Warsaw University Observatory
Al. Ujazdowskie 4, 00-478 Warszawa, Poland
Abstract. The visual observations of 1996 Perseids are reported. Based on over 700 hours of observing time an activity profile from July 15 to August 24 is given. The discovery of a small dip in the activity profile near solar longitude 129o (2000.0) is also presented. The maximal Zenithal Hourly Rates (ZHRs) equal to 16226 were noted on August 12.026 ( = 139.64o). The possibility of the presence of a double peak with maximal values of ZHR at = 139.64o and = 139.66o is also discussed. The minimum of a population index r were observed around the night of the maximum.
In the year 1988 meteor observers noted the appearance of a new peak in addition to the traditional peak in the Perseids' activity profile. This feature preceded the previous maximum by about half a day and its Zenithal Hourly Rates (ZHRs) were comparable with the activity of the older peak. During the next few years these ZHRs were higher and the highest ones were observed in period 1991-1993. In these years fortunate and experienced meteor observer could note over 300 Perseids per hour during the night of the maximum (Rendtel 1993). Predictions made by Williams and Wu (1994) suggested that in the years 1994-1995 the maximal ZHRs could be around 200-300. They were right. On 1994 August 12 at 11h UT the North American observers estimated the Perseids activity around 25045 (Rendtel 1994). Twelve hours later European watchers noted ZHR=13044 (Olech and Woźniak 1996a). Analysis given by Rendtel (1994) indicated that the first and the higher peak in 1995 should occur on August 12 around 17h UT. This time was advantageous for Asian observers. In Europe it was only possible to observe a pit between both maxima with ZHR around 80-90 (Olech and Woźniak 1996b). Only the Ukrainian visual observers noted ZHR=16080 near 18h UT on August 12 (=139.64o) (Rendtel 1995). This result was quickly confirmed by Japanese radio observations (Suzuki 1995).
Predictions for 1996 Perseids' maximum fortunately favored the European observers. The moment of the higher maximum should have occur on August 12 around 0h UT. Additionally the phases of the Moon were almost ideal with New Moon on August 14.
In 1995 a group of 38 Polish meteor observers associated in the Comets and Meteors Workshop (CMW) obtained 448h30m of observing time and counted 2503 meteors from the Perseids stream (Olech and Woźniak 1996b). Taking into account the fact that in 1995 the weather conditions in Poland were excellent, we did not expect a higher number of counts in 1996. Surprisingly and fortunately the reality was different. From 1996 July 15 to 1996 August 25 a group of 50 CMW observers obtained 719h14m of observing time with 6706 meteors from the Perseids stream and 3505 sporadics. The complete list of our observers together with their total time that they observed is given in Appendix A.
This represents a large number of observations but not all of them can be used for ZHR calculations. Using our standard methods (Olech and Woźniak 1996b) we required that the value of the stellar limiting magnitude in a field of view had to be at least 4.80 mag, the correction factor F resulting from clouds cover had to be smaller than 2.0 and the effective time of observations had to be at least 30 minutes. The exception from the last rule was the night from Aug. 11 to 12 where longer runs were divided into periods of 10-30 minutes each. Finally we obtained 686 good ZHR estimates.
3.1 Magnitude distribution and population index r
Although the number of 1995 Perseids in our database was not small, the Full Moon in the vicinity of the maximum made impossible to measure the behavior of a population index r. In 1996 situation was different. The New Moon occured on Aug. 14 i.e. close to the night of maximum and as many as 6290 magnitude estimates for Perseids and 3436 for sporadic meteors were made. The magnitude distributions (without a correction for the altitude of the meteor event) for 1996 Perseids and sporadics are given in Table 1. border-bottom-width
For the purpose of investigate the changes of the population index r during the stream activity we used methods and probabilities of perception given by Koschack and Rendtel (1990). Our results are presented in Fig. 1. At the beginning of August i.e. from Aug. 1 to 7 and at the end of Perseids' activity i.e. from Aug. 15 to 25 r varied from 2.4 to 2.7, which is a typical value for Perseids. The clear minimum of the population index was observed near the maximum of the activity of the stream i.e. during nights Aug. 8-14, when the value of r amounted 2.2. The two lowest values were detected during nights from Aug. 11 to 12 and from Aug. 13 to 14 and amounted to 1.960.05 and 1.880.16, respectively. The second value is even smaller than the value of r during the maximum of activity but its error is large and in reality r might be higher.
Such a small value of the population index during the peak of activity is certainly connected with a large number of bright (massive) meteors. This claim is also visible in Fig. 2 where the mean brightness of meteors from the Perseids stream is plotted versus time for each night. The open squares show the magnitudes for sporadic meteors and the filled circles correspond to the mean magnitudes of the Perseids. The highest point on this graph with value equal to 1.54 mag corresponds to the night of the maximum, confirming that during this night an enhanced number of bright meteors was observed.
The average brightness for all 1996 Perseids was 1.99 mag which is the typical value for this stream and did not change from last years measurements (Olech and Woźniak 1996a, 1996b). For comparison the average value of the brightness of the sporadic meteors was only 2.78 mag. About 23.6% of meteors from Perseid stream had a persistent train and 0.8% ended with a flash. This result is similar to that noted during previous year.
3.2 The activity profile
Using obtained values of the population index r we plotted the whole period activity profile for 1996 Perseids. For nights from Jul. 15/16 to Aug. 7/8 and from Aug. 14/15 to 24/25 we adopted r=2.6, for nights from Aug. 8/9 to 10/11 and from Aug. 12/13 to 13/14 we adopted r=2.2 and for the night of maximum i.e. Aug. 11/12 r=2.0. The zenith exponent was set to 1.0.
The activity of 1996 Perseids is exhibited in Fig. 3. The maximum value of ZHR was equal to 90.55.2 and was noted at the night Aug. 11/12. This point represents the mean value of all proper ZHR estimates made during this night. Because the number of these estimates amounted to 121, we decided to divide its into shorter bins and plot the activity profile only for the night of the maximum. The result of this operation is presented in Fig. 4. It is clearly visible that from Aug. 11.85 UT to Aug. 12.00 UT, ZHRs were low and oscillated around 60. After midnight the activity started to increase and quickly reached the maximal value of 16226 which occured on Aug. 12.026 UT which corresponds to the solar longitude (epoch 2000.0) = 139.64o. After that ZHR dropped to 13420 at = 139.65o and then slightly increased to 13518 at = 139.66o.
The results obtained by International Meteor Organization (IMO) (Rendtel and Arlt, 1996) are marginally different. They noted the maximum activity with ZHR=12117 at = 139.66o0.03o. The peak was not as sharp as in previous years and activity around ZHR120 was observed from = 139.63o to = 139.70o, i.e. it lasted almost one hour and forty minutes. They did not note the high activity at = 139.64o when our ZHRs exceeded 160. Our maximal point is however the mean value of 11 ZHR estimates and it is hard to imagine that it is erroneous. Compiling both results we can notice that the first peak of 1996 Perseids' activity might exhibit double structure with maxima at = 139.64o and = 139.66o.
The older maximum with ZHR=8510 occured at = 140.08o0.04o, according Rendtel and Arlt (1996).
Based on the methods described by Koschack and Rendtel (1990), we computed the spatial densities of the meteor events. For the instant of the maximum i.e. = 139.64o with ZHR=16226 and r=1.960.05 we obtained the spatial density of meteors of magnitude at least 6.5 6.5)=20836 particles/109km3 which corresponds to the spatial density of meteor bodies with mass higher than 10-3g