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Re: (meteorobs) Two questions about Leonids 2000



In a message dated 9/29/00 12:58:51 PM Eastern Daylight Time, 
aakmp.antier@wanadoo.fr writes:

<< I wanted also to ask if there were more accurate estimations of the ZHR 
 of the 2000 Leonids peak. Because if I refered to the NASA site it can 
 be of 30 according to David Asher and Rob McNaught to 215 or 700 for 
 Esko Lyytinen and Tom Van Flandern for the first peak at 7:50 or 7:51 
 UT, and it can be of 20 or 700 respectively for the second peak at 3:40 
 or 3:44 UT. So, I'd like to know if the analyse of the 1999 Leonids 
 storm had permits to precise the estimations. >>

    At last weekend's 10th annual Connecticut Star Party in Colebrook, I 
announced my own "take" on this year's Leonid shower.  I think it is 
interesting to note that this year's display has a potential to be rather 
unique in the recent (modern) history of Leonid annals.  At least within the 
past few hundred years, we have never had a scenario where very strong Leonid 
displays have occurred on consecutive nights.  Perhaps the closest we've had 
to such a circumstance were the Leonids of 1961, when Perseid-type activity 
(ZHR = ~50) was noted by assiduous observers on November 16 and 17.  As we 
all know, the most significant Leonid activity is usually confined to just 
one night.

    In making an attempt to predict activity for 2000, I based my analysis on 
three past Leonid displays to which we have very good activity curves: 1866, 
1966 and 1999.  In the case of the first two I used Peter Brown's "Historical 
Visual Observations" article which appeared last year in Icarus.  For the 
1999 Leonids, IMO data published in the December 1999 WGN (27-6) was 
consulted.  An attempt was then made to match the activity profiles and 
meteor trail encounter data provided by David Asher and Rob McNaught for 
these three cases with the upcoming situation in 2000.
  
    For the morning of November 17, we should pass closest to material shed 
at the 1932 perihelion of 55P/Tempel-Tuttle at 07:53 UT, the distance between 
the Earth and trail separated by -0.00012 a.u. (the trail debris being 
located outside the Earth's orbit).  According to the IMO's Global Analysis 
of 1999, we encountered this 1999 trail at 01:43 UT on November 18 of last 
year.  This year, it would appear that we should again approach close enough 
for some sort of brief outburst or enhanced activity.  I am thus suggesting a 
ZHR of 250 to 500 near the November 17, 07:53 UT time for this year. 

    For the morning of November 18, in deference to Major League Baseball, I 
am referring to this as "Leonid Wild Card Night."  

    On this night, two trails -- a 4 revolution and an 8 revolution trail -- 
with satisfactory ejection velocities may interact with the Earth.  The 
8-revolution trail, in fact, gave rise to the Leonid storm of 1866.  The 
distances of these trails relative to our orbit are only a trifle greater 
than the 1999 value.  

    Unfortunately, as Asher and McNaught have pointed out, the 8 revolution 
trail (from 1733) has apparently undergone some perturbations due to previous 
close encounters with Earth.  In addition, they also point to a case in 1801 
regarding a 2-revolution trail with suitable ejection velocities that in many 
ways is rather similar to the circumstances regarding the 4-revolution trail 
in 2000.  But no outburst or enhanced activity was reported in 1801 (over 
western Europe).  The supposed 1801 enhanced activity would equally be larger 
than any nominal prediction that would be made for 2000.  Such activity could 
certainly have been missed (although historical records from Armagh 
Observatory suggest otherwise), so good activity cannot be totally discounted 
in 2000.  Unfortunately, the available circumstantial evidence looks against 
it, leaving open the possibility that our 4-revolution encounter in 2000 will 
also result in very little Leonid activity.

    Interestingly, Emel'yanenko had predicted that in 1999, additional 
enhanced Leonid activity would occur on November 18, near 17:00 UT.  Enhanced 
activity was indeed observed between 14:24 and 21:36 UT (according to IAUC 
7313), with visual counts of 100-300/hr.  The IMO indicated a "clear second 
outburst" of activity at 16:00 +/-1h UT, although observations made solely 
from China indicate a peak closer to 20:00 UT.  What is important to note 
here is that this 20:00 time is within 5 minutes of the Earth's closest 
passage to the 4-revolution trail by Asher and McNaught.  Unfortunately, Rob 
himself has indicated that the cut off for well-defined predictions regarding 
meteor trails seems to be at around a ZHR of 500 and the distance between 
last year's 4-revolution trail and Earth was a relatively large +0.0016 a.u.  
So there is some question as to whether any of the meteoroids encountered in 
1999s secondary peak may have indeed been related at all to any cometary 
material shed in 1866.  If so, it certainly would seem to enhance our chances 
for a significant display this year.  Unfortunately, I do not have knowledge 
of what trail (or trails) Emel'yanenko was referring to when he made his 
prediction last year, but the 4-revolution (1866) trail is only one of 
several possibilities.   

    So far as what we might expect on the morning of November 18, I would 
thus suggest a rather broad range of possibilities.  Since the 8-revolution 
trail will not be favorable for America as opposed to the 4-revolution trail, 
I only concerned myself with the latter.  

    It would appear that a projection of a ZHR of 20, as put forward by Asher 
and McNaught is certainly valid should a repeat of the 1801 disappointment 
recur this year.  On the other hand, we could go to the opposite end of the 
spectrum and suggest that if everything properly fell into place, that a 
brief outburst with ZHR's as high as 1,400 to 2,800 (the most extreme limits) 
could also be possible near the trail encounter time of 07:51 UT as well!

    With some "guarded optimism," however, I might suggest that the actual 
truth could ultimately lie somewhere in between these two extremes, with 
rates perhaps somewhere in the hundreds per hour.  The prescence of the Last 
Quarter Moon wont help matters any either, but it would certainly be great if 
we have an opportunity to observe unusually high (ZHR >100) activity on two 
consecutive nights in 2000.  But as November 18 is the "Wild Card" night, all 
bets are off.  

    To quote Rob McNaught in a recent e-mail discussion that the two of us 
had on this subject a short while back:  "As with every year, you just have 
to go out and take your chances!"
  
-- joe rao     
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