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Rob McNaught and David Asher's final comments before the 1999 Leonid meteor shower, as shown below, are being sent to the IMO-News e-mailing list, after which the authors are setting off on their travels to observe the Leonids. The authors will reappear after the Leonid shower if the predictions do not turn out to be substantially wrong.- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - Note to readers: if you do not have access to reference , please skip sections that are incomprehensible without it. Things such as the Table 3 predictions should be clear by themselves. Last modified 1999 Nov 9 LEONID DUST TRAILS AND METEOR STORMS: UPDATE 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 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  for further details. The predictive power of the dust trail theory is demonstrated by the following facts. Storms and outbursts over the past 200 years that correspond to the Earth's encounter with a given young dust trail have the calculated peak times (i.e., based on the centre of the Earth reaching the calculated nodal longitude of the trail) and the observed peak times matching to 10 minutes or less , in cases where the observed peak is known to better than that accuracy. A topocentric correction improves the match even further . Data from the past 200 years now indicate that close encounters are predictable with an uncertainty of 5 minutes. The peak time of the 1998 Draconid outburst  was predicted equally accurately by Reznikov  using the same form of dust trail calculations. Moreover, Leonid timings relating to what we term young trail encounters have been independently confirmed by the Russian group that includes Reznikov [5,6] and by Lyytinen . See  for references by the same authors describing work on other streams. 2. Update In , 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  are in r_E-r_D. Reference  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 ). 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 , 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 ). Only the fit centred at r_D (cf. Tables 4 and 9 of ) 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 < ZHR < 2000 give a reasonable fit. 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 . Note that in these figures, the Moon's phase is displayed as seen from the southern hemisphere. Figure 1 (See http://www.atnf.csiro.au/asa_www/images/2001csl.gif [15Kb]) Figure 2 (See http://www.atnf.csiro.au/asa_www/images/2002bsl.gif [15Kb]) 3. Discussion 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. ) 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 , 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 . References  R.H. McNaught, D.J. Asher, `Leonid dust trails and meteor storms.' WGN 27, 1999, pp. 85-102.  R.H. McNaught, D.J. Asher, `Variation of Leonid maximum times with location of observer.' Meteorit. Planet. Sci. 34, 1999, pp. 975-978.  R. Arlt, `Bulletin 13 of the International Leonid Watch: The 1998 Leonid meteor shower.' WGN 26, 1998, pp. 239-248.  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.  E.D. Kondrat'eva, E.A. Reznikov, `Comet Tempel-Tuttle and the Leonid meteor swarm.' Sol. Syst. Res. 19, 1985, pp. 96-101.  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.  E. Lyytinen, `Leonid predictions for the years 1999-2007 with the satellite model of comets.' Meta Res. Bull. 8, 1999, pp. 33-40.  R.H. McNaught, `Visibility of Leonid showers in 1999-2006 and 2034.' WGN 27, 1999, pp. 164-171.  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/info_sheets/leonids.html Acknowledgments DJA thanks Esko Lyytinen for extremely valuable discussions on this work. Authors' addresses Robert H. McNaught, Siding Spring Observatory, Coonabarabran, NSW 2357, Australia (email@example.com) David Asher, Armagh Observatory, College Hill, Armagh, BT61 9DG, N. Ireland, UK (firstname.lastname@example.org)
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