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Leonids 2000

Introduction

Rainer Arlt and Marc Gyssens of the International Meteor Organization (IMO) published a full examination of the Leonid results collected by 230 visual observers around the world in WGN, the Journal of the IMO 28:6 (2000 December), pages 195-208, adding more detail to the preliminary review I discussed on this Webpage soon after the event. In addition, I have completed a review of the radio data submitted to the SPA Meteor Section made during the shower, and give news from that analysis here for the first time. The main finding remains the same as before, that no Leonid storm happened this year, but rates were greatly increased above the normal level. There have been some changes and improvements made to the initial impressions however, which I draw attention to in this revised report.

 

General Overview

In the IMO data Leonid Zenithal Hourly Rates (ZHRs) on November 16-17 were generally between 30-50 +/- 6-10 for most of the night over Europe and North America. A short-lived slight increase to 50-60 +/- 7 occurred around 04h-05h UT, with a further small peak detected from about 06h-07h UT (ZHRs ~ 75-85 +/- 14; note that this level has been revised downwards from the initial results, as more reports have arrived). The radio data suggest an increase in meteor echo rates between about 06h-07h UT particularly, perhaps beginning around 05h UT. This was eclipsed by a stronger radio-visual peak around 08h05m UT, when ZHRs reached 130 +/- 20, a peak now found more clearly and strongly than the first reports implied. This early maximum was probably due to the Leonid dust trail laid down by the shower's parent comet 55P/Tempel-Tuttle two revolutions ago in 1932, as the predicted time for that trail's closest approach to the Earth, and also the time the Earth passed closest to the comet's orbit in 2000, was 07h51m UT.

leo2000-1.jpgNovember 17-18 was a much more active night for the Leonids, with good radio counts and visual ZHRs throughout. ZHRs were 100+ between 00h15m-09h30m UT, the radio results giving clear confirmation that rates were notably increased during this same interval. By about 01h50m UT, ZHRs had exceeded 200, and remained consistently close to or above that level until around 08h25m UT. Within this time, two stronger maxima can be discerned. The initial peak was very ill-defined, building and declining slowly during a roughly one-hour long interval centred on 03h24m UT, when ZHRs of 290 +/- 20 were seen, a period which included the Earth's closest pass of the 1733 Tempel-Tuttle dust trail, predicted by David Asher and Rob McNaught for 03h44m UT. The radio results confirm a peak around this time, with increased echo counts between 02h-05h UT. This was followed by another, much stronger, maximum near 07h12m UT, when ZHRs of 480 +/- 20 were recorded, somewhat higher than first thought. Once more, the radio results concur, with the strongest echo counts during the 2000 Leonids found from 07h-08h UT in virtually all the available datasets where the Leonid radiant was well above the horizon. The predicted time for this peak, which probably resulted from cometary dust shed by Tempel-Tuttle in 1866, was 07h51m UT, about 40m later than was observed. None of the Leonid maxima were sharply-defined this time, all gradually growing from and receding into a strong background level, with ZHRs of 30+ persisting from November 15-16 to 18-19. A graph showing IMO Leonid ZHRs across the peak is given here. The time axis (horizontal) is given in degrees of solar longitude, a fixed time-base used in meteor work which does not suffer problems because the calendar does not fit exactly to the Earth's orbital revolution around the Sun. In November, 1.01 degrees of solar longitude (s.l.) = 24 UT hours, so 0.042 degrees s.l. = one hour. S.l. 234.5 degrees = November 16d13h 33m, s.l. 236 degrees = November 18d01h 11m, and s.l. 237.5 degrees = November 19d12h 50m. For more information on the IMO data, see the Website at: http://www.imo.net or the WGN article I referred to earlier.


 

SPA Meteor Section Reports Discussed

Bright moonlight, poor northern hemisphere weather, and the fact that no storm was seriously expected from the Leonids in 2000 November clearly proved no deterrents to watching! The list of observers follows. As usual, all those who troubled to contact the Section with a report are credited, whether successful or not in their efforts. Visual observers in England have no additional notes given. Those elsewhere are listed with the country they observed from. Non-visual observers are denoted by the letters "R" = radio or "V" = video. Of the radio reports, all the data except that from Dirk Artoos were kindly provided to us by Chris Steyaert in Radio Meteor Observation Bulletins 88 and 89, for December 2000 and January 2001 respectively (contact e-mail: steyaert@vvs.be, Website: http://page.to/rmob/).

Jean-Louis Aillaud (Reunion Island, Indian Ocean; R), Enric Fraile Algeciras (Spain; R), Rainer Arlt (Germany), Dirk Artoos (Belgium; R), Mark Bailey (Northern Ireland), K. Bergen, Mike Boschat (Canada; R), Jay Brausch (North Dakota, USA), Dave Bray, Michael Brooke, Dave Campbell, Maggie Daly, Peter Dean, Maurice de Meyere (Belgium; R), Steve Evans (Spain; V), Ghent University (Belgium; R), Shelagh Godwin (Singapore), Roberto Gorelli (Italy), David Gosnell, "Graeme", Lew Gramer (New England, USA), Valentin Grigore (Romania), Philip Heppenstall, Carl Johannink (Portugal), Will Kelsey (Arkansas, USA; R), Mohammad Ali Khodayari (Iran), "Kieran", Marco Langbroek (Portugal), Jeff Lashley (Scotland), Trevor Law (Egypt), Bob Lunsford (California, USA), Tony Markham, Alastair McBeath, Tom McEwan (Scotland), Steve Milburn, Koen Miskotte (Portugal), Sirko Molau (Germany), Charles Munton, Ben Notarini, Mohammad Odeh and other Jordan AS members (Jordan), Sadao Okamoto (Japan; R), Terry Owen, Peter Phillips (Northern Ireland), Ingo Reimann (Germany; R), Ian Ridpath, Paul Saunders (Wales), Robin Scagell, Ton Schoenmaker (Netherlands; R), Jonathan Shanklin (airborne over the North Atlantic, and Falkland Islands), Nigel Smith, George Spalding, Roger Stapleton (Scotland), Dave Swan (R), Darren Swindells, Ervin Szlanicska (Slovakia; R), Istvan Tepliczky (Hungary; R), Pierre Terrier (France; R), Alistair Thomson, Mihaela Triglav (Slovenia & Italy), Garfield Tsao (Taiwan; R), Fiona Vincent (Scotland), Bruce Young (Queensland, Australia; R).

In Britain, observations on November 16-17 were possible from parts of south Wales and most of central to eastern England, while much of western England, Scotland and Northern Ireland seem to have been caught beneath overcast skies. From the UK-only data, something of the November 17, ~ 06h UT minor peak noted in the IMO results looks to have been visible as dawn twilight was strengthening, when Leonid ZHRs rose from a steady overnight ~ 50-70 +/- 15-20 between at least 02h00m-05h30m UT, to around 100 +/- 30 soon after 06h UT. The IMO ZHRs, drawing on a far larger quantity of data, are much more accurate than these UK-only values, but the British results do show how a small peak like this can be seen even by watchers with often unhelpful skies. A few minor fireballs (magnitude -3) were spotted, with a hint of slightly more of these towards 06h UT, but not especially convincingly. Initial checking of the meteor magnitudes did suggest somewhat more brighter Leonids near this time, but this is less obvious now a better sample of magnitude distributions is available, and no obvious increase in bright Leonids was found in the IMO report close to this time. A couple of observers commented on the rather strange observing circumstances the Moon enforced, where Leonids were coming from "over their shoulder", as they faced towards the northern skies to keep the Moon as far from their line of sight as possible!

Unlike in 1999, southeastern England seems to have been the place to be in Britain on November 17-18, as chiefly positive reports have arrived from this area, including from sites in Bedfordshire, Buckinghamshire, Cambridge, Essex, Kent, Middlesex, Oxfordshire, Surrey and Sussex, plus two reports of clearer skies for a short time around 03h UT further north in South and West Yorkshire. Even so, observers often struggled with clouds, though many reported seeing a superb 22-degree radius halo around the Moon, thanks to a high-altitude cirriform cloud-sheet ahead of an advancing frontal weather system. Healthy Leonid rates of up to minor fireball-class meteors (magnitudes ~ -3 to -8) were viewed by most in spite of this.

By contrast, this frontal system produced generally overcast skies, and sometimes rain, across other parts of Britain. Clouds, rarely punctuated by occasional sightings of a meteor or meteoric flash brightening the overcast, were more typically observed from most places with active watchers in northern England, Northern Ireland, Scotland and Wales on November 17-18. Even so, most people greeted the lack of clear skies philosophically, as November is traditionally one of the worst months for clear weather in the UK, and those who had seen something the night before were glad not to have wasted that opportunity.

Under good skies overseas, several Section correspondents commented that the November 17-18 display seemed similar in strength and persistence to the 1998 Leonid fireball night, but that 2000's Leonid meteors were significantly fainter, with very few notable fireballs. Rainer Arlt in Germany and Mihaela Triglav in Slovenia chased holes in the clouds and fog, driving hundreds of kilometres overnight on November 17-18, to confirm this view. However in Romania, Valentin Grigore commented on how Leonid rates were strengthening to about 3 or 4 a minute until fog descended at 01h26m UT, which then lasted for the next nine hours! The clearest skies were found in Spain, Portugal, Egypt and some parts of the USA in our results. The Jordanian watchers at their Al-Azraq observing camp out in the desert were unlucky to be clouded-out throughout the shower, as skies were clear, though heavily streetlit, only 120 km away in the capital Amman! So much for the myth of clear desert skies!

>From quite a few of the British reports especially, something of a party atmosphere was apparent on both November 16-17 and 17-18. Several people made a real night of it, determined to have a splendid time, for example by observing all five naked-eye planets with the unaided eye in one night (although Mercury was extremely easy from the UK by mid-November, Venus was still setting very early then, especially for sites further north), the Moon, the lunar halo, and managing some Leonid watching as well. Peter Dean with a group in Surrey mentioned good Leonid rates could be counted just within the lunar halo!

Computed ZHRs based on the British reports alone from November 17-18 show a large scatter because of the often poor skies, but were generally around 200+ by 02h UT. There are indications of two clearer submaxima after then, most noticeably around 03h30m-04h15m UT when ZHRs were ~ 330 +/- 100, perhaps with two stronger phases around 03h40m-03h45m and 04h10m-04h15m UT. The radio observations provide some support for the first of these two phases, but not especially the second. Interestingly, the IMO ZHRs show a brief dip to ~ 190 +/- 8 near 04h30m UT, which may have helped give the impression of higher rates just before that in the cloud-affected British results. Drawing on overseas data as well, another possible submaximum may have happened around 02h50m +/- 10m (ZHRs ~ 300 +/- 130) in the SPAMS data, which coincides with a slight ZHR peak of 270 +/- 10 around 02h53m UT in the IMO results. This also looks to have been weakly detected in the radio data. The two bar graphs here illustrate the SPAMS Leonid magnitude and train distributions in 2000. The Leonids' corrected mean magnitude on November 17-18 was +2.6, brighter than their overall mean of +2.9, and both values were notably brighter than the November sporadics at +4.0. IMO data not illustrated here shows more clearly the consistently brighter Leonids seen right across the November 17-18 maxima. About 25% of Leonids left trains, compared to 2% of the sporadics, both values depleted as faint trains were made invisible by the bright Moon.

By November 18-19, skies were overcast above much of southern and western Britain, and very few positive reports have arrived from then. Those that did indicated Leonid ZHRs had fallen back to around 30-40 +/- 15 (IMO results: 25-35 +/- 5). Even that level is two or three times better than the Leonids achieve in their more typical non-outburst years, of course. Steve Evans in southern Spain commented that despite concentrating on his video and photographic equipment then, he was still spotting around 10 Leonids an hour quite easily in casual sky-checks, helped by being at a good, dark-sky rural site.

Some of the early European radio results and comments from radio hams suggested system-saturation had occurred near the three main Leonid peaks. This may have been due to increased atmospheric ionization produced by the Leonids, which may be a type of radio propagation called Sporadic-E, last detected this strongly from the shower in 1996 (as I discussed in WGN 25:1 (1997 February), pages 45-53). However, subsequent enquiries have shown this to be much less widespread in 2000 than was first supposed, with only one radio meteor observer, Ton Schoenmaker in the Netherlands, reporting very severe problems due to this, while no radio hams have come forward with actual evidence to support the few initial claims. Indeed Dave Butler in his monthly "VHF DXER" column in Practical Wireless magazine (77:3, March 2001, pages 56-57) commented that the radio meteor amateurs generally found the 2000 Leonids didn't quite live up to their expectations. He also noted that at times there was so much noise from amateurs failing to observe the correct procedures on some radio frequencies, that making contact using reflections from meteor trails was impossible. It may be this noise which gave the early impression of system saturation for the radio hams, and might even have helped cause part of the difficulties Ton Schoenmaker reported.


 

Conclusion and Looking Ahead

The Leonid return in 2000 provided helpful support for the meteor stream filament models worked out by David Asher and Rob McNaught (who predicted peaks on November 18 at 03h44m and 07h51m UT), and independently by Peter Brown (who predicted Leonid maxima for November 17 at 08h UT and again on November 18 at 08h UT). The time the Earth passed closest to Comet Tempel-Tuttle's orbit, 08h UT on November 17, an event we often use for computing meteor shower maxima generally, also proved a useful guide, but only to the first and weakest of the three chief peaks in 2000. Other proposed peak timings from different theoretical models worked out much less well.

For the Leonid maxima in 2001, the following predictions have been made: November 18 at 10h01m, 17h31m and 18h19m UT (Asher & McNaught; the 18h19m UT peak is expected to be the strongest, possibly with ZHRs around 15,000); November 18 at 11h UT (possible bright meteor peak) and 16h54m UT (main maximum; both these timings by Peter Brown); November 17 at 13h UT (closest approach to the comet's orbit). The timing and strength of any given maximum cannot be definitely known in advance, so it is wise to be alert throughout the nights of November 16-17 and 17-18 at least. The Moon is a waxing crescent, and will give no problems for observers, though unfortunately, none of these expected maxima will be visible from Britain, if they keep to time.

For those planning a trip abroad to see the Leonids this year, the best-placed regions for the various predicted peaks should be North and Central America (10h-11h UT), western North America and the Pacific Ocean (13h UT), and Australia westwards to central Asia (17h-18h UT). If you want to improve your chances of seeing what happens, make sure you check all the available information on likely weather patterns in the region for mid-November before you book to go, and be prepared to move quickly overland to a better site if the forecasts prove unfavourable once you're there.

Good luck, clear skies and don't forget to send your observations in to the SPA afterwards!


 

Acknowledgements

My grateful thanks as always go to everyone who has provided results - whether positive or negative - to make this report possible. If anyone still has not submitted their Leonid 2000 data, please do so as soon as possible. All will be most welcome!

 

Alastair McBeath, SPA Meteor Section Director E-mail: vice_president@imo.net

Added by:  Robin Scagell