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(meteorobs) Where are the faint Leonids?
It is well-known that the Leonid simulations carried out by Asher,
McNaught, and others predicted an outburst of faint meteors on Nov 18, 1999,
2:08 UT. Indeed, a meteor storm with maximum eZHRs in the order of 5,000
was witnessed by numerous observers in Europe at the given time. Fireballs
were almost absent, and the majority of Leonids were of third or fourth
magnitude, thus, quite faint.
However, the preliminary analysis of video observations carried out by
member of the German AKM meteor group draws a slightly different picture.
At our observing site near Malaga/Spain (4 deg W, 37 deg N) two image
intensified video cameras were operated during the storm:
AVIS : 1.5/100 mm lens, fov = 15 deg, lm ~9 mag
CARMEN : 1.8/28 mm lens, fov = 35 deg, lm ~6 mag
The center of fov of both cameras was fixed at 54 deg azimuth and 43 deg
altitude.
The field of view of CARMEN was about 5.5 times as large as the fov of AVIS.
On the other hand, it's limiting magnitude was about 3 mag brighter. Given a
conservative low estimate of 2.0 for the population index of the Leonids at
the maximum one would expect that AVIS recorded (2^3)/5.5 = ~1.5 times
as much Leonids as CARMEN. However, on the contrary, the wide angle camera
recorded more than twice as much Leonids as the one with the better
limiting magnitude:
AVIS : 165 Leonids (01:00-03:00 UT)
CARMEN: 393 Leonids (01:00-03:00 UT)
The difference becomes even more prominent if we consider only meteors whose
begin (124 vs. 373 Leonids) or end point (89 vs. 325 Leonids) was inside the
field of view of the camera.
On the other hand, AVIS recorded about five times as much non-Leonid meteors
(90 vs. 19), which is in good agreement with an average r-value of 3.0 for
those.
How can this contradiction be explained? At www.imodot net/leo99 a graph
with preliminary brightness distributions of the video meteors is shown.
In contrast to visual observers, the detection probability of video systems
is almost one until about two mag above the limiting magnitude. Hence,
the cumulative meteor counts are almost complete for meteors <= 6 mag (AVIS)
and <= 4 mag (CARMEN), respectively.
The derived population index is near 3.0 for bright Leonids (0..2 mag), and
decreases to values below 2.0 for Leonids in the order of 3 mag, and to a
value below 1.5 for Leonids fainter than 5 mag.
From that it seems that smaller Leonid meteoroids are not that abundant
anymore, which is why starting at about 3 mag the number of Leonids
increases much slower towards fainter magnitude classes.
The second graph shows the detailed Leonid counts per minute, averaged in
sliding 3 minute intervalls. The activity profiles obtained from data
of both cameras show a number of distinct peaks of similar strength at 1:52,
1:58, 2:06, 2:12, and 2:18 UT. The 1:52 and 2:12 peaks seem to be most
prominent in both profiles. The times are not corrected for the topocentric
time of the stream encounter.
We would like to know about the results of other video observers to check,
whether our results are confirmed by other cameras. Please, send your video
observing data to video@imodot net. You can also contact me if you need help
for the inspection of your video tapes.
Sirko Molau
PS: At the given web address, you find also a nice sum image of 99 bright
Leonids recorded between 1:44 and 2:15 by CARMEN.
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Sirko Molau -- Video Commission Director
International Meteor Organization
e-mail: video@imodot net
WWW : http://www.imodot net/video
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