(meteorobs) Leonids 1998 ZHR and correction for atmospheric extinction

[Jure Atanackov]

Hello,

first of all, I'd like to thank Mikhail and Esko for their replies on the future
Leonid activity. Now I've been looking at some reports of past activity, namely
the Leonids of 1998. In IMO's extensive analysis, the peak time for the bright
'fireball' component is Nov 17, 1:55UT with ZHR 360. From reports from western
Europe and Canary islands I get the impression that the rates persisted at the
same, if not even more elevated level for at least 4 hours after the peak time.
For example, Victor R. Ruiz (Gran Canaria) reported quite a sharp rise in rates
after 5:40 UT, with his 10-minute counts under LM6.1 skies increasing from ~30
to 50+ Leonids. The final 10-minute count before the onset of twilight, at 06:12
UT is 56 Leonids. That would imply a raw rate of over 300 Leonids per hour.
Furthermore, Alan Fitzsimmons reported high rates observed from La Palma. These
were group counts so they are not really useful, but there are some other
indications from which it appears that the rates were very high, certainly
higher than ~280 as was calculated in IMO final analysis (Rainer, stay calm, I'm
not really disputing your work). For example, Mr. Fitzsimmons wrote:

"A colleague went outside for a smoke at around 05:15 UT, and only got halfway
through his cigarette  before he'd seen 35 meteors, mostly fireballs." - OK, I
had some coffee today with my friends and one of them is a smoker. He agreed to
do an experiment - he had to smoke a cigarette and I timed how long it took. 5
minutes. So it might be reasonable to expect that mr. Fitzsimmons' colleague
observed for 2-3 minutes, indicating a rate of 12-17 Leonids per minute or a raw
rate of 700-1000 Leonids per hour. 

Mr. Fitzsimmons then writes: "I went out again at 05:50, and on average could
see a meteor every 2-3 seconds. You would get a gap for perhaps 10 seconds, then
3 would occur almost simultaneously, quickly followed by another." This again
would imply a rate close to 1000 Leonids per hour.

Ok, so I'm trying to dismantle the results of thorough and well-made analysis by
IMO by saying the rates instead of declining after 1:55UT actually kept rising
until at least 6h UT, by which time they reached 700+ per hour? Not really.
Well, perhaps the rates from properly made observations do indicate a rate of
perhaps more than 280 per hour, but that can be also ascribed to higher
perception observers. And I also get the impression that the people at Roque de
los Muchacos genuinely saw a really high rate. The location is very dark and at
2400m elevation, so two factors come into play. They probably enjoyed much
darker skies than most other observers, so LM7+ is very probable *and* much
reduced atmospheric extinction closer to the horizon probably enabled them to
see many more meteors close to the horizon. Mr. Fitzsimmons writes: "For some
reason, many of the meteors we observed were at low altitude near the horizon."
I had the same impression during the Leonid 2001 storm as observed from 2800m
high Mt. Lemmon in AZ. For example I calculated average extinctions at 20, 10
and 5 degrees elevation as observed from 0m and 2400m elevations. I used
formulae from paper by Daniel Green: Magnitude Corrections for Atmospheric 
Extinction. ICQ 14 (1992): 55-9, which is available at
http://cfa-www.harvard.edu/icq/ICQExtinct.html.

Alt		0m		2400m
20 degrees	0.82 mag	0.43 mag
10 degrees	1.59 mag	0.83 mag
5 degrees	2.91 mag	1.53 mag

Average coefficients were used. Clearly, atmospheric extinction is much reduced,
which should yeild some increase in rates. Can this difference in extinction
produce a significantly different display, activity-wise, from very different
locations. The report from La Palma certainly looks that way. Perhaps this could
also have been a factor in the exceptionally high rates reported from Kitt
Peakin 1966?

Secondly, this is actually a question for Rainer: were reports from further west
(Brazil, Nova Scotia, etc) included in the 5:00-6:00 UT interval (in 1998)?
During the Leonids of 1999 it appeared that the ZHRs at locations with very
different radiant elevations were different, even corrected for radiant
elevation. IIRC from Spain the observed ZHR was ~2000-3000, while in Jordan it
certainly seemed 5000+. If there has been such an analysis, can you point me to
the article?

Clear skies!
Jure A.




____________________
http://www.email.si/