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From Marc Gyssens

I N T E R N A T I O N A L   M E T E O R   O R G A N I Z A T I O N
                       Press release
Leonid meteor storm materializes around expected peak time (UPDATE)

Experienced visual observers watching near Malaga and at the Sierra Nevada Observatory in Spain and near the Gorges du Verdon in the French Provence report that Leonid meteor activity peaked at up to 30 meteors per minute shortly after 2 am Greenwich Mean Time. This activity was characterized by a lot of faint meteors and almost no fireballs.

Meteor astronomers reduce the actual numbers of meteors seen to a standard value, called the Zenithal Hourly Rate (ZHR), which takes into account the quality of the sky as well as the direction from which meteoroids enter the atmosphere. The peak activity reported by the abovementioned groups of observers corresponds with a ZHR around 5000, which is considerably more than what most meteor observers had hoped for (around 1000).

Preliminary reports of other observing groups at Tenerife, Canary Islands, near Valencia in Spain, and in Jordan confirm the picture sketched above.

Radio observations from Japan and the Czech Republic also indicate a peak time between 2:00 and 2:10 am Greenwich Mean Time.

The observed peak time coincides almost perfectly with the peak time of 2:08 am Greenwich Mean Time predicted by Asher and McNaught, indicating that the activity was due to the dust trail created the Leonids' parent comet, Tempel-Tuttle, about 100 years ago (i.e., 3 revolutions ago of the Comet around the Sun).

Marc Gyssens International Meteor Organization

Below is a more technical description of the observed Leonid peak activity.

       I M O   S h o w e r   C i r c u l a r

               LEONID Activity 1999

Visual observations of the 1999 Leonids revealed a distinctive peak with a ZHR of about 5000 on November 18, 2h05m +/-10m UT (solar longitude 235.287 +/- 0.007, eq. 2000.0). ZHR levels were above 1000 from roughly 1h30m UT to 3h00m UT corresponding to 235.26 to 235.32 degrees in solar longitude.

All observers who were able to view the peak under good sky conditions reported an abundance of faint meteors and a relative absence of fireballs. Some observers noticed a drop in the population index (i.e., a larger fraction of brighter meteors) after the peak.

Reports from Mohammad Odeh (Jordanian Astronomical Society) and Casper ter Kuile (Dutch Meteor Society, observing near Valencia, Spain) are very consistent with the picture sketched above.

In addition, radio data from K. Maegawa (Toyokawa Meteor Observatory, Aichi, Japan) reported by Kazuhiro Suzuki and the backscatter radar data from Ondrejov Observatory (Czech Republic) reported by Petr Pridal and Rosta Stork yield a peak time between 2h00m UT and 2h10m UT.

It seems that the peak time of 2h08m UT predicted by Asher/McNaught is confirmed within a margin of at most a few minutes, although the observed activity is significantly higher. It is reasonable to conclude that the peak activity has been caused by the 3-revolutions old dust trail of 55P/Tempel-Tuttle.

The following observers have contributed data immediately after the event, from which the ZHR profile given below has been derived:

Per Aldrich, C.L. Chan, Asdai Diaz, Yuwei Fan, Fei Gao, Lew Gramer, Andre Knoefel, Wen Kou, Alastair McBeath, Tom Roelandts, Sirko Molau, Renke Song, Wanfang Song, Honglin Tao, Dan Xia, Dongyan Zha, Jinghui Zhang, Yan Zhang, Jin Zhu.

(For groups of observers, only the name of the contributing observer has been mentioned.)

Date   Period (UT)  ZHR  +-
Nov 17  0600-1000   16    2
Nov 17  1600-2010   30    5
Nov 17  1900-2200   53   14
Nov 18  0030-0100  130   90
Nov 18  0100-0115  490  230
Nov 18  0115-0130  770  160
Nov 18  0130-0145 1040  660
Nov 18  0145-0202 4100  840
Nov 18  0200-0215 5000 1100
Nov 18  0212-0230 2400  280
Nov 18  0243-0247 1100  160
Nov 18  0320-0330  470   70
Nov 18  0420-0430  180   40

ZHRs are computed with a population index of 2.0, zenithal exponent of 1.0.

Marc Gyssens, 1999 November 18, 7h UT.

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