J. Brit. Astron. Assoc., 109, 5, 1999, pp.237-238
For meteor observers, the long wait is almost over! The Leonids have been building in strength since 1994, and following the February 1998 perihelion return of their parent comet, 55P/Tempel-Tuttle, produced spectacular activity last November 16-17. Observers in western Europe, including a great many of the Meteor Section's regular contributors, witnessed a fine display of Leonids in the early morning hours before dawn on Nov 17, with rates of several meteors per minute in short bursts. The sustained 'plateau' of high activity was marked by an abundance of bright Leonids, many of which lit up sky and ground, and left behind long-duration persistent trains.
The early arrival of the best activity in 1998 took many by surprise. The 'fireball' peak came as much as 16 hours ahead of the expected maximum, which had been anticipated to come at the time of Earth's closest approach to the descending node of the meteor stream orbit on the evening of Nov 17-18. Closest approach was, in fact, attended by only a relatively small enhancement in Leonid rates: no storm was seen to rival those recorded in the past, and, indeed, observers at the ostensibly more favourable Asian longitudes enjoyed rates only about a third those of those found over Europe the previous morning, and with few bright meteors in evidence.
Modelling of the stream by several professional workers in recent months has led to the conclusion that we ran through a concentration of larger meteoroids ahead of node-passage in 1998. This material, ejected from the comet several revolutions ago, is held in a more concentrated cloud by gravitational effects resulting from resonance between the comet/stream orbit and that of Jupiter. In other streams, such meteoroids would long-since have dispersed.
The Leonid storms seen through history result from smaller debris in a narrow filament extending downstream from the comet. Again, modelling has led to the prediction that this cannot be encountered until after the comet's perihelion. Another critical factor in these encounters is just how closely the comet's orbit approaches that of Earth. Gravitational push and pull by the planets means that some passages are closer than others. At the present epoch, closest approach is about three times more distant than it was in 1966, which may reduce the likelihood of a full-blown storm to rival those of the past. Nonetheless, it is still probable that we will experience major activity from the Leonids close to the time of node-passage in 1999. Past experience suggests that the 'storm' peak, if we see one, will be short-lived - perhaps only lasting for an hour or so at the very most. Activity should be at least as high as that of the Perseids at maximum in an average year for some hours to either side, and watches on the Wednesday night to Thursday morning of November 17-18 are likely to be productive whatever transpires. After last year's surprise early activity, serious observers will need no further reminders that watch data from the nights to either side are also of importance!
The shower circumstances are very favourable for western European observers this year. Closest approach to the stream node comes at solar longitude(2000) = 235°.26, around 1999 Nov 18d 02h UT. If a storm or 'grand display' is to occur, it should come within a few hours of this time. The Leonid radiant rises around 2240 local time from UK locations: observers are advised to be ready to start their watches no later than this time. An early onset of high activity may result in observers being treated to a display of long-pathed (near-grazing) meteors early in the night when the radiant is low. By the time of closest approach to the meteor stream, the radiant will be well up in UK skies, continuing to climb until dawn.
Moonlight is slightly more of a consideration than it was last year. The Moon is eight days old on November 17, and will not set until about 0030 local time. If the Leonids produce an outburst at the forecast time, the Moon will have set, leaving dark skies. Certainly, the best dark sky conditions for observing should be found in the post-midnight hours. The storm peaks of the past have seen a fair number of bright Leonids, but most of the activity is produced by smaller meteoroids. It seems unlikely that the abundance of fireballs enjoyed last year will recur in 1999. Rather, we can expect to see any major outburst being comprised largely of faint meteors with a smattering of bright events. To either side, the 'background' component of the Leonids probably will show a higher proportion of bright events. Rates permitting, observers should pay careful attention to Leonid magnitudes during watches this November.
The possibility of exceptionally high activity begs the question of accurate counting. The harsh reality is that few, if any, of the current generation of meteor observers - even the most experienced among us - have experience of observing under full-blown storm conditions. If rates climb past about 20 per minute, we will probably be into the realms of guesswork! Low-light video observations being planned by several groups worldwide may hold out the best hope of getting reliable counts in the event of extremely high Leonid activity. Timed photographic exposures of precisely one minute's duration offer the chance to assess bright meteor activity during any outburst. Similarly, visual observers may attempt counting the numbers of Leonids in restricted areas of sky - the 'Bowl' of the Plough, say - during timed one-minute intervals. Fuller observing instructions are available from the Meteor Section; details are available on the Section pages at the BAA Internet site. Reports should be sent to the Director at the address on the inside cover of the Journal as soon as possible after the event.
Whatever happens in 1999, the Leonids have already left a big impression on meteor observers in the late 1990s. The 1998 return provided a most memorable observing night for those of us fortunate to have seen them at their best in the pre-dawn hours of November 17. Clear skies and a major display in 1999 would be the icing on the cake.
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