Impressions from the big Leonids conference in Israel

By Daniel Fischer.

Here is a quick report about my favorite highlights from the exciting Leonids conference in Tel Aviv last week - you can find a web version at (story 3), but there are not many links: Most of the science is so new that it only exists on the speakers' viewgraphs, Powerpoint files or video tapes...

Bewildering torrent of data collected during 1999 Leonid storm

Unique records of rare celestial event on parade at first major conference / Value of amateur observations hailed / USAF drops out of Leonid business

It was one of those Woodstock-style events that punktuate the steady flow of scientific progress now and then: The first scientific conference after a rare space event can sometimes turn into a fast-paced presentation of one mysterious and/or spectacular observation after the other, with often little theoretical understanding - but everyone present feels that something special has happened and that years of scientific work will be initiated by the encounter with the unknown. The sessions on the Great Comet Crash of 1994 at the IAU General Assembly in The Hague were such a special event - and the Leonid MAC 2000 Workshop in mid-April in Tel Aviv was no less exciting.

"MAC" stands for the Multi Instrument Aircraft Campaign that the U.S. Air Force, NASA and other agencies had run during the 1999 Leonid storm, and the meeting at Tel Aviv University was the first occasion for a joint presentation of the observations made from the two aircraft that had flown "through" the storm near Italy. But many groundbased teams, associated with MAC'99 or independent, were represented, too, as were amateur astronomers from Israel (who had observed the storm in the Negev desert) and Germany (who had been in Tenerife and Jordan).

The theme running through most of the conference could be described as: "The meteors are coming - let's try something new!" A number of speakers had used instruments during the meteor storm that had rarely or never been used for meteor work before: big radar dishes, networks of radio antennae, HDTV cameras etc. Almost every other speaker had a video cassette to show with sometimes strange but more often breathtaking views of the celestial spectacle. The "best picture" award would clearly go to the wide-angle HDTV tape shot by Japanese researchers from one of the airplanes which really showed the rain of several meteors per second during the peak of the storm (H. Yano). Almost as captivating was a Czech video of meteor spectra in realtime (J. Borovicka), also shot from the air.

Here are a few of the highlights of the meeting and some early discoveries - a collection of refereed papers should be published as a special issue of Earth, Moon, and Planets as well as a book this November:

You see 7 times as many meteors from the air than from the ground! This surprising 'discovery' by the airborne observers has been analyzed theoretically and found to be perfectly logical (D. Koschny): From, say, 10 km altitude you can look through a much larger volume of the atmosphere with less extinction than from the ground, so the number of meteors near the horizon rises dramatically. This fact was also exploited by the above-mentioned Japanese HDTV system.

How real is the fine structure in the ZHR profile of the peak? There was considerable debate about the reality of several peaks of the meteor rate before and after the main peak, with P. Jenniskens arguing for a remarkably smooth ZHR profile (with a Lorentzian shape) as generated from multiple airborne video cameras, but several others (among them I. Manulis, H. Yano, S. Molau and the author) saw clear evidence for at least one peak 20 minutes before the maximum that shows up in the airborne video as well as in the Israeli and Jordanian visual and video data (see also the report about the Radebeul conference in Update # 182 story 3).

Are there two layers in the atmosphere where the meteoroids burn up? Puzzling Israeli radar data (N. Brosch) could indicate that one class of dust particles decays at 250 km and the other at 120 km altitude, but the result remained controversial and the technical details of the (military) radar system unknown. The Leonid activity had also been monitored by a worldwide network of identical mobile radar antennae (W. Hocking), but many of the detailled recordings have still to be made public (and could prove the fine structure mentioned above).

What can we learn from the radio emission of meteors? An array of radio antennae normally used for lightning research had been placed in the Israeli desert - and recorded up to 18 000 VLF radio signatures an hour during the Leonid storm that are characteristic for meteors (C. Price). The tons of data collected have hardly been looked at, but the tentative activity profile from the meteor emissions is puzzling: There are both the main and the above-mentioned pre-peak 1/2 hour earlier - but also a huge number of signatures 1 1/2 hours before the peak that have no visual counterparts whatsoever...

Nebulous meteors are real! A few researchers had obtained video recordings of meteors with high angular resolution - and sometimes meteors would show an extended V-shaped halo (I. Murray, M. Taylor). The detailled hydrodynamics of how meteoroids decay in the upper atmosphere, one could learn at the conference, are not that well understood, as are the subtleties of the emission spectra from the meteors. Those, in turn, might eventually even teach us about how cosmic dust could bring organic molecules to Earth - a reason why even "astrobiologists" show some interest in the Leonid data.

Will there be more observing campaigns like MAC'98 and '99? The USAF has concluded that the Leonids are no significant threat to satellites and that further large enpenditures on observing campaigns would not be justified from the 'threat' perspective (M. Treu). It had also become obvious to the military brass that amateur astronomers are very well capable of monitoring the meteor activity in the sky. The MAC scientists, though, are most interested in further air campaigns, in 2001 and 2002 (the Moon is less of a problem when you're airborne than on the ground), and perhaps even in 2000.

This year's Leonid activity will be a decisive test whether there will really be more storms in 2001 and 2002 and should thus be monitored well, many speakers emphasized. The leading theoretician of the Leonid dust trails, D. Asher, stated once more that all reasonable cometary dust ejection models make Leonid storms in 2001 and 2002 all but inevitable, and P. Jenniskens is already qualifying his doubts (mentioned in the just-released June issue of Sky & Telescope) and is now hoping for ZHRs of 6000 and more in those two years. Again, new and ususal observing methods are likely to be brought into action, followed by years of data analysis...

Daniel Fischer

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