Leonids: update to WGN 27, 85-102

R.H. McNaught and D.J. Asher (Last modified 1999 Nov 9)

Note to readers: if you do not have access to reference [1], please skip sections that are incomprehensible without it. Things such as the Table 3 predictions should be clear by themselves.

The original mail was sent by David Asher to the "imo-news" mailinglist on november 9, 1999


R.H. McNaught and D.J. Asher

In WGN 27, 85-102 (1999), details of the Earth's encounters with Leonid dust trails were presented. Moderately close encounters will lead to substantial meteor outbursts during the 1999 and perhaps the 2000 Leonids, while even closer encounters will produce storm level activity in 2001 and 2002. Here we summarise the predictions of the dust trail model for 1999 and the following few years, for the last time before observations of the 1999 Leonids afford the first test of these predictions. The updated analysis here is a little more comprehensive than presented previously.

  1. Introduction
  2. The highest ZHR Leonid storms of the 19th and 20th centuries, as well as many other sharp outbursts, have occurred when the Earth encountered a young dust trail within the Leonid stream. Such a trail is generated every 33 years or so when Comet 55P/Tempel-Tuttle returns to perihelion. Each trail progressively lengthens while remaining narrow and dense (density dilution being due to lengthening alone, not broadening) until it is scattered into the Leonid background after a few centuries. Readers should refer to [1] for further details.

    The predictive power of the dust trail theory is demonstrated by the following facts.

  3. Update
  4. In [1], a desire to avoid excessive effort led us to make our calculations comprehensive (covering the past 200 years) only for trails 6 or less revolutions old; three encounters with slightly older trails were considered as special cases. Also an error in the calculations caused very small inaccuracies (less than 0.0001 AU) in the determination of trails' nodal distances.

    Now we provide an updated list of encounters (covering the next few years only, these being of greatest interest for meteor observers) for trails up to 9 revolutions old. Changes to the ZHR fit due to the error are not substantial but a new fit is done.

    Table 1 shows the data for past trail encounters. The only corrections to the entries in Tables 2 and 3 of [1] are in r_E-r_D. Reference [1] can be consulted for more details, but in summary, the strength of an outburst is affected by Delta a_0 (which effectively corresponds to the ejection velocity, this in turn being related to the mass distribution), r_E-r_D (the miss distance of the Earth from the trail node) and f_M (which measures the change in density due to trail lengthening). Table 2 is for encounters over the next few years (cf. Table 5 of [1]). Roughly, f_M is expected to be inversely related to the age of the trail, but for example, the value for the part of the 9-rev trail that is encountered in 2001 shows that gravitational perturbations can cause deviations from this simplistic relationship, after a few revolutions.

    Table 1 - Past trail encounters
                                                    Observed  Calculated
    Year  Trail   Node   Delta a_0  r_E-r_D   f_M    ZHR/f_M   ZHR/f_M
                 (J2000)    (AU)      (AU)
    1966  2-rev  235.158   +0.168  -0.00013   0.52   170,000   100,000
    1833  1-rev  233.184   +0.174  -0.00021   0.95    63,000    76,000
    1866  4-rev  233.333   +0.059  -0.00029   0.37    22,000    22,000
    1867  1-rev  233.420   +0.373  -0.00014   1.00     4,500     4,600
    1869  3-rev  233.536   +0.320  -0.00047   0.44     2,300     2,200
    1969  1-rev  235.272   +0.934  -0.00004   0.95         -         -
    Table 2 - Future trail encounters
    Year  Trail   Node   Delta a_0  r_E-r_D   f_M
                 (J2000)    (AU)      (AU)
    1999  3-rev  235.291   +0.138  -0.00066   0.38
    2000  8-rev  236.103   +0.064  +0.00076   0.27
    2000  4-rev  236.276   +0.114  +0.00077   0.13
    2001  7-rev  236.114   +0.081  -0.00043  ~0.14
    2001  9-rev  236.429   +0.041  +0.00015   0.43
    2001  4-rev  236.463   +0.142  +0.00022   0.13
    2002  7-rev  236.610   +0.113  -0.00015   0.13
    2002  4-rev  236.888   +0.172  -0.00005   0.15
    2006  2-rev  236.615   +0.961  -0.00009   0.53

    A model in which ZHR/f_M (ZHR being the observed peak ZHR in past encounters) is fitted as a function of Delta a_0 and r_E-r_D, as described in [1], is done, and applied to the future years. The five storm years in Table 1 (1969 excluded) are used in the fit, to interpolate ZHR estimates for 1999-2002. It is inappropriate to apply exactly the same model to very different values of Delta a_0 and so 1969 alone is used to predict 2006 alone. The predictions are in Table 3 (cf. Table 6 of [1]). Only the fit centred at r_D (cf. Tables 4 and 9 of [1]) is given, the formal uncertainty in the fit being 20%. Whilst the overall fit is reasonable, there is now a discrepancy between the calculated ZHR values for 1833 and 1966. For 1966 the calculated ZHR is 53,000. Despite the uncertainty in the observed ZHR in 1966, it is probably one of the more reliable data points used in the fit and 1833 the least reliable. In 1999, the formal ZHR prediction is 500 and this appears fairly robust, regardless of whether the 1833, 1966 or both are used in the fit, but values 200

    Table 3 - Predictions
    Time (UT)           Trail  Estimated  Moon   Visible from
                                  ZHR     age
    1999 Nov 18, 02:08  3-rev      500     10  Africa, Europe
    2000 Nov 18, 03:44  8-rev       30?    22  W. Africa, W. Europe, 
                                               NE S. America
    2000 Nov 18, 07:51  4-rev       20?    22  N. America, Central America
                                               &  NW S. America
    2001 Nov 18, 10:01  7-rev    1,500?     3  N. & Central America
    2001 Nov 18, 17:31  9-rev   15,000      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, 
                                               N. Canada, NE S. America
    2002 Nov 19, 10:36  4-rev   25,000     15  N. America
    2006 Nov 19, 04:45  2-rev      100     28  W. Europe, W. Africa

    Figures 1 and 2 are the visibility maps that are not in [8]. Note that in these figures, the Moon's phase is displayed as seen from the southern hemisphere.

  5. Discussion
  6. Revised values of the nodal distance required a reassessment of the ZHR predictions from the dust trail density model. Overall, the rates have not changed substantially, although the uncertainty in the fit has increased. For 1999, the predicted ZHR for the 3-rev dust trail encounter is probably of the order of 500. This value requires some elaboration. Activity from this dust trail will be additional to background activity, which could itself have a ZHR in the hundreds. Thus, the observed ZHR at the time of the peak will probably lie in the range 500-1000, if the dust trail contributes a ZHR of 500. This value is entirely consistent with past data, given that there are uncertainties in the past peak ZHRs used in the fit, and different fits can be done (cf. [1]) centred on slightly different values of r_D.

    Given that some older trails have been demonstrated to be capable of delivering high rates (e.g. the 9-rev trail of high f_M in 2001), it will be necessary to check that for the years of the storm data used in the ZHR fit, no additional old dust trails were contaminating the ZHR. In future analyses, we shall also remove the background component from the peak ZHRs, to more truly represent the contribution of the dust trails alone.

    One interesting change to the ZHR fit, is that the predicted ZHR in 1801 from the 2-rev trail is now 300. This is much smaller than our original predictions that suggested a minor storm had occurred. Thus, this potential anomaly of an unobserved storm in the last 200 years over western Europe, is no longer a problem. A short lived peak ZHR of around 300 would probably not have attracted much attention in those years. However, we are aware of no data that refute the possibility of a storm in that year.

    There are no additional encounters with trails up to 19 revolutions old in 1999; therefore, unpredicted high activity is unlikely. There appear to be no other encounters of significance in the following years up to this age, although in 2001, an encounter with a disrupted 10-rev trail of uncertain density could produce a peak ZHR of around 1,000 on Nov 18, 18:01 UT. Although the disrupted nature of this section of the 10-rev trail makes this time unreliable (pending more detailed simulations), it appears to be during the 48 minute gap between the stronger encounters. These three encounters in close succession, with no interference from the Moon, suggest the highest observable rates will occur in 2001, although the ZHR in 2002 is likely to be higher.

    For the 3-rev trail encounter in 1999, the time of maximum is predicted to be at Nov 18 02:08 UT in the Mediterranean region, with an uncertainty of around 5 minutes. The time of maximum is dependent on location [2], with the peak predicted at 01:58 in South Africa and 02:14 in northern Scandanavia. The dust trail model does not make any prediction about the time or intensity of the background activity maximum, but in the past 200 years the highest Leonid rates outside young dust trail encounters, can approach a ZHR of 500.

    Further information is available in [9].

  7. References
    1. R.H. McNaught, D.J. Asher, `Leonid dust trails and meteor storms.' WGN 27, 1999, pp. 85-102.
    2. R.H. McNaught, D.J. Asher, `Variation of Leonid maximum times with location of observer.' Meteorit. Planet. Sci. 34, 1999, pp. 975-978.
    3. R. Arlt, `Bulletin 13 of the International Leonid Watch: The 1998 Leonid meteor shower.' WGN 26, 1998, pp. 239-248.
    4. E.A. Reznikov, `The Giacobini-Zinner Comet and Giacobinid meteor stream.' Trudy Kazan. Gor. Astron. Obs. 53, 1993, pp. 80-101 (in Russian). See also IMO-News mailing list, 1998 Sept 9.
    5. E.D. Kondrat'eva, E.A. Reznikov, `Comet Tempel-Tuttle and the Leonid meteor swarm.' Sol. Syst. Res. 19, 1985, pp. 96-101.
    6. E.D. Kondrat'eva, I.N. Murav'eva, E.A. Reznikov, `On the forthcoming return of the Leonid meteoric swarm.' Sol. Syst. Res. 31, 1997, pp. 489-492.
    7. E. Lyytinen, `Leonid predictions for the years 1999-2007 with the satellite model of comets.' Meta Res. Bull. 8, 1999, pp. 33-40.
    8. R.H. McNaught, `Visibility of Leonid showers in 1999-2006 and 2034.' WGN 27, 1999, pp. 164-171.
    9. Armagh Observatory Leonid WWW pages are http://www.arm.ac.uk/leonid/ and general notes for the public are at http://www.atnf.csiro.au/asa_www/leonids.html

  8. Acknowledgments
  9. DJA thanks Esko Lyytinen for extremely valuable discussions on this work.

  10. Authors' addresses:
  11. Robert H. McNaught, Siding Spring Observatory, Coonabarabran, NSW 2357, Australia (rmn@aaocbn.aao.gov.au)

    David Asher, Armagh Observatory, College Hill, Armagh, BT61 9DG, N. Ireland, UK (dja@star.arm.ac.uk)

The original mail was sent by David Asher to the "imo-news" mailinglist on november 9, 1999

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