Leonid trail encounters

Work by the Kazan research group, Kondrat'eva, Reznikov and colleagues (applied to various meteor streams, not just the Leonids) showed that meteor storms or sharp meteor outbursts are generally associated with the passage of the Earth through dense regions of material sometimes called dust trails on account of their long, narrow structure. Each dust trail consists of solid particles released at a single return of the stream's parent comet to perihelion. These trails successively created from any one comet have a separate existence in space within the stream.

2006 outburst

In 2006 the Earth encounters the Leonid trail created at the 1932 return of Comet 55P/Tempel-Tuttle. The following estimate by McNaught and Asher is for the meteor outburst that will result from this trail encounter.

Time (GMT)          Trail  Estimated  Moon   Visible from
                              ZHR     age
2006 Nov 19, 04:45  2-rev      120     28  W. Europe, W. Africa

For more details on this estimate, read the further information on 2006 Leonids.


What follows was written before the 2002 Leonids. It is being kept here for reference (e.g. if you want to see what estimates were done before the information of what actually happened in 2002 became available).

It is well known that in 2002 November the Earth encounters Leonid trails created at the 1866 and 1767 returns of Comet 55P/Tempel-Tuttle. These trails are respectively 4 and 7 revolutions old (Tempel-Tuttle orbits the sun every 33 years or so). Many meteors from these trails will be visible (albeit with the full moon in the sky), and independent calculations by various astronomers have verified what parts of the world will be favoured.

Final estimates before 2002 encounters

A huge amount of significance has been attached by some to the game of guessing what the peak ZHR will be for the various trail encounters. ZHR = Zenithal Hourly Rate, basically the number of meteors visible to an average observer in perfect viewing conditions, scaled up to a time interval of an hour. There are various sensible estimates available (i.e., based on independent models by different researchers), compared at this Leonid page by Bill Cooke, which also contains information of interest to satellite operators, and estimates of observable meteor rates from different cities (i.e., in realistic rather than perfect viewing conditions).

The following estimates are by Robert H. McNaught (Research School of Astronomy and Astrophysics, Australian National University) and David Asher (Armagh Observatory), and constitute their entry for the above mentioned ZHR guessing game, superseding model fits done previously. Reference: WGN Vol. 30 (No. 5), pp. 132-143, 2002. This paper is downloadable in postscript or pdf format, and explains why on balance the authors prefer the peak ZHR estimate of 1,000 to a possible alternative of 2,000 for the first maximum, and 6,000 to a possible alternative of 2,900 for the second maximum. The estimated FWHM (Full Width Half Maximum) gives an idea of the trail encounter durations that are expected according to the model.

Time (GMT)          Trail  Estimated  Estimated  Moon   Visible from
                              ZHR       FWHM     age
2002 Nov 19, 03:56  7-rev    1,000     130 min    15  W. Africa, W. Europe,
                                                      NE Canada, NE S. America
2002 Nov 19, 10:34  4-rev    6,000      71 min    15  N. America

Meteor brightnesses in trail encounters

Quantitative estimates are provided above for time, ZHR and FWHM of the two main Leonid trail encounters in 2002. It is also possible to say something, albeit less quantitative as far as our current models go, about the expected brightness of the meteors. This is because typical brightnesses observed in any given trail encounter have been seen to relate quite well to the parameter Delta a_0, which is essentially a measure of the meteoroids' orbital period around the Sun (for those who like technical definitions: Delta a_0 = difference in semi-major axis between meteoroids and comet at time of ejection). Smaller meteoroids (solid grains) are more affected by solar radiation pressure as they orbit the Sun, solar radiation pressure increases the orbital period, and larger meteoroids produce brighter meteors when they impact the atmosphere. Therefore smaller Delta a_0 corresponds to brighter meteors. Note that this is not an exact relation - in any encounter there is plenty of dispersion in the meteor brightnesses. But Delta a_0 does give a reasonable idea of the typical brightnesses to be expected. Here are some examples:

                   Delta a_0
     1966 (2-rev)    +0.17
     1969 (1-rev)    +0.93
     1998 (20-rev)   -0.02
     1999 (3-rev)    +0.14
     2001 (7-rev)    +0.08
     2001 (9-rev)    +0.04
     2001 (4-rev)    +0.14
     2002 (7-rev)    +0.11
     2002 (4-rev)    +0.17
     2006 (2-rev)    +0.96

For example, the 1998 Leonid outburst was rich in fireballs (bright meteors), and Delta a_0 was small. In 2001, the 9-rev and 4-rev trail encounters were overlapping in time, both observable from east Asian longitudes. But the duration of the 9-rev encounter was longer, with bright meteors being observable for many hours. Superimposed on this was a sharper peak due to the 4-rev encounter. The meteors in that sharp peak were generally fainter because Delta a_0 was larger. In 2002, both peaks will be affected by the full moon, but the 7-rev (European/African) one slightly less than the 4-rev (American) one because of the slightly higher proportion of brighter meteors in the former. Of course, if the ZHR of the 4-rev peak is higher, this may more than compensate, as far as observable rates are concerned.

Old estimates

For reference, here are some older estimates by the same authors (their entry for the ZHR guessing game 2001), reference WGN Vol. 29 (No. 5), pp. 156-164, 2001:

---LAST YEAR'S ESTIMATES---
Time (GMT)          Trail  Estimated  Moon   Visible from
                              ZHR     age
2001 Nov 18, 09:55  7-rev      800      3  N. & Central America
2001 Nov 18, 17:24  9-rev    2,000      3  Australia, E. Asia
2001 Nov 18, 18:13  4-rev    8,000      3  W. Australia, E., SE & Central Asia
2002 Nov 19, 03:53  7-rev    3,000     15  W. Africa, W. Europe, NE Canada,
                                              NE S. America
2002 Nov 19, 10:29  4-rev   10,000     15  N. America
---LAST YEAR'S ESTIMATES---

And for reference, even older estimates previously given on this page are not deleted yet and were as shown in the table below. The main differences are (1) the data below were calculated before the 2000 Leonids (i.e., with 2000 and 2001 data unavailable for calibration); and (2) while it is known that gravitational perturbations or solar radiation pressure would not have caused these parts of these trails to become wider with time, the current estimates use an empirical aging parameter to allow for the possibility that other radiative forces can cause a slight dispersion with time. Radiative forces have been investigated by Lyytinen et al. Also, tiny uncertainties in the past orbit of Comet 55P/Tempel-Tuttle can lead to differences in calculated timings such as between the tables above and below. Thus timings are inherently uncertain at that level.

---THE YEAR BEFORE LAST'S ESTIMATES---
Time (GMT)          Trail  Estimated  Moon   Visible from
                              ZHR     age
2000 Nov 18, 03:44  8-rev      100?    22  W. Africa, W. Europe, NE S. America
2000 Nov 18, 07:51  4-rev      100?    22  N. America, Central America &
                                              NW S. America
2001 Nov 18, 10:01  7-rev    2,500?     3  N. & Central America
2001 Nov 18, 17:31  9-rev    9,000      3  Australia, E. Asia
2001 Nov 18, 18:19  4-rev   15,000      3  W. Australia, E., SE & Central Asia
2002 Nov 19, 04:00  7-rev   15,000     15  W. Africa, W. Europe, NE Canada,
                                              NE S. America
2002 Nov 19, 10:36  4-rev   30,000     15  N. America
---THE YEAR BEFORE LAST'S ESTIMATES---

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Last Revised: 2006 August 22nd WWW contact:webmaster@arm.ac.uk

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