J. Brit. Astron. Assoc., 109, 1, 1999, p.3
A spectacular Leonid fireball at 04.32 UT on the morning of 1998 November 17. A 60s exposure with an MX5 CCD camera and 12mm lens. T. C. Platt/The Astronomer
The past year has been one of many surprises for meteor observers, starting with an unexpected 'revival' of the Pons-Winneckids (presumed extinct since the late 1920s) on June 27-28, when rates of roughly one per minute were seen throughout the night. A minor outburst was seen from the periodic Giacobinid shower around 13-14h UT on October 8 by observers in the Far East, with rates of 50-100/hr coming some eight hours ahead of forecasts. Observers in the British Isles later reported rates of one or two Giacobinids per hour, rather lower than the popular media would have led us to expect. The major attraction, however, was the return of the Leonids around November 17.
As outlined in the October Journal, expectations were high that the 1998 Leonids, quite closely following the perihelion return of the stream parent (Comet 55P/Tempel-Tuttle), would produce exceptional rates, and perhaps even a storm. Very much enhanced activity was, indeed, seen - unexpectedly early, in the morning hours of Tuesday November 17, and favourably for the UK.
For once, the mid-November weather proved reasonably cooperative for observers in the British Isles. High pressure to the east maintained a cold northerly airflow, bringing reasonably clear skies for a couple of nights around the time of maximum, at least on the eastern side of the country. Some observers were able to put in good watches under the best skies for the period in the post-midnight hours of Nov 15-16, finding Leonid activity present at a modest level of 3-5/hr.
The bulk of UK observations come from Nov 16-17, which was cold and frosty at most locations. Thin, patchy cirrus cloud was a problem for many, while - particularly in the Midlands - fog caused some difficulties. Nevertheless, those who were patient and waited for the cloud present around midnight to clear in the early morning hours, and heeded the Meteor Section's admonitions to go out and use whatever clear sky materialised on the nights to either side of, as well as on, the expected Leonid maximum, were rewarded with the finest meteor display most of us have seen. East Anglia seems to have had the best skies around midnight UT on Nov 16-17. Even this early, with the radiant low in the eastern sky, it was obvious that the Leonids had become much more active. Bursts of several meteors in quick succession were noted, and many of the events were bright with persistent ionisation trains. Several observers have reported a couple of notable fireballs, at 0015 and 0051 UT, each of which left a long-duration train which distorted in upper-atmosphere winds over the course of a few minutes.
As the night went on, this activity continued unabated. Typical rates for a single, experienced observer under clear skies were of the order of 80 Leonids/hr, up to perhaps as many as 120-140/hr. Bursts of eight meteors in a minute, with events often appearing simultaneously or in very quick succession, were common. Provisional corrected Zenithal Hourly Rates come out around 230-250 throughout the night (more than ten times the Leonids' 'quiet-time' 1980s peak level, and three times that for a typical Perseid maximum). Reports from overseas suggest that this level of activity was evident from as early as Nov 16d 17h UT, and continued until dawn broke over the western United States around Nov 17d 12h UT.
Not only was the activity high, it was also marked by a pronounced abundance of bright events as shown in the accompanying histogram, which compares Leonid and sporadic magnitude estimates from experienced observers. From 2850 Leonids and 107 sporadics, respective rough mean magnitudes of 0 and +2 are found. Most showers have a mean closer to mag +1.5-2.0. Many observers commented on a dearth of Leonids fainter than about mag +3.
There were numerous exceptionally bright fireballs. Another facet of the night's activity on which many have commented was the frequent occurrence of flashes produced by events outside the field of view, often lighting up sky and ground. A particularly major event occurred at 0345 UT, being recorded from Scotland, Devon, Berks. and the east Midlands. Several of the brighter meteors have been photographed. Steve Evans and his collaborators managed to obtain a lot of data for detailed positional analysis and triangulation. As part of this work, Andrew Elliott at Reading ran his low-light video system, capturing numerous fireballs and their persistent trains, and also confirming several out-of-field flashes. Terry Platt's wide-field CCD imagery recorded a very impressive Leonid fireball at 0429 UT and the subsequent dispersal of its long-duration train.
For observers in the south of England, probably the single most impressive event was a mag. -8 to -10 fireball at 0459 UT. From Chichester, West Sussex, the Director saw this close to the radiant. The meteor ended in a huge sky- and ground-illuminating flash, leaving behind an intense, knotted train with a couple of spiral loops. This remained visible to the naked eye for the next 18 minutes! Over time, the initial bright core spread out to form a loop - like a 'close-up planetary nebula' - gradually moving eastwards from Gamma Leonis towards Denebola. A few miles away in Selsey, John Randall had the presence of mind to swing a camera onto the train, recording a magnificent series of colour 30-second exposures of its development. At least 46% of the Leonids recorded on this night left behind persistent trains. Most were of a few seconds' duration, but there were many whose fade took several minutes.
The high activity plateau on Nov 16-17 arrived earlier than anyone had forecast. The expectation had been that we might see activity begin to build around dawn (for western Europe) on Nov 17, with a broad span of Perseid-level activity surrounding a sharp peak around closest passage to the stream orbit's node around Nov 17d 20h UT. Observers who travelled to Asian longitudes - including a party in India, led by the Section's Assistant Director John Mason - found lower rates at this time than had been seen 16 hours earlier! Leonid activity had dropped from the plateau to about 40/hr by the evening of Nov 17 - far from the storm or 'grand display' rates which many had hoped for, but still above the levels found in a more normal year.
The night of Nov 17-18 proved frustrating for those of us back in Britain, too, with cloud and patchy rain afflicting most areas except East Anglia and Kent. Where skies were clear, rates of 10-15 Leonids/hr were found after midnight, clearly further diminished from those seen by observers in Asia. Bright events were also much less common on this night. A handful of reports show Leonid activity dwindling away up to Nov 19-20.
In light of the unexpectedly early, high activity on November 16-17, many are now revising their forecasts as to whether there will be a major display in 1999. I remain confident, however, that we should see at least a grand display around the time of node-passage (roughly Nov 18d 02h UT). The very high activity outbursts are associated with narrow filaments of material within the more extensive near-comet debris cloud. Historical studies suggest that the 'storm' filament lies outside 55P/Tempel-Tuttle's orbit, and will be encountered increasingly late in numbers of days behind the comet with increasing distance between Earth and the node at the time of closest approach. In 1998 we did not arrive far enough, in days, behind the comet to encounter this filament. A broadly similar situation obtained in the 1960s, such that although Earth passed closest in days behind the comet to the node in 1965, the highest activity was actually a year later. The volume of near-comet space through which we passed in 1998 contained mainly larger meteoroids, ejected by the comet several revolutions ago; the still more abundant, smaller material probably lies further downstream in the region we should pass next November 18.
Overall, 1998 must go down as perhaps the most successful observing run the BAA Meteor Section has ever enjoyed for the Leonids. Up to the time of writing, in early December, more than eighty sets of results had been received, amounting to many thousands of meteors. Time has not yet permitted a detailed formal analysis; this will have to wait until the spring when all the data have been gathered together. There can be little doubt, however, that many will be talking about the morning of 1998 November 17 - the 'Night of the Fireballs'! - for a long time to come.
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