Leonids 1994: Bay Area, report I

High Leonid activity November 17/18 and 18/19 - Part I by Peter Jenniskens

The Bay Area has had fortunate weather conditions during this years Leonid return. Clear skies prevailed in the period 15/16 until Nov. 19/20 except for the period between 8 and 14 UT on Nov. 17, when we attempted to photograph the Leonids from three sites (we includes Tom & Ingeborg Rice, Rick Morales, Duncan McNeill, Frank Dibbel, Kari Salomaa, Kathy Black, and Mike Wilson). Unfortunately, that night was spoiled by a continuous cloud cover and occasional rain showers. Further south, Bob Lunsford (San Diego, CA) saw 5, 1, 6 and 14 Leonids in periods of one hour at Lm = 5.1. The next night, November 17/18, was clear except for some scattered clouds in the beginning of the night. A large workload allowed me to start observing only at 12:34 UT. I immediately saw two bright Leonids in the first two minutes. A regular watch started at 12:40 UT and I continued until twilight set in at 14:00 UT. The sky conditions were poor because of a full moon and also because I was observing from within the city of Mountain View. I estimated the limiting magnitude at 5.1, rising to 5.3 at the end of the observing period. These are my raw counts:

The minus 2 or so meteor had a typical Leonid-like appearance: a symmetric light curve with a flare in the middle. The meteor appeared in the zenith and had a persistent train that was visible in the full moon glow for about 4 seconds. The meteors radiated from a point in the head of Leo, at about RA = 151, DEC = +21 (2000.0). I did not spot a meteoric glow in the direction of the true radiant (note however: bright sky background). The rate of Leonids was surprisingly high and continuous during the observations. A message was spread two hours after the event, hoping that other observers might confirm the high rates. The night after this event, Nov. 18/19, was clear again. I observed from a location outside the city, mainly to establish a comparisson of sky conditions with a similar near full moon. The sky appeared clearly more transparant. Indeed, I estimated a limiting magnitude of 5.6-5.8 which probably compares well with the 5.1-5.3 from downtown Mountain View. The sporadic rate was also a bit higher. The Leonids were still active:

Here are the ZHRs calculated for the data available to me on the eve of Nov.19:

These are preliminary results. I assumed x = 2.15 for 17/18, x=2.35 for 16/17 and 18/19. I have taken y=1.4 in radiant altitude correction: sin(hr)^-y. I assume a perception of 1.0 for both observers. I conclude that the activity in the night November 17/18 was a factor of 9 higher than normal (ZHR = 9 - ref.A&A 287,990 1994), while rates on 16/17 were close to normal (ZHR about 23). The rate in the night 18/19 was still a factor of three higher than normal (ZHR = 3). This suggests that the activity extended for more than a day. This is consistent with the observations prior to the return of the comet in 1966. However, this year the time of maximum seems to have been after passage of the node of the comet, which is at solar longitude 234.55-234.58.
Tentative confirmation of high Leonid rates comes from a visual observer in Spain (ZHR close to 100 at 235.0 - IMO) and from meteor scatter data by Peter Bus, The Netherlands (- data DMS). No details available yet. These observations provide the first indication of the comet's return to perihelion. The high activity classifies as a meteor outburst, which is likely the first in a series of many to come.

Dr. Peter Jenniskens

This message was prepared on behalf of the Dutch Meteor Society.

My address:

NASA/Ames Research Center
Mail Stop 239-4
Moffett Field
CA 94035-1000.
work: 415-6043086
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Casper ter Kuile and Peter Jenniskens