The 1999 Leonid Storm.

Mark Kidger

    Despite a great deal of understandable scepticism, an important Leonid storm was observed during the early morning of November 18th. From Tenerife, more than half an hour before maximum, a meteor per minute could be seen, even with the radiant below 10 degrees altitude. The resultant shower was as spectacular as it was unexpected for most of us.

2. Observations.

    I was observing from the south of the island at a site called "El Contador" close to the village of Arico, at around 1200-m altitude. Conditions were mediocre, with a limiting magnitude initially of 6.1, improving to 6.4 by the end of the watch. I recorded almost 800 Leonids in something over 3.5 hours and, although there were few very bright meteors, almost all Leonids brighter than magnitude +4 showed an enduring train. Other observers in other sites though had a very different experience and, although there were not so many very bright meteors as in 1998, many bright fireballs were seen. Just why Tenerife did not see any particularly bright meteors (this was the observational experience of at least three independent groups) is uncertain, although one expert has suggested that it was just due to "bad luck".

    These were the first observations of the Tenerife Meteor Group, formed of a group of meteor observing friends. The aim was to enjoy ourselves and do some really serious observing. We were joined at our observing site by Richard Miles and two other BAA members and by a group of three American amateurs, as well as two classes of secondary school children and their teachers. In the car down the TF-1 motorway I had commented to Oswaldo González, one of the members of the group that the low activity the previous night made me optimistic that there would be a storm (previously I had always been sceptical) - this prediction proved to be fortuitous.

    My own activity curve for the observation is shown below. It is very close to the IMO activity curve and shows a pronounced knee just before maximum which in higher time-resolution data is seen to be the double maximum discovered by IMO.

    When the results from Dulce Plasencia (IMO code - PLADU), Oswaldo González (GONOS) and Javier Sánchez (SANJA) are added in (I have not yet got round to adding the data from David Hernández (HERDA) to the plot). We see a fairly consistent activity curve. This was generated using a population index of r=2.5 however, for most intervals, except the majority of Javier Sánchez's observations, the naked eye limit was close to 6.5, so a change in "r" has little effect on the observed equivalent zenithal hourly rate. The peak EZHR from our observations was 4010+/-760 at 02:06 approximately. Note that different places met the centre of the meteor tube at different times so this may not agree with your local time of maximum (the differences are a few minutes from place to place on the Earth). The purple line in the plot is the mean of the data at each interval of time from 01:15UT to 04:45UT when observations stopped.

    The International Meteor Organisation (IMO) has produced several initial summaries of the observations that they have received. The plots below are based on their results (produced by Marc Gyssens who has been very helpful with comments and suggestions), my own data and data available on the Internet, particularly from the Nippon Meteor Society (NMS).

    The overall activity curve shows just how sharp the Leonid maximum was. The normal activity in the nights prior to and after maximum barely registers as a blip on the horizontal axis. Note that an important maximum (to 350 meteors per hour) was observed from Japan some 16 hours after the main maximum. The error bars are large because these are provisional results based on a relatively few observers and on a provisional data reduction.

    Looking at the maximum on an expanded scale we see various interesting features. There are two clearly visible peaks - the main one at 02:01UT and a smaller peak around 01:53UT, with a dip in activity between. This is clearly seen in the IMO data. From a personal point of view I have the satisfaction of seeing that my own data is very close to the IMO curve. The NMS data does not show up here as the period covered is only about 7 hours around maximum. There is a very high point at maximum, with a particularly large errorbar. Looking at the data, it looks like this high point at ZHR=5400 is an outlier and that the true maximum ZHR was about 4000, although the IMO points out that this point is already the mean of a number of observers.

    [Note added later (January 9th 2000): I was later proved right on this one - the final IMO maximum ZHR (see below) was just 3700. There was also no large peak observed at the maximum. In fact, there was no double peak either, most of the unusual structures in the provisional activity curve disappeared on re-analysis and on correcting for the local shift in maximum. This was caused by observers at different sites seeing the maximum at different moments in time as each site entered the centre of the stream. There was a difference of several minutes in the observed moment of maximum between, for example, Finland and the Canary Islands due to the time taken for the Earth to cross its own radius in its orbit].

    There is an evident dip in the data just before the maximum and a resultant double peak at 01:53 and 02:01UT. Richard Miles of the BAA team here in Tenerife suggests that this may be due to a gap in the Leonid stream rather than a genuine drop in activity. They registered gaps in the appearance of meteors of 26, 20, and 29 second interval at this time - when meteors were appearing at about one every 2-3 seconds - so that the gap is far above what would be expected statistically. The three gaps in the meteor activity occurred close together: the 29 second gap at 02:02:58UT, the 20 second gap at 02:03:37 and the 26 second gap at 02:04:24UT, which would explain why 5 minute counts show a sharp reduction in activity at this time. The IMO report suggests that there may have been a great deal of structure with several filaments of material giving brief maxima only a minute or less wide.

    The following plot shows an expanded scale in time from November 18.05-18.13UT (01:12-03:07UT), showing just the peak of the outburst. Note how the very high point looks inconsistent with the flat-topped maximum seen in the rest of the data.

    When we plot the data as the logarithm of the ZHR against time we see several components in the shower activity as shown below. We see the sharp narrow maximum of the storm component. This is superimposed on a broad maximum of rapid rise and slow decline which lasts for several days and is the broad component of activity. Sixteen hours after the main maximum observers in Hawaii and the NMS report a maximum to around ZHR=350 that lasted for 3 hours approximately. There is even evidence of another broad maximum to around ZHR=40 on the afternoon of November 16th in NMS data.

    The main maximum shows a doubling of activity every 21 minutes. This is typical of a storm. However, it is much slower than the usual rate of increase of Leonid activity. In 1866, 1867, 1966 and 1969 the doubling time was about 12 minutes. In terms of rate of increase per degree of solar longitude the previously mentioned Leonid outbursts had b=30; the 1999 storm shows B+=B-=19.4, thus the 1999 Leonid outburst was much broader than previous outbursts seen from the Leonids.

The 1999 Leonid outburst has confirmed the Asher-McNaught model of Leonid outbursts. Their model shows that the 1999 outburst came from material released from the comet in 1899. Weaker activity is expected in the year 2000 when the Earth will meet dust released in 1733 (as seen from Western Europe) and 1866 (from the Americas) although it is not certain that there will be a storm and there is severe Moon interference. However, the model then predicts a major activity in 2001 as dust trails released in 1866 and 1699 coincide for observers in the Far East. Western Europe may be lucky again, despite the Full Moon, in 2002 when the 1767 dust trail crosses the Earth’s orbit and a major storm from the 1866 return of Comet 55P/Tempel-Tuttle should be seen from the Americas.

3. Definitive IMO results:

    The International Meteor Organisation has reported the definitive results from 434 observers who reported 277172 Leonids in 10806 observing periods. The peak of the storm happened at a solar longitude of lambda=235.285°±0.001°, corresponding to November 18, 1999, 2h02m±2m UT with a peak equivalent ZHR of 3700±100.

You can find full report of results here.

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