Leonids 1994: Bay Area, report II

High activity of Leonids on November 17/18 and 18/19 - Part II by Peter Jenniskens

By Nov. 20th, details became available of the meteor scatter observations of Peter Bus (Groningen, The Netherlands), and the visual observations by Spanish observer Josep M. Trigo from Castellon (Spain). The analysis of the raw data is still preliminary. However, the raw data itself do confirm high activity of Leonids on Nov. 17/18.
The radio-MS data by Eisse Pieter Bus were forewarded to me by Hans Betlem of the Dutch Meteor Society. There is no clear increase of numbers visible in these counts. However, the reflections on Nov. 17/18 lasted typically much longer than those on the other nights. The total dead-time (in minutes) is given below:

The receiver was listening to a radio station in Poland and the geometry antenna-receiver was most favourable around 6:30 UT. Later in the observing run not only the geometry became less favourable, but also the Leonid radiant altitude decreased. Peter Bus calculated the reflection rates below, assuming an activity of zero at 18/19, which counts have been subtracted. This analysis is preliminary and awaits a more careful reduction of the data. The table lists the calculated rate and between square brackets the number of reflections on which they were based. Values between normal brackets are uncertain because of a low radiant altitude.

The feature that stands out in this table is that the number of strong reflections decreases in the night 16/17 with a decreasing radiant altitude and less favourable antenna geometry, but remains constant on Nov. 17/18. The result is an increase of rates during the night 17/18 after correction for antenna geometry. This effect is only strengthened when also a radiant altitude correction is applied. The radio data, therefore, show an increase in rates between 6 and 13 UT.
The visual observations by Josep M. Trigo were kindly forewarded to me by Luis Bellot of the International Meteor Organisation. The observing site was Castellon (Spain), at Long = 0W, Lat. = 40N and the observer has a reported perception well above the average (around 1.5). The raw counts are (where F = correction for sky obstruction due to clouds):

The reported Lm values (between 3 and 4 in steps of 0.5) are much less than my estimates from downtown Mountain View (with a thick smog layer). I assume that the sky condition estimate is not on the same scale as my estimates. Therefore, I have calculated zenith hourly rates assuming x = 2.15, Lm as given and Cp = 1.5 (A) and an alternative set of values (B) for x = 2.35, Lm = 5.6 rising to 5.8 and Cp = 1.0.

In both cases, the resulting ZHR values are significantly above the annual rate (about ZHR = 13) and they, therefore, confirm the high Leonid activity on Nov. 17/18. The uncertainty in the limiting magnitude leaves some doubt about the time of maximum of the Leonid peak. If the limiting magnitude estimates are taken at face value, then Josep Trigo would have the higher rates and the peak would be at about 5 UT. However, that would contradict the meteor-scatter account of increasing rates. Also, a significant increase of radio rates over the counts on Nov. 16/17 is expected if the maximum was as early as 5 UT when receiver and antenna were in a favourable position. The counts by Bob Lunsford on Nov. 16/17 are too low by a factor of two to allow a maximum as early as 5 UT, Nov. 18, and a symmetric profile (as usual - A&A 1995 in press). From these arguments, I opt for the alternative ZHR values (B), which are in agreement with Bob's counts and qualitatively in agreement with the MS data. In combination with the observations of Nov. 18/19, these data put the maximum at about 14h UT, at the time of the observations in Californie.
I conclude at this point that the maximum probably fell at around 14h UT (solar longitude L = 235.4 - all values in Equinox 1950.0). The decrease away from maximum had slope B = 1.1 and ZHRmax about 70, where ZHR(outburst component) = ZHRmax x 10^(- B x | L - 235.4 |). This corresponds to a duration of 0.8 days above the 1/e times maximum activity value, which is the same duration as in 1961 and 1965 (A&A 1995). I hope more information will soon become available.

Peter Jenniskens,

November 20, 1994.

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