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Leonids: 1998
What follows is a brief synopsis of observations of the 1998 Leonids. They started out as "Alerts" as I tried to keep everyone posted on the much earlier than expected startup of the Leonids. Following my own observations, and the resulting reports of numerous other people during the week following the display, I put this summary together for the American Meteor Society on November 25.
Leonid Summary
The 1998 Leonid meteor shower was generally predicted to reach storm potential around 19:43 UT on November 17, the time of Earth's closest approach to comet Tempel-Tuttle's orbit. The International Meteor Organization published extensive charts and predictions of Leonid rates for observers around the world which generally indicated hourly rates would be above 10 per hour for 12 hours before and after maximum. With this in mind, the Author closely monitored observer reports beginning on November 14 looking for the moment of the re-awakening of the Leonids. The dates and times given in the following summary are all given in Universal Time (UT).
Leonid observations on November 14 and 15 were not widely made, but observers were consistent in giving estimates ranging from 0 to 7 per hour. Observations were very scarce at the beginning of the 16th, but Mark Mikutis of Kellogg, Iowa, observed from 10:15 to 12:15 and recorded 42 Leonids. Exactly 20 of these were of negative magnitude with the brightest being -4. An independent confirmation of Mikutis' observations was actually being made by Robert Lunsford and George Zay at Descanso Observatory in California. Lunsford observed from 9:15 to 13:25 and recorded 89 Leonids while Zay observed from about 6:00 to 13:30 and recorded 77 Leonids, most of which were seen after 8:00. Both observers noted a large number of fireballs and each recorded some brighter than magnitude -10. As twilight ended observations in the United States, it was obvious the Leonids were already producing 10 to 20 meteors an hour.
This early start-up of the Leonids caught observers off guard, and as the radiant rose over the Pacific Ocean few observations were made. Robert McNaught of Australia was observing between 17:00 and 19:30 and although he provided no hourly counts he did note "High rates of Leonid fireballs...with about half the Leonids of negative magnitude." He noted several brighter than magnitude -7 with one at magnitude -14.
Numerous east Asian observers were active by the time the Leonids made an appearance in their skies. Although over 60 reports were available to the Author at the time of this writing, only 12 individuals gave hourly rates based on actual counts. These individuals observed in China, Japan, Korea, and Thailand and mostly covered the hours of 19:00 to 22:00. Hourly rates were then generally given as 100 to 110, with extremes of 70 to 140.
Observers in India reported seeing fireballs, but cloudy and hazy skies prevented any accurate counts. The earliest counts came from Jordan and Israel where observers consistently counted over 160 meteors per hour during the period of 21:00 on the 16th until 03:00 on the 17th. Most impressive, however, was the fact that rates were typically between 200 and 250 for the period of 01:00 to 03:00.
More than 100 observations are available from Europe, but where the observations from the United States, through Asia, and into the Middle East generally indicated a steady increase in rates, the European observations were all over the place. These observers ultimately indicated a range of 80 to 600 meteors per hour! The highest rates were not based on actual counts, but were rough estimates or group counts. As discussions in Lew Gramer's meteor discussion list proved during the week following the display, estimates tended to be greatly exaggerated for the Leonids. Meanwhile, group counts do not enable an accurate determination of a meteor shower's activity rates because of the potential for some meteors to be counted more than once. One group count came from the Observatoire de la Cote d'Azur in France where it was said that some 300 meteors were seen during the 45 minutes following 04:35. Astronomer Brett Gladman observed with that group and e-mail from him indicated an individual observer's rate would probably have been one-third that total, or about 100 meteors. Since this was a 45-minute interval, this indicates an hourly rate of 133. Several experienced Italian amateur astronomers were observing during the period of 23:00 to 03:30 and obtained accurate counts. The best skies were present for Stomeo Enrico on Mt. Tomba and his highest rates were near 125 per hour around 00:00 and 01:00. A little later, Alfredo Pereira of Cabo da Roca, Portugal, observed from 03:10 to 05:46. His highest rates were 90 per hour during the interval of 05:08 to 05:36.
The last European observers to watch the Leonids were on the Canary Islands. Accurate 10-minute counts were obtained by Maximum Suárez, Fracisco Rodríguez, and Victor R. Ruiz. Their highest 10-minute totals came after 05:00 when counts reached 50 or more. Ruiz said the 40 minutes following 05:17 indicated the hourly rate was over 300 per hour. Other rates were reported from the Canary Islands, but these were group rates made to try and derive an all-sky total. The rates of 1000 and 2000 per hour published on International Astronomical Union Circular 7052 were derived from 2-minute intervals. Ruiz said the high rate of fireballs was continuing as he logged over 24 of magnitude -6 or brighter. The Canary Islands are known for their transparent skies and the reported limiting magnitudes were better than those reported in Italy and Portugal. Therefore, although the above European accounts indicate a peak for the longitude running through Israel and Jordon and another peak for the Canary Islands, it is quite possible that rates across Europe remained above 200 per hour for several hours for people observing under very transparent skies.
By the time the radiant was observed over the East Coast of North America, rates were off from the European totals. Observers generally submitted reports within the time frame of 05:20 to 11:05. The most easterly reports came from observers in Nova Scotia. They observed from 05:50 until 08:45 and obtained 15- and 30-minute counts. Their highest counts indicated hourly rates of 86 to 164. By the time the radiant could be seen by observers in the United States the rates seemed to have declined still further, with reports from Delaware, New York, New Jersey, Maryland, Virginia, South Carolina, and Florida giving rates of 50-70 per hour. There was also a noticeable decline in the number of extremely bright meteors.
Hourly rates were basically unchanged for observers in the Midwestern United States. My group in Illinois consistently estimated rates of 45-60 per hour during the middle of our 7 hour watch. The highest hourly rates received so far from Midwestern observers was near 70.
Interestingly, rates seemed to jump again for observers in Oklahoma, Texas, Utah, and Arizona. Unfortunately, the true nature of the increase can not be fully realized because many of the submitted counts were by groups of people. However, two independent reports from the Davis Mountains in west Texas did provide half-hour and hourly counts indicating rates of 120 or more during the period of 07:00 to 09:00. By the time California and Oregon came into view, the rates had again dropped, with most observers indicating hourly rates of 50-60.
The period of 12:02 to 15:35 was covered by veteran meteor observer George Gliba in Hale Pohaku, Hawaii. Under extremely transparent skies he saw maximum hourly rates around 110.
Since the maximum was predicted for 19:43, Asian observers eagerly awaited the radiant's appearance in the morning sky. But they quickly realized the expected storm was not coming. Although one estimate exists indicating an hourly rate of about 100, most observers indicate rates of 20-40 per hour. All of these reports noted a lack of bright meteors as compared to the previous morning.
By the time the radiant was again observable in Europe many observers failed to see more than 10 meteors per hour.
In summary, it would seem the 1998 Leonids surprised the experts by occurring between 14 and 19 hours early. The resulting maximum occurred over Europe and seems to have persisted for at least five hours. Unfortunately, the decline of the maximum was missed as Canary Island observers indicated no sign of weakening as morning twilight began.
Read about the Leonids of 1999
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