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A long-term high-resolution study of the visual activity curve of the Perseid meteor stream 1953–1983
Planetary and Space Science 48 (2000) 905–909
www.elsevier.nl/locate/planspasci
A long-term high-resolution study of the visual activity curve of the Perseid meteor stream 1953–1983 B.A. Lindblad ∗ Lund Observatory, P.O. Box 43, S-22100 Lund, Sweden Received 1 October 1999; accepted 10 November 1999
Abstract Perseid and sporadic meteor hourly rates and magnitudes observed in 1953–1983 by a team of visual observers at the Onsala Space Observatory, Sweden, are analysed. A high-resolution study of the zenithal hourly rates of bright Perseid meteors (m 6 ◦ 2:5) versus solar longitude has been made using a step length of 0:05 in solar longitude. The present study, which is based on ◦ ◦ 147 Perseid hourly rates, observed between solar longitudes 138:70 –141:65 (equinox 2000) covers the period of maximum Perseid activity. Somewhat surprisingly it reveals a multi-peak structure of the Perseid maximum with at least four separate peaks in ◦ the activity curve of bright Perseids. The rst peak located at 139:38 corresponds to the crossing of the nodal plane of the par◦ ◦ ent comet, the second at 139:68 is the so-called “new” Perseid maximum, the third peak at 140:20 is the “old” or “traditional” maximum of the shower, while the fourth peak although less intense than the previous three peaks indicates a well-de ned activ◦ ity maximum centred on 140:78 . It is interesting to note that, for bright Perseids, the nodal maximum is of the same intensity as the “new” and “old” maxima, but is of a somewhat shorter duration than these two maxima. A similar study of the activity curve of all observed Perseid meteors (i.e. independent of apparent magnitude) shows the same multi-peak structure, but with slightly less pronounced peaks. In a previous study of the Perseid activity curve based on 605 photographic Perseid orbits obtained in various two-station programs 1937–1985 the multi-peak structure can be recognised in a number versus solar longitude diagram (Lindblad and Porubcan, 1994. Planet Space Sci. 42, 117–122.). The various peaks in the photographic data are located at the same solar longitudes as in the visual data. This agreement between the results of the present long-term visual study and a long-term photographic study of the Perseid activity curve strongly supports our conclusions as to the multi-peak structure of the Perseid c 2000 Elsevier Science Ltd. All rights reserved. shower.
1. Introduction The Perseid meteor shower is amongst the strongest and most frequently observed annual meteor showers in the Northern Hemisphere. Ancient records of the shower have been summarised by Olivier (1925), Kronk (1988) and Hasegawa (1993). Since its documentation by Herrick (1838) and Quetelet (1839) several hundreds of reports have been published by visual observers describing the activity of the Perseids. For references to these papers see Lovell (1954) and Kronk (1988). Many visual studies have suggested large year to year variations in the intensity of the Perseid shower maximum (Lovell, 1954). The highest Perseid rates listed in Lovell’s compilation occurred in 1921 when observers at the Tashkent ∗ Fax: +46-46-222-4614. E-mail address: [email protected] (B.A. Lindblad).
Observatory reported a mean hourly rate of 250 Perseids per h for the night of 11=12 August 1921 (Opik, 1923). However, it is important to note that this observation was made at local midnight, with no interference from moon◦ light, and at solar longitude 139:9 (equinox 2000), which corresponds to the solar longitude of the main maximum of the annual Perseid shower (Lindblad and Simek, 1986). Exceptional observing conditions can therefore explain this anomalous rate. In appraising these reports one should note that Northern Hemisphere observers can only record Perseid activity during a few hours each night centred on local midnight. This limited night-time window reduces the observational coverage of Perseid activity to some 20 – 40% of the total diurnal duration of the shower. It follows that even under perfect weather and lunation conditions a visual observer has to collect data over many years in order to obtain a complete picture of the Perseid hourly rate variation along the Earth’s path.
c 2000 Elsevier Science Ltd. All rights reserved. 0032-0633/00/$ - see front matter PII: S 0 0 3 2 - 0 6 3 3 ( 0 0 ) 0 0 0 5 5 - 6
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B.A. Lindblad / Planetary and Space Science 48 (2000) 905–909
1.1. Single-station data One of the rst attempts to obtain a complete Perseid activity pro le was made by Denning (1899a,b). In his study, Denning analysed 2409 Perseids observed between the dates July 11 and August 19, 1869 –1998. Unfortunately, Denning (and his contemporaries) did not refer their observed hourly rates to a speci c solar longitude, nor did they correct the observed hourly rates to a zenithal value. Perseid activity was instead plotted as mean observed hourly rate versus date. The use of date introduced an unnecessary smoothing of the activity curve. The extremely smooth character of Denning 1899 Perseid curve suggests that other smoothing procedures were also introduced. Denning also studied radiant right ascension and declination versus date. This is one of the rst studies where the daily radiant motion of a meteor shower is clearly demonstrated. During the 20th century detailed visual and photographic studies of the structure and activity of the Perseid stream have been made by Guigay (1947/1948), Ahnert-Rohlfs (1952), Wright and Whipple (1953), Lindblad (1986) and others. Ahnert-Rohlfs analysed 853 Perseid meteors observed in Sonneberg in 1931–1936. Hourly rates were corrected to a zenithal value and the rates were presented as a function of solar longitude (equinox 1925). Ahnert-Rohlfs interpreted the derived Perseid activity pro le as the superposition of two components: one broad component consisting of meteors perturbed by the Earth during previous Perseid returns and a second, spike-like component caused by unperturbed meteors. This interpretation has been questioned by Brown and Jones (1998). In the 1940s and 1950s teams of 4 –8 visual observers were often organised to study the hourly rate curves and magnitude distributions of various meteor showers. Unfortunately the magnitude distributions, hourly rate curves and other results of these teams are often dicult to compare because of di erences in the observing techniques (Stohl and Lindblad, 1982). A diculty in the analysis of the hourly rate data and in the meteor magnitudes collected by these large groups is that sometimes the individual magnitude scales and perception coecients of the various team members had not been checked or mutually compared. For example, in the Ottawa visual observations (with up to 8 observers) magnitude estimates from several of the observers were later rejected because of unrealistic magnitude scales (Stohl and Millman, 1973). In 1944 –1953 a team of experienced meteor observers at the Skalnate Pleso observatory recorded more than 17,000 Perseids. Zenithal corrected Perseid rates per one observer were later computed, taking into consideration corrections for cloudiness, lunation e ects and personal coecients of the observers (Zvolankova, 1984). The published data give an interesting pro le of Perseid activity. Unfortunately, there is a gap in the Skalnate Pleso records during the Perseid maximum, with no observations
◦
◦
reported between solar longitudes 139:80 and 140:44 (equinox 1950). In Lindblad (1986) a preliminary study based on Perseid visual observations 1953–1979 is presented. Large apparent changes in the Perseid hourly rates from one year to the next were noticed. The accumulated data were therefore smoothed using a sliding mean of 5 data points. The resulting zenithal hourly rate curve was ◦ extremely smooth peaking at a solar longitude of 139:9 (equinox 2000). The use of a sliding mean was, however, rather unfortunate since it obliterated any information on a possible ne structure in the hourly rate data. It also produced a spurious wave-like structure in the sporadic meteor rates. Various other methods of analysing the visual hourly rate pro les of meteor showers have been proposed. Jenniskens (1990,1994) introduced a method based on a study of the logarithm of the corrected zenithal rates versus solar longitude. The advantage of this method is that in a logarithmic scale the rates are to a rst order represented by linear increasing and decreasing branches. However, in general this method is not applicable to the Perseids or any other meteor shower which exhibits a multipeak structure. 1.2. Global network data An inherent diculty in the analysis of all longterm programs of this type is that any variations in the number of meteoroid particles along the stream orbit are neglected. An alternative to long-term, single-station observing of meteor stream activity is to organise a global network of observers. One of the rst attempts to construct a Perseid zenithal hourly rate pro le based on world-wide observations during a single year was carried out in 1988 by amateur meteor observers associated with the International Meteor Organisation (IMO) (Roggemans, 1989; Roggemans et al., 1991). Additional IMO campaigns during the 1995 and 1996 Perseids were reported by Rendtel and Arlt (1996). The logistics of co-ordinating observations on a global scale and analysing the often disparate hourly rates reported by amateur groups is a very dicult task. A second problem in global network studies is the occurrence at some stations of meteor counts made at very large zenith distances of the radiant. At these stations the choice of the exponent n in the secn z correction factor becomes very critical (Roggemans, 1987). 2. Present study 2.1. Onsala visual observations A long-term visual and radar study of the Perseid meteor stream was carried out at the Onsala Space Observa◦ tory in Sweden (longitude = 0h 48m E, latitude = 57 240 N) in the period 1953–1983. During this study a total of 8705 meteors were observed, of which 5974 were
B.A. Lindblad / Planetary and Space Science 48 (2000) 905–909
Perseids, 2226 were sporadics, 458 were Aquarids, 21 were Cygnids and 26 were other minor shower meteors. The observing team consisted of 3 experienced observers plus a timekeeper. Two of the observers watched the beamed sky area of the meteor radar, the third observer covered the adjacent eld extending to the zenith. Each meteor was normally recorded by only one of the observers, who plotted the meteor path on a star map and estimated the apparent magnitude to the nearest 0:5m . In order to avoid observer dead-time while plotting a meteor path the observers were instructed to assist each other by temporarily expanding their regular observing elds. This temporary expansion of the observing eld was monitored by the timekeeper (recorder). The system e ectively prevented losses in the hourly rate count. Owing to the restricted eld of view of each observer the total hourly rate count of the Onsala team was similar to that of a single observer who recorded all meteors he sees. For a full description of the observing techniques see Lindblad (1963). The Onsala visual observations were co-ordinated with the radar recordings thus allowing identi cation of time coincidences between visual meteors and radar-echoes. For a description of methods and results from the combined radar and visual program see Lindblad (1963, 1967, 1998), Lindblad and Stohl (1983). 2.2. Data analysis The raw hourly rates of the team were rst corrected for gaps in the records. Data from observing periods of 20 min or more were prorated to 1 h rates. Observing periods of less than 20 min were rejected. Next a correction for varying seeing and weather conditions was introduced. The weather conditions were estimated for each night during the Perseid campaign by the present writer on an arbitrary scale of 1–9, where 1 was overcast conditions with only occasional patches of clear sky and 9 was perfect weather. In the present study only those nights where the weather was classi ed as 6 –9 were considered. For a discussion of how the weather correction factors were derived see Lindblad (1986). Finally, the hourly rates of the team were corrected to a zenithal rate using a simple sec z law. Since the night-time visual Perseid observing window at the Onsala station was of length approximately 4.5 –5 h correction factors to zenith did not vary drastically during the observational run. The solar longitude of the midpoint of each observing hour was read from an astronomical almanac and rounded o to two decimals. This solar longitude together with date and raw hourly rates for Perseid and sporadic meteors was used as the input le to our study. Next various computer programs introduced the corrections discussed above. Finally, the corrected zenithal hourly rates of the team were ◦ sorted into bins of width 0:10 in solar longitude. This bin ◦ was then moved forward using a step length of 0:05 in
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Fig. 1. Zenithal hourly rates of bright meteors (m 6 2:5) observed during the Perseid maximum 1953–1983 versus solar longitude (equinox 2000.0). Top curve: Perseid zenithal hourly rates. Lower curve: Sporadic hourly rates.
solar longitude and a mean hourly rate was computed for each step. Although the input le consisted of 570 individual hourly rates observed during 170 individual nights in the period 1953–1983, only 160 hourly rates referred to the period of maximum activity, here arbitrarily de ned ◦ ◦ by the computer program as the interval 137:70 –143:65 in solar longitude (equinox 2000). 2.3. Activity proÿle of bright Perseids Fig. 1 depicts the zenithal hourly rate versus solar longitude curve for Perseids observed in 1953–1983 and of apparent magnitude 2:5m or brighter. In order to emphasise the main features of the Perseid maximum only hourly ◦ ◦ rates in the interval 138:70 to 141:65 are shown in Fig. 1. Each point in the diagram represents an average Perseid ◦ hourly rate in a bin width of 0:10 in solar longitude. In ◦ the study a step length of 0:05 in solar longitude has been used between the points corresponding approximately to a time interval of 1h 15m . The number of hourly rates averaged in each bin of Fig. 1 varied between 1 and 10 with a mean of 5.2. The reason for selecting the brighter meteors for study is that they are less scattered and in general delineate the main features of a meteor stream better than the fainter particles. A second reason is that the bright meteor counts are less dependent on lunation e ects and on temporary variations in the limiting stellar magnitude at the observing site. Inspection of Fig. 1 shows that during the period 1953– 1983 there is considerable ne structure in the Perseid activity curve. In particular, four well-de ned peaks of very nearly the same hourly rate are evident. The rst peak at ◦ 139:38 corresponds to the crossing of the orbital plane of ◦ the parent comet 109 P=Swift Tuttle, the second at 139:68 is the so-called “new” Perseid maximum, the third peak at
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B.A. Lindblad / Planetary and Space Science 48 (2000) 905–909
Table 1 Solar longitude of Perseid meteor stream main maxima 1953–1983 Peak no.
Designation name
1 2 3
Nodal “New” Traditional
4
Post maximum
(Bright visual meteors)
ZHR
Longitude 1950
Longitude 2000
138.68 138.98 139.34 139.50 140.08
139.38 139.68 140.04 140.20 140.78
◦
140:20 is the “old” or “traditional” maximum, the fourth ◦ peak at 140:78 we will call the post-maximum peak. Fig. 1 shows that the nodal peak is fairly sharp, indicating a stable structure, while the “new” peak is fairly broad. The existence of a nodal peak in the Perseid activity curve was discussed by Lindblad and Porubcan (1994) and Lindblad (1998). Bus (1999) reports that this peak was noticed by Dutch visual observers in 1993 and 1994. The appear◦ ◦ ance of a “new” Perseid peak in the interval 139:5 –139:7 was rst reported by Watanabe et al. (1992) from radar observations and by Yabu et al. (1993) from visual observations. Of the four main peaks, the rst three peaks are of approximately the same height. The fourth peak ◦ at longitude 140:78 (equinox 2000) is of smaller width and has not been discussed previously in the literature. However, its existence has been evident for some time in photographic and radar studies of Perseid rates (Lindblad and Porubcan, 1994; Lindblad and Simek, 1986; Simek, 1993). An interesting feature of the Perseid activity curve in Fig. 1 is a possible double peak in the traditional Perseid maximum; the rst peak occurring approximately at ◦ solar longitude 140:04 and the second peak at longitude ◦ 140:20 . Although there may be some doubt about the statistical signi cance of this feature, it is interesting to note that this double peak is also evident in Roggemans (1989) global analysis of the 1988 Perseids. The positions in solar longitude of the four main peaks are summarised in Table 1. The position in solar longitude of each peak was computed as a weighted mean solar longitude of the three highest mean hourly rates de ning the peak. The un◦ certainty in this method is approximately ±0:02 in solar longitude. 2.4. Activity proÿle of all Perseids The zenithal hourly rate pro le of all Perseids (i.e. independent of magnitude) was studied in the same way as for the bright meteors. The above-mentioned peaks are clearly discernible also in the total Perseid meteor sample, but with somewhat reduced signal to noise ratio compared with the peaks in the bright meteor data. There is also a very slight discrepancy in the position of the “new” maximum between the two samples.
28.6 23.8 29.7 26.0 27.8
Comments
Broad peak Double peak?
Photographic (Longitude 2000)
139.38 139.67 140.31 140.80
2.5. Sporadic meteors During the meteor observations 1953–1983 numerous Aquarids, Alpha Capricornids, Cygnids and sporadic meteors were observed within the solar longitude interval ◦ ◦ 137:00 to 141:50 (equinox 2000). For simplicity in the data handling the Aquarids, Alpha Capricornids, Cygnids and other minor showers in the present study are included ◦ in the sporadic group. As before a bin of 0:10 in so◦ lar longitude was moved forward in steps of 0:05 . Fig. 1 (lower curve) depicts the resulting hourly rate vs solar longitude diagram for all non-Perseid meteors of apparent magnitude 2:5m and brighter. It is seen that the zenithal hourly rate of bright sporadic and minor shower meteors has varied in a random manner between 0.0 and 5.7 meteors per hour. Smallness of sample prevents us from drawing any further conclusions. The corresponding hourly rate variation for all non-Perseid meteors (i.e. independent of magnitude) was 2.5 to 10.0, with slightly higher rates occurring before the Perseid maximum. This higher rate was mainly caused by activity from several radiants in Aquarius. 2.6. Activity proÿle of photographic Perseids In a study of 605 double-station photographic Perseid meteor orbits observed in 1937–1985 Lindblad and Porubcan (1994) showed a histogram (Fig. 4 of the 1994 paper) describing how the number of photographic Perseids varied with solar longitude (it was not possible to compute hourly rates for these very bright objects since the operating schedules of the various photographic stations was not known). The histogram (Fig. 4) showed the existence of the nodal, the “new” and the traditional peaks in the Perseid activity curve. A fourth peak at solar longitude ◦ 140:78 was also discernable. The positions of the various peaks in the photographic data were determined by the authors in the same way as in the visual data, i.e. as a weighted mean of the three highest rates de ning the peak. The resulting positions in the number versus solar longitude diagram of the four main peaks are listed in Table 1. There is a surprisingly good agreement between the photographic meteor data and the bright visual-meteor
B.A. Lindblad / Planetary and Space Science 48 (2000) 905–909
data. However, there is a minor discrepancy in the position of the traditional Perseid maximum. This discrepancy is ascribable to the slightly bi-modal character of the traditional maximum, which makes it dicult to precisely determine the solar longitude of this maximum. 3. Future work The Perseid radar recordings at Onsala were carried out independently of the visual observations for 12 h per day. (One night-time run 22-04 and one daytime run 08-14 MET). Radar recordings were also carried out during various background runs in August and September in order to establish a reliable background rate. The total number of August recordings 1953–1978 was 149 night-time runs and 113 daytime runs, comprising 1939 individual one-hour rates. Owing to severe cuts in funding data for 1979 –1983 have not been fully reduced. Preliminary studies of the Perseid radar rate data for long duration echoes 1953–1978 have been published (Lindblad and Simek (1983,1986)) and for short duration echoes (Simek and Lindblad, 1989). These studies, however, have used sliding means or other smoothing procedures to obtain a mean Perseid activity pro le. This procedure has smoothed out nearly all detail in the resulting Perseid activity curve. It is hoped that in the future it will be possible to re-analyse the complete radar rate data (1953–1983) using the same high-resolution technique as in the present study. Acknowledgements The long-term Perseid observing program at the Onsala Observatory was supported by grants from the Swedish Natural Science Research Council and from Alice och Knut Wallenbergs Stiftelse. Support for data reduction of the visual observations has been provided by Kgl. Fysiogra ska Sallskapet, Lund and from Crafoordska Stiftelsen, Lund. The author gratefully acknowledges this support and also the skilful help provided by the sta of the Onsala Observatory and also by the meteor observers who participated in the observing program. References Ahnert-Rohlfs, E., 1952. Vero entl. der Sternwarte in Sonnenberg. Akad. Verlag Berlin 2 (2), 5–38.
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Brown, P., Jones, J., 1998. Icarus 133, 36–68. Bus, E.P., 1999. Radiant 21, 76. Denning, W.F., 1899a. Mem. Roy. Astron. Soc. 53, 203–292. Denning, W.F., 1899b. Astron. Nachricht. 148, 284. Guigay, G., 1947/1948. J. des Observateurs, 30 –31. Hasegawa, I., 1993. Historical records of meteor showers. In: Stohl, J., Williams, I.P. (Eds.), Meteoroids and their Parent Bodies. Slovak Acad. Sciences, Bratislava, pp. 209–229. Herrick, E.C., 1838. Am. J. Sci. 33 (176), 354–461. Jenniskens, P., 1990. Meteor Stream Activity Pro les from naked eye counts. In: Lagerkvist, C.-I., Rickman, H., Lindblad, B.A., Lindgren, M. (Eds.), Asteroids, Comets and Meteors III, Uppsala, 1983. Jenniskens, P., 1994. Astron. Astrophys. 287, 990–1013. Kronk, G.W., 1988. Meteor Sohwers, A Descriptive Catalog. Enslow Publishers, New Jersey. Lindblad, B.A., 1963. The Relation between Visual Magnitudes of Meteors and Durations of Radar Echoes. Smithson. Contr. Astrophysics, 7, 27–39. Lindblad, B.A., 1967. Smithson. Contr. Astrophys. 11, 171–182. Lindblad, B.A., 1986. Structure and Activity of the Perseid Meteor Stream from Visual Observations 1953-1981. In: Lagerkvist, C.-I., Lindblad, B.A., Lundstedt, H., Rickman, H. (Eds.), Asteroids, Comets, Meteors II, Uppsala, 1986. Lindblad, B.A., 1998. Planet Space Sci. 46, 865–868. Lindblad, B.A., Porubcan, V., 1994. Planet Space Sci. 42, 117–122. Lindblad, B.A., Simek, M., 1986. In: Lagerkvist, C.-I., Lindblad, B.A., Lundstedt, H., Rickman, H. (Eds.), Asteroids, Comets and Meteors II, Uppsala, 1986. Lindblad, B.A., Stohl, J., 1983. Errors in visual meteor observations. In: Lagerkvist, C.-I., Rickman, H. (Eds.), Asteroids, Comets and Meteors, Uppsala, 1983, pp. 451–455. Lovell, A.C.B., 1954. Meteor Astronomy. Oxford University Press, Oxford. Olivier, C.P., 1925. Meteors. Williams and Wilkins, Baltimore. Opik, E., 1923. Publ. Tartu Obs. 25 (4), 1–70. Quetelet, A., 1839. Catalogue der Principales. Apparitions d’Etoile Filiantes, Bruselles. Rendtel, J., Arlt, R., 1996. WGN 24, 141–147. Roggemans, P., 1987. WGN, 15, pp. 181–183. Roggemans, P., 1989. WGN, 17, pp. 127–137. Roggemans, P., Gyssens, M., Rendtel, J., 1991. WGN, 19, pp. 181–184. Simek, M., 1993. WGN, 21, pp. 17–18. Simek, M., Lindblad, B.A., 1989. In: Lagerkvist, C.-I., Rickman, H, Lindblad, B.A., Lindgren, M. (Eds.), Asteroids, Comets and Meteors III, Uppsala, 1989. Stohl, J., Lindblad, B.A., 1982. Bull. Astron. Inst. Czechosl. 33, 129. Stohl, J., Millman, P., 1973. Bull. Astron. Inst. Czechosl. 24, 321–330. Watanabe, J., Nakamura, T., Tsutsumi, M., Tsuda, T., 1992. Publ. Astron. Soc. Japan 44, 677–685. Wright, F., Whipple, F., 1953. Technical Report no. 11, Harvard College Obs. Yabu, Y., Ueda, M., Zuzuki, K., 1993. Outburst of the Perseids 1991 observed in Japan. In: Stohl, J., Williams, I.P. (Eds.), Meteoroids and Their Parent Bodies. Slovak Acad. Sciences, Bratislava, pp. 201–204. Zvolankova, J., 1984. Contr. Astron. Obs. Skalnate Pleso 12, 45–63.
www.elsevier.nl/locate/planspasci
A long-term high-resolution study of the visual activity curve of the Perseid meteor stream 1953–1983 B.A. Lindblad ∗ Lund Observatory, P.O. Box 43, S-22100 Lund, Sweden Received 1 October 1999; accepted 10 November 1999
Abstract Perseid and sporadic meteor hourly rates and magnitudes observed in 1953–1983 by a team of visual observers at the Onsala Space Observatory, Sweden, are analysed. A high-resolution study of the zenithal hourly rates of bright Perseid meteors (m 6 ◦ 2:5) versus solar longitude has been made using a step length of 0:05 in solar longitude. The present study, which is based on ◦ ◦ 147 Perseid hourly rates, observed between solar longitudes 138:70 –141:65 (equinox 2000) covers the period of maximum Perseid activity. Somewhat surprisingly it reveals a multi-peak structure of the Perseid maximum with at least four separate peaks in ◦ the activity curve of bright Perseids. The rst peak located at 139:38 corresponds to the crossing of the nodal plane of the par◦ ◦ ent comet, the second at 139:68 is the so-called “new” Perseid maximum, the third peak at 140:20 is the “old” or “traditional” maximum of the shower, while the fourth peak although less intense than the previous three peaks indicates a well-de ned activ◦ ity maximum centred on 140:78 . It is interesting to note that, for bright Perseids, the nodal maximum is of the same intensity as the “new” and “old” maxima, but is of a somewhat shorter duration than these two maxima. A similar study of the activity curve of all observed Perseid meteors (i.e. independent of apparent magnitude) shows the same multi-peak structure, but with slightly less pronounced peaks. In a previous study of the Perseid activity curve based on 605 photographic Perseid orbits obtained in various two-station programs 1937–1985 the multi-peak structure can be recognised in a number versus solar longitude diagram (Lindblad and Porubcan, 1994. Planet Space Sci. 42, 117–122.). The various peaks in the photographic data are located at the same solar longitudes as in the visual data. This agreement between the results of the present long-term visual study and a long-term photographic study of the Perseid activity curve strongly supports our conclusions as to the multi-peak structure of the Perseid c 2000 Elsevier Science Ltd. All rights reserved. shower.
1. Introduction The Perseid meteor shower is amongst the strongest and most frequently observed annual meteor showers in the Northern Hemisphere. Ancient records of the shower have been summarised by Olivier (1925), Kronk (1988) and Hasegawa (1993). Since its documentation by Herrick (1838) and Quetelet (1839) several hundreds of reports have been published by visual observers describing the activity of the Perseids. For references to these papers see Lovell (1954) and Kronk (1988). Many visual studies have suggested large year to year variations in the intensity of the Perseid shower maximum (Lovell, 1954). The highest Perseid rates listed in Lovell’s compilation occurred in 1921 when observers at the Tashkent ∗ Fax: +46-46-222-4614. E-mail address: [email protected] (B.A. Lindblad).
Observatory reported a mean hourly rate of 250 Perseids per h for the night of 11=12 August 1921 (Opik, 1923). However, it is important to note that this observation was made at local midnight, with no interference from moon◦ light, and at solar longitude 139:9 (equinox 2000), which corresponds to the solar longitude of the main maximum of the annual Perseid shower (Lindblad and Simek, 1986). Exceptional observing conditions can therefore explain this anomalous rate. In appraising these reports one should note that Northern Hemisphere observers can only record Perseid activity during a few hours each night centred on local midnight. This limited night-time window reduces the observational coverage of Perseid activity to some 20 – 40% of the total diurnal duration of the shower. It follows that even under perfect weather and lunation conditions a visual observer has to collect data over many years in order to obtain a complete picture of the Perseid hourly rate variation along the Earth’s path.
c 2000 Elsevier Science Ltd. All rights reserved. 0032-0633/00/$ - see front matter PII: S 0 0 3 2 - 0 6 3 3 ( 0 0 ) 0 0 0 5 5 - 6
906
B.A. Lindblad / Planetary and Space Science 48 (2000) 905–909
1.1. Single-station data One of the rst attempts to obtain a complete Perseid activity pro le was made by Denning (1899a,b). In his study, Denning analysed 2409 Perseids observed between the dates July 11 and August 19, 1869 –1998. Unfortunately, Denning (and his contemporaries) did not refer their observed hourly rates to a speci c solar longitude, nor did they correct the observed hourly rates to a zenithal value. Perseid activity was instead plotted as mean observed hourly rate versus date. The use of date introduced an unnecessary smoothing of the activity curve. The extremely smooth character of Denning 1899 Perseid curve suggests that other smoothing procedures were also introduced. Denning also studied radiant right ascension and declination versus date. This is one of the rst studies where the daily radiant motion of a meteor shower is clearly demonstrated. During the 20th century detailed visual and photographic studies of the structure and activity of the Perseid stream have been made by Guigay (1947/1948), Ahnert-Rohlfs (1952), Wright and Whipple (1953), Lindblad (1986) and others. Ahnert-Rohlfs analysed 853 Perseid meteors observed in Sonneberg in 1931–1936. Hourly rates were corrected to a zenithal value and the rates were presented as a function of solar longitude (equinox 1925). Ahnert-Rohlfs interpreted the derived Perseid activity pro le as the superposition of two components: one broad component consisting of meteors perturbed by the Earth during previous Perseid returns and a second, spike-like component caused by unperturbed meteors. This interpretation has been questioned by Brown and Jones (1998). In the 1940s and 1950s teams of 4 –8 visual observers were often organised to study the hourly rate curves and magnitude distributions of various meteor showers. Unfortunately the magnitude distributions, hourly rate curves and other results of these teams are often dicult to compare because of di erences in the observing techniques (Stohl and Lindblad, 1982). A diculty in the analysis of the hourly rate data and in the meteor magnitudes collected by these large groups is that sometimes the individual magnitude scales and perception coecients of the various team members had not been checked or mutually compared. For example, in the Ottawa visual observations (with up to 8 observers) magnitude estimates from several of the observers were later rejected because of unrealistic magnitude scales (Stohl and Millman, 1973). In 1944 –1953 a team of experienced meteor observers at the Skalnate Pleso observatory recorded more than 17,000 Perseids. Zenithal corrected Perseid rates per one observer were later computed, taking into consideration corrections for cloudiness, lunation e ects and personal coecients of the observers (Zvolankova, 1984). The published data give an interesting pro le of Perseid activity. Unfortunately, there is a gap in the Skalnate Pleso records during the Perseid maximum, with no observations
◦
◦
reported between solar longitudes 139:80 and 140:44 (equinox 1950). In Lindblad (1986) a preliminary study based on Perseid visual observations 1953–1979 is presented. Large apparent changes in the Perseid hourly rates from one year to the next were noticed. The accumulated data were therefore smoothed using a sliding mean of 5 data points. The resulting zenithal hourly rate curve was ◦ extremely smooth peaking at a solar longitude of 139:9 (equinox 2000). The use of a sliding mean was, however, rather unfortunate since it obliterated any information on a possible ne structure in the hourly rate data. It also produced a spurious wave-like structure in the sporadic meteor rates. Various other methods of analysing the visual hourly rate pro les of meteor showers have been proposed. Jenniskens (1990,1994) introduced a method based on a study of the logarithm of the corrected zenithal rates versus solar longitude. The advantage of this method is that in a logarithmic scale the rates are to a rst order represented by linear increasing and decreasing branches. However, in general this method is not applicable to the Perseids or any other meteor shower which exhibits a multipeak structure. 1.2. Global network data An inherent diculty in the analysis of all longterm programs of this type is that any variations in the number of meteoroid particles along the stream orbit are neglected. An alternative to long-term, single-station observing of meteor stream activity is to organise a global network of observers. One of the rst attempts to construct a Perseid zenithal hourly rate pro le based on world-wide observations during a single year was carried out in 1988 by amateur meteor observers associated with the International Meteor Organisation (IMO) (Roggemans, 1989; Roggemans et al., 1991). Additional IMO campaigns during the 1995 and 1996 Perseids were reported by Rendtel and Arlt (1996). The logistics of co-ordinating observations on a global scale and analysing the often disparate hourly rates reported by amateur groups is a very dicult task. A second problem in global network studies is the occurrence at some stations of meteor counts made at very large zenith distances of the radiant. At these stations the choice of the exponent n in the secn z correction factor becomes very critical (Roggemans, 1987). 2. Present study 2.1. Onsala visual observations A long-term visual and radar study of the Perseid meteor stream was carried out at the Onsala Space Observa◦ tory in Sweden (longitude = 0h 48m E, latitude = 57 240 N) in the period 1953–1983. During this study a total of 8705 meteors were observed, of which 5974 were
B.A. Lindblad / Planetary and Space Science 48 (2000) 905–909
Perseids, 2226 were sporadics, 458 were Aquarids, 21 were Cygnids and 26 were other minor shower meteors. The observing team consisted of 3 experienced observers plus a timekeeper. Two of the observers watched the beamed sky area of the meteor radar, the third observer covered the adjacent eld extending to the zenith. Each meteor was normally recorded by only one of the observers, who plotted the meteor path on a star map and estimated the apparent magnitude to the nearest 0:5m . In order to avoid observer dead-time while plotting a meteor path the observers were instructed to assist each other by temporarily expanding their regular observing elds. This temporary expansion of the observing eld was monitored by the timekeeper (recorder). The system e ectively prevented losses in the hourly rate count. Owing to the restricted eld of view of each observer the total hourly rate count of the Onsala team was similar to that of a single observer who recorded all meteors he sees. For a full description of the observing techniques see Lindblad (1963). The Onsala visual observations were co-ordinated with the radar recordings thus allowing identi cation of time coincidences between visual meteors and radar-echoes. For a description of methods and results from the combined radar and visual program see Lindblad (1963, 1967, 1998), Lindblad and Stohl (1983). 2.2. Data analysis The raw hourly rates of the team were rst corrected for gaps in the records. Data from observing periods of 20 min or more were prorated to 1 h rates. Observing periods of less than 20 min were rejected. Next a correction for varying seeing and weather conditions was introduced. The weather conditions were estimated for each night during the Perseid campaign by the present writer on an arbitrary scale of 1–9, where 1 was overcast conditions with only occasional patches of clear sky and 9 was perfect weather. In the present study only those nights where the weather was classi ed as 6 –9 were considered. For a discussion of how the weather correction factors were derived see Lindblad (1986). Finally, the hourly rates of the team were corrected to a zenithal rate using a simple sec z law. Since the night-time visual Perseid observing window at the Onsala station was of length approximately 4.5 –5 h correction factors to zenith did not vary drastically during the observational run. The solar longitude of the midpoint of each observing hour was read from an astronomical almanac and rounded o to two decimals. This solar longitude together with date and raw hourly rates for Perseid and sporadic meteors was used as the input le to our study. Next various computer programs introduced the corrections discussed above. Finally, the corrected zenithal hourly rates of the team were ◦ sorted into bins of width 0:10 in solar longitude. This bin ◦ was then moved forward using a step length of 0:05 in
907
Fig. 1. Zenithal hourly rates of bright meteors (m 6 2:5) observed during the Perseid maximum 1953–1983 versus solar longitude (equinox 2000.0). Top curve: Perseid zenithal hourly rates. Lower curve: Sporadic hourly rates.
solar longitude and a mean hourly rate was computed for each step. Although the input le consisted of 570 individual hourly rates observed during 170 individual nights in the period 1953–1983, only 160 hourly rates referred to the period of maximum activity, here arbitrarily de ned ◦ ◦ by the computer program as the interval 137:70 –143:65 in solar longitude (equinox 2000). 2.3. Activity proÿle of bright Perseids Fig. 1 depicts the zenithal hourly rate versus solar longitude curve for Perseids observed in 1953–1983 and of apparent magnitude 2:5m or brighter. In order to emphasise the main features of the Perseid maximum only hourly ◦ ◦ rates in the interval 138:70 to 141:65 are shown in Fig. 1. Each point in the diagram represents an average Perseid ◦ hourly rate in a bin width of 0:10 in solar longitude. In ◦ the study a step length of 0:05 in solar longitude has been used between the points corresponding approximately to a time interval of 1h 15m . The number of hourly rates averaged in each bin of Fig. 1 varied between 1 and 10 with a mean of 5.2. The reason for selecting the brighter meteors for study is that they are less scattered and in general delineate the main features of a meteor stream better than the fainter particles. A second reason is that the bright meteor counts are less dependent on lunation e ects and on temporary variations in the limiting stellar magnitude at the observing site. Inspection of Fig. 1 shows that during the period 1953– 1983 there is considerable ne structure in the Perseid activity curve. In particular, four well-de ned peaks of very nearly the same hourly rate are evident. The rst peak at ◦ 139:38 corresponds to the crossing of the orbital plane of ◦ the parent comet 109 P=Swift Tuttle, the second at 139:68 is the so-called “new” Perseid maximum, the third peak at
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B.A. Lindblad / Planetary and Space Science 48 (2000) 905–909
Table 1 Solar longitude of Perseid meteor stream main maxima 1953–1983 Peak no.
Designation name
1 2 3
Nodal “New” Traditional
4
Post maximum
(Bright visual meteors)
ZHR
Longitude 1950
Longitude 2000
138.68 138.98 139.34 139.50 140.08
139.38 139.68 140.04 140.20 140.78
◦
140:20 is the “old” or “traditional” maximum, the fourth ◦ peak at 140:78 we will call the post-maximum peak. Fig. 1 shows that the nodal peak is fairly sharp, indicating a stable structure, while the “new” peak is fairly broad. The existence of a nodal peak in the Perseid activity curve was discussed by Lindblad and Porubcan (1994) and Lindblad (1998). Bus (1999) reports that this peak was noticed by Dutch visual observers in 1993 and 1994. The appear◦ ◦ ance of a “new” Perseid peak in the interval 139:5 –139:7 was rst reported by Watanabe et al. (1992) from radar observations and by Yabu et al. (1993) from visual observations. Of the four main peaks, the rst three peaks are of approximately the same height. The fourth peak ◦ at longitude 140:78 (equinox 2000) is of smaller width and has not been discussed previously in the literature. However, its existence has been evident for some time in photographic and radar studies of Perseid rates (Lindblad and Porubcan, 1994; Lindblad and Simek, 1986; Simek, 1993). An interesting feature of the Perseid activity curve in Fig. 1 is a possible double peak in the traditional Perseid maximum; the rst peak occurring approximately at ◦ solar longitude 140:04 and the second peak at longitude ◦ 140:20 . Although there may be some doubt about the statistical signi cance of this feature, it is interesting to note that this double peak is also evident in Roggemans (1989) global analysis of the 1988 Perseids. The positions in solar longitude of the four main peaks are summarised in Table 1. The position in solar longitude of each peak was computed as a weighted mean solar longitude of the three highest mean hourly rates de ning the peak. The un◦ certainty in this method is approximately ±0:02 in solar longitude. 2.4. Activity proÿle of all Perseids The zenithal hourly rate pro le of all Perseids (i.e. independent of magnitude) was studied in the same way as for the bright meteors. The above-mentioned peaks are clearly discernible also in the total Perseid meteor sample, but with somewhat reduced signal to noise ratio compared with the peaks in the bright meteor data. There is also a very slight discrepancy in the position of the “new” maximum between the two samples.
28.6 23.8 29.7 26.0 27.8
Comments
Broad peak Double peak?
Photographic (Longitude 2000)
139.38 139.67 140.31 140.80
2.5. Sporadic meteors During the meteor observations 1953–1983 numerous Aquarids, Alpha Capricornids, Cygnids and sporadic meteors were observed within the solar longitude interval ◦ ◦ 137:00 to 141:50 (equinox 2000). For simplicity in the data handling the Aquarids, Alpha Capricornids, Cygnids and other minor showers in the present study are included ◦ in the sporadic group. As before a bin of 0:10 in so◦ lar longitude was moved forward in steps of 0:05 . Fig. 1 (lower curve) depicts the resulting hourly rate vs solar longitude diagram for all non-Perseid meteors of apparent magnitude 2:5m and brighter. It is seen that the zenithal hourly rate of bright sporadic and minor shower meteors has varied in a random manner between 0.0 and 5.7 meteors per hour. Smallness of sample prevents us from drawing any further conclusions. The corresponding hourly rate variation for all non-Perseid meteors (i.e. independent of magnitude) was 2.5 to 10.0, with slightly higher rates occurring before the Perseid maximum. This higher rate was mainly caused by activity from several radiants in Aquarius. 2.6. Activity proÿle of photographic Perseids In a study of 605 double-station photographic Perseid meteor orbits observed in 1937–1985 Lindblad and Porubcan (1994) showed a histogram (Fig. 4 of the 1994 paper) describing how the number of photographic Perseids varied with solar longitude (it was not possible to compute hourly rates for these very bright objects since the operating schedules of the various photographic stations was not known). The histogram (Fig. 4) showed the existence of the nodal, the “new” and the traditional peaks in the Perseid activity curve. A fourth peak at solar longitude ◦ 140:78 was also discernable. The positions of the various peaks in the photographic data were determined by the authors in the same way as in the visual data, i.e. as a weighted mean of the three highest rates de ning the peak. The resulting positions in the number versus solar longitude diagram of the four main peaks are listed in Table 1. There is a surprisingly good agreement between the photographic meteor data and the bright visual-meteor
B.A. Lindblad / Planetary and Space Science 48 (2000) 905–909
data. However, there is a minor discrepancy in the position of the traditional Perseid maximum. This discrepancy is ascribable to the slightly bi-modal character of the traditional maximum, which makes it dicult to precisely determine the solar longitude of this maximum. 3. Future work The Perseid radar recordings at Onsala were carried out independently of the visual observations for 12 h per day. (One night-time run 22-04 and one daytime run 08-14 MET). Radar recordings were also carried out during various background runs in August and September in order to establish a reliable background rate. The total number of August recordings 1953–1978 was 149 night-time runs and 113 daytime runs, comprising 1939 individual one-hour rates. Owing to severe cuts in funding data for 1979 –1983 have not been fully reduced. Preliminary studies of the Perseid radar rate data for long duration echoes 1953–1978 have been published (Lindblad and Simek (1983,1986)) and for short duration echoes (Simek and Lindblad, 1989). These studies, however, have used sliding means or other smoothing procedures to obtain a mean Perseid activity pro le. This procedure has smoothed out nearly all detail in the resulting Perseid activity curve. It is hoped that in the future it will be possible to re-analyse the complete radar rate data (1953–1983) using the same high-resolution technique as in the present study. Acknowledgements The long-term Perseid observing program at the Onsala Observatory was supported by grants from the Swedish Natural Science Research Council and from Alice och Knut Wallenbergs Stiftelse. Support for data reduction of the visual observations has been provided by Kgl. Fysiogra ska Sallskapet, Lund and from Crafoordska Stiftelsen, Lund. The author gratefully acknowledges this support and also the skilful help provided by the sta of the Onsala Observatory and also by the meteor observers who participated in the observing program. References Ahnert-Rohlfs, E., 1952. Vero entl. der Sternwarte in Sonnenberg. Akad. Verlag Berlin 2 (2), 5–38.
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