luck!
Cheers, Rob
Robert H. McNaught
rmn@aaocbn.aaodot gov.au
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