From: Amara Graps (amara@amara.com)
Date: Mon Apr 14 2003 - 03:42:19 MDT
Hi Folks,
Last November 20, 2002, we had this brief conversation
(Subject- "some (early) Leonids results") on extropians:
Hal Finney:
>I don't know if it was just the bright moon washing out the dimmer
>meteors, or if the predicted rates of 3000-6000 per hour were vast
>exaggerations.
Amara Graps
>The moon was almost full, which had something to do with it. However
>'vast exaggerations' is not really fair. The scientists making the
>predictions had a model that gave meteor rates. This year, the rates
>were 1/3, or less, of the predictions. The (sophisticated) Leonids
>models had been getting better and better at matching data, up to
>now.
[...]
>Therefore, while it is sad for all of the people who did not see a
>show, it means that there _might_ be some new physics, that wasn't
>included in the models before. That's really interesting.
I was wrong, the predictions and models were in very good agreement.
What was different this last (big) shower, was that there were many
faint meteors. These faint meteors were predicted in earlier years
too, but not seen, which is one of the puzzles for the Leonids.
To recap:
The Leonids are caused by a stream of predominantly very small
particles, less than 1 mm in size, which orbit the Sun with a period
of 33 years, together with their parent comet, Tempel-Tuttle. The
orbit of the Leonid particles intersect the Earth's orbit. Each year
around November 17, when the Earth is at this intersection, Leonid
particles enter the Earth's atmosphere and cause meteors. Along the
larger part of Comet Tempel-Tuttle's orbit, Leonid particles are
scattered sparsely, so that, in most years, we see only a few Leonid
meteors per hour. Only in the vicinity of the comet, the density of
Leonid particles is much higher. Consequently, every 33 years,
during the years that Comet Tempel-Tuttle revisits our region of the
Solar System, much higher Leonid activity is recorded.
In 2002 November, the Earth encountered Leonid trails created at the
1866 and 1767 returns of Comet 55P/Tempel-Tuttle. These trails are
respectively 4 and 7 revolutions old. The 1767 return ("left" peak)
was favored by the W. Africa, W. Europe, NE Canada, NE S. America
observers, and the 1866 return of the comet trail was favored by N.
America observers.
See the following for descriptions of the predictions:
http://www.spaceweather.com/leonids/
http://www.imcce.fr/s2p/leonides/predictions/Leonid_forecast.html
You can see some results here:
Leonids Multi-aircraft Campaign (The USAF FISTA and NASA DC-8
aircrafts flew from Spain to Nebraska during the shower).
http://leonid.arc.nasa.gov/
[... scroll down the page to the Zenith Hourly Rate (ZHR) versus Time
plot]
Two Leonid storms were seen on Nov 19 2002. In each case, the peak
occurred 10-20 minutes later than predicted, but observations
matched prediction. European observers saw the peak at 04:09 UT (ZHR
= 2,300/hr), while observers in the America's witnessed a storm
peaking at 10:50 UT (ZHR = 2,600/hr). Both peaks were narrow, with a
full-width-at-half-maximum of only 0.52 and 0.50 hours,
respectively. Both peaks were also rich in faint meteors. As a
result, the near-full Moon and bad weather at prime observing sites
made visual observations from the ground difficult.
Last Thursday at the joint European Geophysics Society (EGS) and
American Geophysical Union (AGU) meeting in Nice, France, (*), I
had the pleasure to watch Peter Jenniskens' presentation showing ~15
minutes of video of the first Leonids 2002 peak (that trail from the
1767 return of Comet 55P/Tempel-Tuttle). The meteor storm looked
like a canopy of diamonds gently falling from the sky. Lovely!
The new interesting physics is why were the peaks so narrow? I asked
during the question session why Poynting-Robertson drag did not
spread out the peaks, and Peter didn't know. There's some interesting
physics occurring here to confine the peaks. Here we have work for
some new PhD theses, so those of you with a love for physics and an
itch to go to grad school ...
Amara
(*) This year was the first joint meeting of the two huge research
societies, and I hope that they do not combine their meetings again.
Even with the U.S. cancellations, there were too many people
(>10,000), elbow to elbow, ~2000 posters every day, too many parallel
sessions, poorly organized and very expensive. However my Heidelberg
dust colleagues organized a great dust session, the best we've ever had
at the EGS, and that made the conference frustrations worthwhile. Oh,
and Nice is a beautiful city, even when it rains.
-- Amara Graps, PhD Istituto di Fisica delle Spazio Interplanetario (IFSI) Consiglio Nazionale delle Ricerche (CNR), Roma, ITALIA Amara.Graps@ifsi.rm.cnr.it
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