Published in WGN, the Journal of IMO 24:5, p. 141 (1996)
Until now 14092 Perseids of 1995 and 7557 Perseids of 1996 are available in the Visual Meteor Database (VMDB). The following observers contributed to the analysis of the Perseid maxima in 1995 and 1996:
Vida Angel, Rainer Arlt, Mária Bartolomejová, Jozef Bezak, Nikola Biliskov, Lucian Boboc, Grzegorz Bonikowski, Emil Brezina, Bob Brown, Marek Bujdos, Jacek Burda, Branko Burmaz, Jaroslava Cablkova, Anja Cervek, Jiang Chang-gui, Vratislav Cillik, Koen Clement, Peter Craven, Jozef Csipes, Mark Davis, David de Wolf, Monika Diallova, Marta Dikova, Iveta Dobrovolna, Joachim Draeger, Radek Dreveny, Jozef Drga, Tomasz Dziubinski, Bert Everaert, Andrea Friebel, Josipa Friscic, Marcin Gajos, Slaven Garaj, Pawel Gembara, Jaroslav Gerbos, Ivanka Getsova, Benny Geys, Vincent Giovannone, George W. Gliba, John Glover, Lew Gramer, Neven Grbac, Valentin Grigore, Andrey I. Grishchenyuk, Adam Grzeszuk, José Luis Guixeras Romero, Andrej Gulis, Peter S. Gural, Wayne T. Hally, Pavol Hanzlícek, Peter Harmady, Yukiti Hattori, Robert Hays, Monika Hazukowa, Veerle Herrygers, Sylwia Holowacz, Kamil Hornoch, Filip Hroch, Vladimír Hrusovsky, Su Hua, Richard Huziak, Oomi Iiyama, Daiyu Ito, Jan Janca, Miroslav Jedlicka, Jaroslava Jelchova, Liu Jing, Michal Jurek, Vaclav Kalas, Stanislav Kaniansky, Fumihiko Kanno, Niladri Kar, Jana Kasparova, Kevin Kilkenny, Timo Kinnunen, Hitomi Kisanuki, André Knöfel, Lubica Kobová, Ralf Koschack, Detlef Koschny, Jaroslav Kovarik, Ales Kratochvil, Dita Krcmarova, Gotfred M. Kristensen, \O{}yvind Kristiansen, Silvija Krizak, Jan Kucera, Martin Kundrat, Alexander Kupco, Lívia Kusá, Ralf Kuschnik, Jari Kuula, Maciej Kwinta, Jan Kysely, Juraj Lacko, Jean-Christophe Lernould, Inge Leyssens, Vladimir Lukic, Robert Lunsford, Kouji Maeda, Peter Majchrak, Urszula Majewska, Veikko Mäkelä, Miroslava Mala, Stefan Malár, Radek Maly, Katuhiko Mameta, Petr Masek, Jan Masiar, Alastair McBeath, Tom McEwan, Norman McLeod, Jana Micikova, Vasile Micu, Carl B. Miller, Koen Miskotte, Radovan Misovic, Hidekatu Mizoguchi, Jan Mojzsis, Sirko Molau, Ivelina Momcheva, Tibor Mrmus, Adrian Mrska, Hisayuki Nagai, Tomas Nasku, Dragana Okolic, Arkadiusz Olech, Jens O. Olesen, Jan Ondrus, Artyom E. Oreshonok, John Penner, Christian Pinter, Jiri Polak, Mila Popovic, Lilia Porozhanova, Tim Printy, Wen Qingliu, Leo Rajala, Pavol Rapavy, Ina Rendtel, Jürgen Rendtel, Maciej Reszelski, Alberto J. Roldán Piracés, Manca Rotner, Julián Ruiz-Garrido Zabala, Lukasz Sanocki, Koetu Sato, Branislav Savic, René Scurbecq, Peter Sedlak, Miguel Serra Martin, Francisco Sevilla, Gregory Shanos, Yasuo Shiba, Anna Sikchina, Eva Skvarkova, Zbynek Slama, Jana Slizova, Lukas Smahel, Alexander Smetanko, James N. Smith, Milos Sochan, Manuel Angel Solano Vinuesa, Manuel Solano Ruiz, Zdeno Sovcik, Ulrich Sperberg, Jiri Srba, Elisa Stefani, Katarina Stefanikova, Svetozár Stefecek, Enrico Stomeo, Wesley Stone, Niko Stritof, Marta Svancarova, Pavel Svozil, David Swann, Richard Taibi, Marko Toivonen, Jiri Tomcik, Daniel Toth, Manuela Trenn, Mihaela Triglav, Josep M. Trigo Rodriguez, Juraj Trojak, Peter Trojak, Elena Valero Rodriguez, Hendrik Vandenbruaene, Michel Vandeputte, Maarten Vanleenhove, Cis Verbeeck, Jan Verbert, Marco Virsek, Bruno Wagner, Thomas Westphal, Linda Wilson, Jean-Marc Wislez, Nikolai Wünsche, Zhou Xingming, Yasuo Yabu, Satiko Yamaguti, Vasilij Yaremchuk, Hiromiti Yosidome, Ilkka Yrjölä, Jerzy Zagrodnik, George Zay, Goran Zgrablic, Peter Zimnikoval, Beata Zimnikovalova, Krzysztof Zurek
A detailed investigation of the recent Perseid activity revealed that it consists of three major parts [1]:
Traces of the outburst Perseids situated roughly 12 hours before the regular Perseid maximum near sol=140° were found in the 1988 and 1989 Perseid analyses [2]. A peak of very high ZHRs was first observed in 1991 and in all subsequent years. Generally, the peak ZHR decreased since 1991 from about 400 to 120 in 1996. This is in agreement with model calculations of Wu and Williams [3] who predicted enhanced rates for the remainder of the century. However, the activity level may decrease to a level which makes its separation from the regular Perseid rates very difficult.
The position of this peak varied from one return to the next, and the pattern of the variations seemed to be unpredictable. The 1996 peak occurred quite close to the 1994 and 1995 positions.
The population index r determined from the magnitude data showed no peculiarities during the activity of the outburst Perseids. However, there seems to occur a significant local maximum of r in the 1996 data. It has to be checked with the complete data whether this feature is really a new phenomenon associated with the increasing distance from the high activity part of this component of the Perseids.
In our analysis, we restrict ourselves to the time period sol=139.5° to 140.1°, i.e., the outburst Perseids and the ascending branch of the regular maximum, designated as core Perseids.
We already mentioned a feature found in the 1996 data analyzed for this report (Figure 1). Here, the value of the population index r seems to steadily decrease from sol=138° from 2.1 towards 1.9 close to 139.0°. Coinciding with the position of the peak, we find r=2.03±0.02, while the later figures can be interpreted as a continuation of a general dip with an ascending branch reaching r=2.1 just after the regular Perseid maximum. If this is a significant structure, this indicates a change in the particle population as compared to the previous passages of the Earth through this region with an increasing portion of fainter meteors. Profiles of the population index r [1] did not show any local structure in the vicinity of the outburst peak.
Figure 1 - Profile of the population index r for the 1996 Perseids obtained from all available magnitude data (as of September 1996). The point at sol=138.911° coincides with the peak period and indicates that the particle size distribution may be different from the surrounding region where r shows a wider dip, just interrupted by the mentioned value. Considering the error bars, the higher figure of r at this moment seems to be significant.
Figure 2 - Profile of the ZHR for the 1995 Perseid peak and the ascending branch to the "regular maximum". The small sample did not allow a better temporal resolution.
The typical profile of the peak as observed in the years 1991 to 1994, particularly well observed in 1993 and 1994, consisted of an ascending branch lasting for some hours, a short peak, and a quite rapid decrease of the ZHR [1]. Leaving out the value of 1992, average full width at half maximum (FWHM) was 0.11°, or 2.8 hours.
Since the ascending and descending branches were different, we distinguish between the half widths at half maximum (HWHM) for the two periods. These are 0.06° (1.5 hours) and 0.04° (1.0 hours) for the ascending and descending branches, respectively.
Figure 3 - ZHR profile of the 1996 Perseid peak and the ascending branch to the "regular maximum". The larger error bars during the peak period result from the scatter of the individual ZHRs of the short count intervals used here.
The 1996 peak showed a slightly different shape (Figure 3). Until sol=139.55°, there was almost no increase in the ZHR, while, at sol=139.60°, an almost immediate rise has been reported. This ZHR rise coincided with the occurrence of a number of fireballs as well, but according to the magnitude data and the population index r this part of the activity also included a large portion of fainter meteors. Here the ZHR reached a level of 120. Most interestingly, this enhanced ZHR lasted for more than one hour. The FWHM is almost 3~hours, the respective HWHMs are 1.3 and 1.7 hours for the ascending and descending branches again. This also shows the rapid increase of the ZHR mentioned above.
Although disturbed by moonlight and hence of less weight, we can detect a similar trend already in the 1995 data, when the duration of the ascending and descending branches was of about the same lengths, considering the HWHM values (see Figure 2). If we use the FWHM as a measure for the width of the peak, we refer to a rate profile consisting of three components (see above). Since the ZHR of the background and core Perseids can be regarded as constant for each return, we suggest to use the width of the graph at a ZHR level above that of the other components as a measure for the width of the outburst component. This method also avoids errors caused by the uncertainty of the peak ZHR values, which may have been influenced by a possibly incorrect value of the population index r and fluctuations in the observers' perception due to varying moonlight influences. We chose a ZHR of 80 as a reference level. This is sufficiently above the ZHR which occurred near the outburst peak position before the peak itself appeared, say before 1988. Except for 1990, where the ZHR of the peak remained below 80, the average duration of the period with a ZHR exceeding 80 is 0.20°±0.12° (about 5±3 hours). The scatter of the individual values is remarkable, and there is no trend within the series between 1988 and 1996.
So, the main feature of the 1996 Perseid ZHR profile is the different relative duration of the ascending and descending branches as compared to the previous returns. The duration of the peak does not significantly differ from the 1991-1995 averages, neither in exceeding a given ZHR level nor considering the FWHM.
Summary of Perseid peak data for the period 1988 to 1994 [1] and this work. The 1990, 1992, and 1995 results should be considered as rough estimates only since these severely suffered from the full moon disturbance.
| Year | sol (outburst) | r | ZHR | S6.5 | sol (max) | r | ZHR | S6.5 |
|---|---|---|---|---|---|---|---|---|
| 1988 | 139.78±0.03 | 2.0 | 86±4 | 97±16 | 140.08±0.04 | 2.1 | 106±22 | 94±14 |
| 1989 | 139.56±0.03 | 2.1 | 102±10 | 127±23 | 139.80±0.09 | 2.1 | 94±6 | 120±20 |
| 1990 | 139.55±0.05 | 1.8 | 75±10 | 45±35 | 140.54±0.2 | 2.1 | 81±61 | 66±5 |
| 1991 | 139.55±0.03 | 2.2 | 284±63 | 494±150 | 139.94±0.04 | 2.1 | 97±2 | 124±20 |
| 1992 | 139.48±0.02 | (2.1) | 220±22 | 257±60 | 140.13±0.2 | 2.0 | 84±34 | 96±15 |
| 1993 | 139.53±0.01 | 2.0 | 264±17 | 242±62 | 139.91±0.04 | 1.9 | 98±5 | 79±34 |
| 1994 | 139.59±0.01 | 1.8 | 238±17 | 151±28 | 139.84±0.04 | 1.9 | 86±2 | 69±12 |
| 1995 | 139.62±0.05 | (2.2) | 171±30 | 290±90 | 139.90±0.15 | 2.1 | 65±20 | 95±20 |
| 1996 | 139.66±0.03 | 2.0 | 121±17 | 114±24 | 140.08±0.04 | 1.7 | 85±10 | 76±20 |
The high figure of the number density of meteoroids causing meteors of magnitude at least 6.5mag (S6.5 in Table 1) for the 1995 Perseid peak is probably an artifact caused by the value of r=2.2 which has been discussed above. Since this is very uncertain, the number density itself is an upper limit at best.
While there are known problems with the determination of limiting magnitudes under moonlit conditions, there are obviously further effects which reduce the value of such observations for detailed analyses. These effects are expected to act in a systematic way. One might think about perception differences particularly of meteors close to the given limiting magnitudes and about selection effects in the procedures to determine the population index r. It is known that counts of high, observable rates under good conditions suffer from a kind of saturation [5], whereas in the case of disturbance by moonlight the number of visible meteors remains low even during the peak period. We suspect that the derived ZHRs during the moonlight-disturbed periods are closer to an upper limit than under "regular" conditions.
The uncertainties of quantities obtained from moonlight-disturbed observations, such as 1992 and 1995, underline that such data can only be used for deriving upper/lower limits of some parameters. Obviously, there are systematic effects from the moonlight disturbance which are difficult to separate. It seems that both the meteor magnitude data (hence the population index r) and the limiting magnitude estimates (hence counts and ZHRs) are affected. Of course, the effects on the values of r and the ZHR do also yield erroneous figures of the spatial number density S.
The average of the 1991-1994 outburst peaks yields HWHMs of the ascending and descending branches of 0.06° (1.5 hours) and 0.04° (1.0 hours), respectively. For the 1996 Perseid ZHR profile, we find a different relation between these branches with 1.3 hours and 1.7 hours, respectively. With caution, this can be suspected also from the 1995 data. However, the duration of the 1995 and 1996 peaks is not significantly differing from the 1991-1994 averages, neither in exceeding a given ZHR level nor considering the FWHM.
As shown in Table 1, the position of the peak varied from one return to the next. Some years ago, the changes in the position looked rather accidental, but seen the entire series as given in Table 1 (plotted as Figure 4), there seems to be a systematic decrease in the solar longitude from 139.78° in 1988 back to 139.48° in 1992, and a subsequent increase in the solar longitude of the peak arriving at 139.66° in 1996. The ascending node of the comet 109P/Swift-Tuttle is at 139.44° [6], hence the 1992 passage was the closest to the orbit of the Perseid parent comet. We also find the longest duration of the outburst peak in 1992: The ZHR exceeded 80 for 0.40°, i.e. roughly 10 hours, and the FWHM was of the order of 7 hours. However, these figures have been derived from moonlight-disturbed data, and one should regard the values as additional information.
Figure 4 - Position of the outburst peak of the Perseids observed in the period 1988 to 1996. There is a systematic drift of the peak position towards the node of the Perseids' parent comet, 109P/Swift-Tuttle, at 139.44° until 1992, and a subsequent drift to a later position again until 1996.
Both the change in the solar longitude of the peak and the peak activity level indicate that the new peak might fall below the detection limit again within the next few years. Perhaps the solar longitude of the peak further increases, thus shifting the peak to a position with a higher ZHR of the core Perseids, making its detection even more difficult.
[1] Brown P., Rendtel J.: The Perseid Meteoroid Stream: Characterization of Recent Activity from Visual Observations. Icarus 124, 1996, pp. 414-428
[2] Roggemans P.: The Perseid Meteor Stream in 1988: A Double Maximum! WGN 17, 1989, pp. 127-137
[3] Wu Z., Williams I.P.: The Perseid meteor shower at the current time. MNRAS 264, 1993, pp. 980-990
[4] Rendtel J.: A first global analysis of the 1994 Perseids. WGN 22, 1994, pp. 205-209
[5] Koschack R., Arlt R., Rendtel J.: Global analysis of the 1991 and 1992 Perseids. WGN 21, 1993, pp. 152-168
[6] Marsden B.G., Williams G.: Catalogue of cometary orbits. Cambridge (Massachusetts), 10th edition, 1995