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(meteorobs) McNaught - Leonids 2000



At 18:12 2000/10/19 +1000, you wrote:
>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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