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Astron. Astrophys. 357, 777-781 (2000)

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3. Activity profiles

The background activity hourly rates for each hour in a particular year were determined for 13 echo duration intervals in the echo duration range [FORMULA] s by means of the iterative method introduced by Simek (1985). Such hourly rates were subtracted to obtain hourly shower echo rates for further analysis. Hourly shower echo rates for echo duration class [FORMULA] s are presented in Figs. 1-3. Sporadic background rates were deduced from the observations in 1965-67 preceded the solar longitude [FORMULA] and followed [FORMULA] (J2000.0) (see Table 1) using a mean background model as described in Simek (1985). This approach is generally accepted but we feel that it would be worth to comment the nature of echo counts referred to recorded particular meteor echo duration.

[FIGURE] Fig. 1. The observed hourly rate of Leonid 1965 shower, represented by asterisks, and the hourly rate corrected for diurnal variation, depicted by open squares, as a function of solar longitude in degrees (J2000.0)

[FIGURE] Fig. 2. The same as in Fig. 1 but for 1966

[FIGURE] Fig. 3. The same as in Fig. 1 but for 1967


[TABLE]

Table 1. Schedule of observations. Hours designate the beginnings of the entire hour.


Verniani (1973) has derived from the statistics of observed sample of radar meteors the following expression for maximum electron line density in meteor trail, [FORMULA], expressed in el/m,

[EQUATION]

where [FORMULA] is the mass of the meteoroid before entering the atmosphere expressed in grams, V its velocity in km s-1, and [FORMULA] is the zenith distance of the shower radiant. According to McKinley (1961)

[EQUATION]

where [FORMULA] represents the diffusion-controlled duration of the radar echo, [FORMULA] stands for the wavelength of the transmitted wave, and D denotes the ambipolar diffusion coefficient. Combining Eq. (1) and (2) (Simek 1978) we have

[EQUATION]

which shows pre-atmospheric mass of a meteoroid of given velocity as a function of [FORMULA]. The echo duration produced by a meteoroid of discrete mass is then

[EQUATION]

Therefore, it is obvious that a meteoroid of discrete mass and velocity produces an echo duration controlled by the ratio [FORMULA] where D depends on the height of the reflecting point. It follows that the recorded number of echoes with the same duration is produced by a conglomerate of meteoroids having different mass. Echo durations in our set of data appeared in the range of 0.20 s [FORMULA] s, calculated according to Eq. (4) for [FORMULA] g where the Leonid entering velocity [FORMULA] km [FORMULA], [FORMULA] m for Ondejov radar, and coefficient of ambipolar diffusion [FORMULA] [FORMULA] for constant height of 105 km (see McKinley 1961). Since this duration interval of [FORMULA] includes very short overdense echo durations we neglect the small variability of echo heights affecting D. When comparing the series of hourly rates of a particular echo duration limit, they must be related to the same controlling conditions, i. e. for [FORMULA] and D. The [FORMULA] coefficients correcting recorded echo counts are presented in Table 2. They are the product of the iterative process (Simek 1985, and Simek & McIntosh 1986) applied to long-term data from observations of the Perseids. Since only cumulative hourly rates for relevant echo duration and time are the starting parameters of the iterative process, [FORMULA] are products of all effects affecting meteor echo counts de facto involved in the distribution of observed hourly rates.


[TABLE]

Table 2. Leonid zenith radiant angle, [FORMULA], multiplying factors, [FORMULA], eliminating the diurnal variation of the Leonid mass-distribution index, s, and inverse response function of the radar, [FORMULA]. Time is given in the local time (LT=UT+1). Since [FORMULA] factors for 0h, 13h and 23h designated by * are not available for relevant radiant zenith angles, corresponding values are estimated by extrapolation. Resulting corrected hourly rates are, therefore, uncertain.


McIntosh & Millman (1970) normalized recorded rates "to remove the variation caused by changing elevation angle of the radiant h. For reasons discussed by McIntosh (1966) a simple sine function has been used and rates are normalized to [FORMULA]".

Corrected hourly shower echo rates are presented in Figs. 1-3. We can distinguish in 1965 two well pronounced maxima at [FORMULA], and [FORMULA] the widths of which at half maximum level are [FORMULA] and [FORMULA], respectively. Two peaks have also been recognized by McIntosh & Millman (1970). Since their observations were limited by the location of the Leonid radiant, both peak patterns were not especially pronounced, and McIntosh and Millman interpreted the resulting activity pattern as a single one having its peak within the observational break. Such a feature was not confirmed by our observation, and second activity peak at [FORMULA] indicates the maximum for 1965 display. Nevertheless, it should be mentioned that a single observation run of the Leonids is limited at our latitude to 14 hours which makes a description of whole activity pattern of the shower impossible.

The results of observations in 1966 were affected by the position of shower radiant near horizon when culminated Leonid activity similarly as for Canadian radar observations in 1965. The position of highest activity was found at [FORMULA].

The modest display of the Leonids 1967 is apparent from Fig. 3. The low rates as well as their fluctuations around the hourly rate of 25 remain rather the nature of the shower during intermediate Leonid period discussed by Brown et al. (1997). The shower radiant was below the horizon at the time of activity culminations in 1965 and 1966 at [FORMULA]. There are not many available reports of visual observations because of not good observational conditions due to the moonlight (see Brown 1999).

The cluster of Leonid particles encountering the Earth atmosphere in 1965-66 is characterized by the non-uniform structure along the parent comet orbit. Its nature was analyzed by McIntosh (1973) who explains it by the change of spacings between the comet and allocations of observed meteor showers along its orbit.

The maximum hourly rate in 1965 occurred at the same solar longitude as in 1966. The latter corrected value was higher than that in 1965 by the ratio [FORMULA].

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Online publication: June 5, 2000
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