Leonids 2003: What Happened
Despite expressions of disappointment from many visual observers, the 2003 Leonids appear to taken place largely as predicted. Enhanced numbers of meteors were seen at times corresponding to Earth's encounters with the 1499 and 1533 dust trails, with a possible contribution from the 1733 stream as well.
An optimistic scenario involving the broad dust component known as the Filament failed to materialize. It had been hoped that the Filament would contribute a population of bright meteors to produce a more visually appealing display.
Instead, each of the Leonid stream encounters between Nov. 13 and 23 yielded only relatively faint meteors. The reason lies in a non-gravitational force affecting small particles.
" In order for us to be intercepting these streams so long after comet Tempel-Tuttle passed through, the particles had to be retarded in their orbital motion," said Bill Cooke, a meteor expert with NASA's Marshall Space Flight Center in Huntsville, Alabama.
" The most important force holding them back is radiation pressure from sunlight, and that acts most strongly on the smallest particles," Cooke said. The smaller the particle, the fainter the meteor it creates.
The 2003 Leonids as forecast (red curve) and observed visually by members of the International Meteor Organization (blue points). Enhanced activity appears evident on Nov. 13–14, but the moon hampered observations and the 1499 peak was not resolved. A broad activity maximum on Nov. 19 may represent the combined effect of the 1533 and 1733 trails, but no sharp peak is seen. |
" The first maximum on Nov. 13 is particularly interesting," said Rainer Arlt, an astronomer with the Astrophysical Institute of Potsdam, Germany, and a member of the International Meteor Organization (IMO). The IMO coordinates and analyzes amateur meteor observations from around the world.
" During 'normal' Leonid times, nothing is seen then," he said. " The fact that enhanced rates were seen … might have a link to the predicted dust trail of 1499."
According to the IMO, this first Leonid encounter peaked with a Zenithal Hourly Rate, or ZHR, of about 20 meteors. Beyond that, Arlt noted, " I doubt much more can be said about the Nov. 13 maximum, since the moon destroyed a lot."
Meteor radars also observed the Nov. 13 peak. A preliminary analysis by Peter Brown of the University of Western Ontario, Canada, compared well with the IMO estimate.
Leonid activity (circled) stands out clearly in these plots of 38-Mhz radar meteor counts from the University of Western Ontario in Canada. Above: Nov. 13, 2003. Below: Nov. 19, 2003. The radiant of each detected meteor is plotted on an all-sky map and brighter colors indicate higher meteor counts. |
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The most widely anticipated Leonid outburst occurred on Nov. 19. The timing favored observers along the U.S. east coast, but the weather did not and clouds obscured most of the region. As a result, " too few reports have been received to resolve smaller features" of the event, noted Arlt.
Visual observations " are still producing a noisy picture," he said, but a clear maximum of between 40 and 50 meteors an hour occurred on Nov. 19 between 0h UT and 22h UT. Arlt suggested that the long-lasting maximum may be due to the combined contributions of the 1533 and 1733 trails, but observations are too sparse to reveal the individual peaks.
Preliminary radar results also indicate a ZHR of around 50 for the Nov. 19 peak, and a more thorough analysis will probably shed light on many details that visual observations cannot resolve.
" One could argue this was really a radar event," Cooke said, " and that has some bearing on how we'll perceive the next outburst all of the forecasters agree on, in 2006."
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