International Meteor Organization (IMO)

Global Analysis of the 1997 Perseids

Rainer Arlt

Published in WGN, the Journal of IMO 26:2, p. 61 (1998)

Abstract: A total number of 79730 Perseids seen by 520 observers in 5061 h observing time were available for a global activity analysis of the Perseids. The traditional maximum occurred over eastern Asian sites at sol=140.03°±0.03° (eq. 2000.0) with a maximum ZHR of 94±2. The new filament, which has been observed since 1988, showed its maximum at sol=139.71°±0.01° with a ZHR of 137±5. A third distinct activity peak was monitored by European observers at sol=140.35°±0.03° reaching a ZHR of 68±5. When the ZHR profile is converted into a meteoroid flux profile using the population index profile, two distinct maxima before and after the traditional maximum occur, having about the same strength of about 0.02 particles/(km2 h). These maxima are rich in faint meteors.

Introduction

A fair First-Quarter Moon at low declinations spared observers lunar disturbances during the maximum of the 1997 Perseids. A whole week of brilliant weather covered the maximum nights at many European sites. Observers in the western US also enjoyed good conditions, whereas the eastern part saw some cloudy periods and rain. An impressive community of 520 observers from 28 countries monitored the Perseid activity between mid-July and the end of August. We are very grateful to the following observers for their efforts, as well as to those whose observations could not be used, because they were not lucky with the observing conditions:

S. Abdo, A. Al-Niamat, J. Ambroz, M. Andrejko, R. Arlt, J. Assmus, L. Babarikova, B. Baca, A. Bajc, L. Bakmann, L. Balint, A. Bankovic, M. Bares, L. Bastiaens, P. Becvar, G. Beeckman, P. Belov, P. Belak, P. Belcak, I. Benyo, L. Benner, R. Beres, R. Berg, V. Berlecky, F. Bettonvil, D. Bidlen, B. Biller, N. Biliskov, Z. Biliskov, L. Binder, M. Blaho, R. Blatak, R. Bödefeld, E. Bojurova, N. Bone, F. Bové, E. Brezina, M. Broncek, S. Broos, B. Brown, M. Broz, N. Bucek, A. Budovicova, M. Bujdos, I. Buljan, M. Cabala, P. Caillian, P. Campbell, A. Cervek, L. Cerveny, M. Cernak, A. Cesen, D. Chakarov, N. Chambers, P. Chladny, P. Ciljak, V. Cillik, O. Cioroianu, K. Cisar, K. Clement, M. Collier, B. Colyn, U. Cotar, H. Dalee, G. Deconink, W. Deconinck, J. de Hert, S. de Jonge, M. de Lignie, V. Desmarais, P. Detterline, M. Diallova, D. Dielen, L. Diko, M. Djordjevic, D. Dlhopolcekova, L. Dobrovoda, P. Dolinsky, I. Donik, D. Dotzinsky, T. Drandeva, P. Drengubiak, R. Dreveny, J. Drga, M. Dujava, J. Dygos, O. Dzafic, T. Dziubinski, D. Edwards, F. Enzlein, F. Erben, B. Everaert, T. Fajfer, J. Fedor, L. Fekete, M. Fenovcik, D. Ferdinandy, R. Fernandez, K. Fialova, T. Fodor, R. Formanek, C. Foyt, A. Friebel, K. Fukui, N. Fukuda, Y. Fuyube, K. Gaarder, M. Gajos, V. Gajdos, R. Gehlhaar, M. Geltner, J. Gerbos, I. Getsova, B. Geys, T. Giguere, M. Gillis, G. Gliba, I. Goethals, R. Gorelli, L. Gramer, J. Griscik, M. Growe, E. Guetens, P. Gural, P. Habuda, M. Hadidi, C. Hall, J. Halkova, K. Halir, W. Hally, H. Handjiiski, J. Hancar, K. Hanusova, T. Hansen, P. Harmady, T. Hashimoto, R. Haver, R. Hays, L. Heen, B. Heinrich, B. Held, A. Hemsy, B. Hendrickx, U. Henning, V. Herrygers, M. Hevesi, Z. Hevesi, T. Hillestad, W. Hinz, M. Hiriak, A. Hirv, D. Holman, R. Holodnak, N. Hontelé, D. Hostetter, S. Hribar, Z. Hrotekova, V. Hrusovsky, D. Hubner, J. Hudecek, R. Hughes, M. Husnaj, R. Huziak, O. Iiyama, O. Imamura, C. Ishikawa, M. Isii, N. Ishiwata, D. Ito, M. Ivanovic, P. Ivanov, K.\ Izumi, S. Izuhara, Y. Izuhara, V. Jankov, M. Jarski, M. Jedlicka, C. Johannink, W. Jonderko, H.-S. Jung, J. Kac, V. Kalas, K. Kaminski, M. Kania, P. Kanuk, J. Karabas, J. Kasparová, H. Kawaguchi, K. Kawabata, T. Kawasima, Á. Kereszturi, K. Kerekesova, M. Keresztessy, K. Kilkenny, L. Kirby, A. Knöfel, J. Kohout, H. Koide, Z. Komarek, K. Konsul, M. Konopka, M. Korec, R. Koromhaz, R. Koschack, D. Koschny, G. Koschny, N. Kosiyama, M. Kotur, A. Kovalova, J. Kovarik, J. Kozak, A. Krajcírová, A. Kratochvil, A. Krawietz, L. Krajci, L. Kral, D. Krcmarova, A. Kremzer, I. Krestianko, Ø. Kristiansen, V. Krumov, R. Kucman, T. Kucharski, G. Kudor, A. Kupco, Y. Kurosawa, P. Kusnirák, R. Kuschnik, M. Kwinta, J. Lacko, M. Lacko, M. Langbroek, A. Latkoczy, A. Latini, L. Lenza, M. Leustek, M. Limpens, R. Liska, M. Litavsky, R. Löwenherz, R. Lunsford, H. Luthen, G. Maciejewski, K. Maeda, T. Maets, G. Mahres, S. Majnik, M. Mala, T. Malek, K. Mameta, R. Manak, T. Mancic, A. Marek, B. Martinak, J. dos Reis Martins, P. Martin, R. Marecek, J. Masiar, M. Maturkanic jr., Y. Matumoto, M. Mazak, A. McBeath, N. McLeod, S. McLeod, L. Mecir, R. Medlín, D. Metakhov, H.-J. Mettig, I. Micek, J. Micikova, V. Micu, P. Mikulka, R. Mikusinec, T. Miklos, I. Miljacki, N. Milutinovic, V. Miovic, I. Miseje, J. Miskuf, K.\ Miskotte, H. Mizoguchi, J. Mocek, M. Mocak, H. Mokrisova, S. Molau, I. Momcheva, M. Morrow, S. Mori, S. Moravcik, T. Morgan, M. Mraz, T. Morikawa, W. Murakami, J. Murin, M. Muraki, S. Nakayama, Y. Nakayama, T. Nasku, S. Näther, S. Nedeljkovic, K. Nicasi, D. Nikolic, M. Nitschke, T. Nonay, H. Nose, M. Novak, D. Ocenas, M. Odeh, I. Odwan, M. Oka, A. Olech, J. Olesen, J. Ondrus, P. Onufrak, A. Oreshonok, Z. Orsag, D. Ortmanns, E. Ortmanns, K. Osada, K. Osaki, K. Pagacova, P. Panda, A. Panos Moya, A. Papista, T. Pavlovic, L. Pekarik, S. Pelckmans, M. Penev, K. Perunska, L. Petersen, N. Petelin, K. Piekarzova, P. Pisara, R. Pitaluga, J. Plazar, H. Plott, I. Polakova, J. Polak, K. Popanastasov, M. Popovic, L. Porozhanova, L. Pospieszny, Z. Pospechova, P. Potucek, L. Pozdisek, J. Prudic, M. Rankin, P. Rapavy, L. Rashkova, T. Rattei, A. Rendtel, I. Rendtel, J. Rendtel, P. Rendtel, J. Richter, J. Ridzyova, V. Rodiger, D. Rombauts, M. Rosseel, M. Rosina, M. Rudolph, V. Ruiz Ruiz, B. Ruzickova, S. Ruzicka, J. Sajdl, M. Sakaguchi, R. Sampson, J. Sandel, L. Sanocki, K. Sárneczky, K. Sato, T. Sato, T. Sato, M. Schmidhuber, T. Schreyer, R. Scurbecq, P. Sedlak, M. Sefara, H. Seifert, T. Sekiguchi, I. Sergey, M. Serra Martin, B. Shulist, G. Sill, H. Sioi, A. Skoczewski, I. Skokic, K. Skoczewska, M. Skreka, J. Skvarka, V. Slavkovic, J. Slizová, J. Sliz, L. Smahel, J. Smith, T. Sobczak, K. Socha, M. Sochan, J. Solomon, M. Solano Ruiz, A. Sosik, P. Spanik, J. Srba, J. Stancel, J. Stas, U. Stagno, J. Stefecek, J. Stehlik, K. Stefanikova, L. Steensgaard, S. Stefecek, C. Stijn, T. Satomi, E. Stomeo, S. Stomeo, W. Stone, E. Strivinska, N. Stritof, S. Sullivan, B. Susmak, E. Suskova, M. Sustr, M. Suzuki, P. Svozil, R. Svrcina, R. Sykora, A. Szaruga, G. Szasz, K. Szaruga, R. Szczerba, R. Taibi, H. Takiguchi, M. Takanasi, M. Takanasi, K. Tanaka, S. Tanaka, K. Tell, M. Tirpak, M. Tkacik, M. Toda, R. Togni, J. Tomcik, Y. Tonomura, T. Tornyos, D. Tóoth, T. Tóth, M. Trenn, G. Triglav, M. Triglav, J. Trigo Rodriguez, P. Trybus, S. Uehara, M. Uhlar, H. Ulbricht, J. Urban, D. Vajda, B. Vajdova, E. van Ballegoy, K. Van Beurden, M. Van den Broeck, H. Vandenbruaene, J. Vandenbruaene, G. Van de Weyer, K. van Gorp, S. van Impe, M. Vanko, F. van Loo, P. van Loo, B. van Opstal, C. van Olmen, G. van Olmen, J. Vansteelandt, A. van Weerden, P. Vargovic, J. Varju, V. Velkov, C. Verbeeck, J. Verbert, S. Veren, G. Vince, E. Vinceova, M. Vingerhoets, W. Vinken, A. Vlasaty, M. Vucelja, F. Wáchter, S. Wáchter, B. Wagner, G. Wagner, J. Wagner, T. Weiland, N. Werner, T. Westphal, B. Wilson, R. Winkler, G. Witzler, K. Wtorek, N. Wünsche, O. Wusk, Y. Yabu, S. Yanagi, H. Yamashita, N. Yamashita, Y. Yonekura, N. Yosimura, K. Yosino, K. Yosizaki, S. Zabic, J. Zacek, M. Zapletal, E. Zapletalova, H. Zaunick, G. Zay, M. Zibar, B. Zimnikovalova, M. Znásik, I. Zsolnai, and T. Zywczak

The observers are from the following countries:

Austria, Belarus, Belgium, Bulgaria, Canada, Croatia, Czech Republic, Denmark, Estonia, Germany, Gibraltar, Hungary, Italy, Japan, Jordan, Korea, Malta, the Netherlands, Norway, Poland, Portugal, Romania, Slovakia, Slovenia, Spain, United Kingdom, United States, and Yugoslavia.

Besides the traditional maximum around a solar longitude of sol=140°, a new peak of variable activity has been observed since 1988, which is mainly associated with the perihelion passage of the parent comet, 109P/Swift-Tuttle. The position of this new peak was expected to occur several hours before the traditional maximum, at sol=139.71° (eq. 2000.0), or August 12, 8h30m UT)[1]. We will refer to this maximum by the terms "first" peak or "new-filament" peak.

Method of analysis

The calculation of a population index profile should precede any serious ZHR calculation. The population index r describes the increase of meteor numbers from one magnitude class to the next fainter one, and is needed to correct observations with other than the standard limiting magnitude (lm) of 6.5. The r-value of the Perseids can be computed from the magnitude distribution when the perception probabilities for meteors of each magnitude are known. These probabilities were derived from a considerable meteor sample in [2].

Individual magnitude distributions should fulfil the following significance criteria:

The meteor numbers observed per magnitude are converted into true meteor numbers using the perception probabilities. The population index results from the regression line through the logarithm of the true meteor numbers versus magnitude. The individual r-values are averaged for a population index profile.

The Zenithal Hourly Rate (ZHR) of a meteor shower at a certain time is calculated by

ZHR=r(6.5-lm-Deltalm) F n / (sin hR Teff)

where r is the population index, lm the limiting magnitude, F the correction for obstructions of the field of view (clouds), $n$ the number of shower meteors, hR the radiant elevation, and Teff the effective observing time. The value of Deltalm corrects the observer's individual perception of meteors and must be derived in the course of the analysis. The correction for perception should not be confused with the perception probabilities which give the fraction of the true meteor number seen by a typical observer per magnitude class. In the case of correction of perception, we deal with the systematic, individual deviation in the total number of meteors seen.

In this analysis, the values of Deltalm were derived from a ZHR profile computed without perception correction (Deltalm=0.0). Periods of constant or slowly increasing activity were used to obtain several perception factors for each observer, considering the average ZHR as the true rate. These factors are then averaged and expressed as a difference in stellar limiting magnitude and meteor limiting magnitude, Deltalm. The computation of perpection from the ZHR profile is preferred to a calibration by sporadic rates, since the meteor numbers the ZHR profile is based on are much larger than the sporadic meteor numbers, which suffer considerably more from Poissonian errors.

Individual rates selected for the ZHR profile have to fulfil the following criteria:

An empirical exponent in the zenith correction was suggested by Öpik [3]. The zenith correction factor he proposed is singamma hR, with gamma the so-called zenithal exponent. Bellot Rubio [4] and Koschack [5] showed that this exponent is close to 1.0 for visual Perseid data. Hence, we ignore the zenith exponent in our analysis; the lower limit for the radiant height prevents us from too large errors by doing so.

The averaging procedures for both the population index profile and the ZHR profile include an outlier rejection algorithm. The position of the averaged value in the diagram is the mean solar longitude of the values involved in the average. All solar longitudes in this paper refer to equinox 2000.0.

Population index profile

The full profile of the population index between July 28 and August 16 is shown in Figure 1. Too few meteors were recorded before and after these dates to derive a reliable r-value. Most of the averages before the activity maximum lie between roughly 2.1 and 2.2, except for two fairly reliable values at sol=135.5°-136° (August 8), when the population index drops to 2.03±0.04. Whilst these points are based on 44 and 69 individual observations, respectively, the two higher values before are averages of only 21 and 23 observations, respectively. Such variations seem to be a feature of just one particular year, since, e.g., the 1989 profile [6] shows a significant increase in r up to 2.3 at the same time. The same holds for the 1992 profile, where r reached 2.5. In 1991, the population index was almost constant at 2.2 around sol=135.5° [7].

Figure 2 shows a magnification of the population index profile between sol=138.2° (August 10, 19h UT) and sol=140.6° (August 13, 7h UT). A strong decrease of r down to r=1.80±0.04 can be seen around sol approx. 139.7°, where we expect the first, sharp activity peak to take place. No population index information is given at solar longitudes sol=139° and sol=140°, since no magnitude distributions are available from Japanese observers. Hence, we have no r-value where the traditional maximum is expected. At sol=140.31°, a sharp peak in the population index reaches r=2.24±0.06. This increase is linked with a feature in the activity profile described below.

Figure 1 - Complete profile of the population index r of the 1997 Perseids.

Figure 2 - Magnification of Figure 1 around the maximum of the 1997 Perseids.

ZHR profile

The activity profile of the Perseids includes the application of a personal perception correction which is derived from activity averages of periods with constant or only gradually changing activity. Since the perceptions of individual observers were computed within this analysis, they may be quite representative for the observers' capabilities at that time, but leave us with rather large uncertainties due to the limited sample. The correction values are given in Table~1: "Obs" is the number of perception estimates being averaged, cp is the perception coefficient, which is the mean factor by which the observer deviated from the average ZHR, and Deltalm is the resulting difference in meteor limiting magnitude and stellar limiting magnitude.

An alteration in the limiting magnitude is more appropriate than a correction factor, since observers usually differ in their abilities to detect faint meteors. Hence, their perception correction should depend on the r-value, which is properly reproduced by a Deltalm. Positive values mean that the observer saw too many meteors, and that his or her actual meteor limiting magnitude is higher than what was given on the observing report. Negative values reduce the limiting magnitude estimate, since the observer saw too few meteors. Note that not all observers listed in the Introduction were active in the periods selected for perception estimates. Moreover, only observers with more than 3 perception estimates were selected.

Figure 3 shows the complete activity profile of the 1997 Perseids from sol=110° (July 12) to sol=152° (August 25). A minor increase of the ZHR up to values around 15 seems significant around sol=123° (July 24-26). Off-maximum features of major-shower profiles are difficult to interpret as they usually appear in only one year.

The main structures of the activity profile were already given in [8]. The most prominent feature in the detail of the activity profile shown in Figure 4 is the new Perseid peak at sol=139.72±0.01° with a maximum ZHR of about 130. This activity level is not lower than the 1996 one, despite a gradual decrease between 1993 and 1996 [9,10].

Figure 3 - ZHR-profile of the 1997 Perseids.

Figure 4 - Magnification of the ZHR profile of Figure 3 around the maximum of the 1997 Perseids.

Table 1 - Perceptions

Observer#ObscPDelta lmObserver#ObscPDelta lm
Abdo Sana'a41.06+0.03±0.37Gerbos Jaroslav201.34+0.38±0.22
Al-Niamat Ahmad41.46+0.51±0.18Giguere Tom 40.90-0.16±0.19
Ambroz Jaroslav240.99-0.05±0.33Gillis Maarten100.87-0.19±0.21
Andrejko Marcel60.91-0.17±0.34Gliba George W. 41.33+0.35±0.33
Arlt Rainer271.12+0.08±0.42Goethals Ivan121.02+0.02±0.13
Assmus Joseph D.80.92-0.14±0.10Gorelli Roberto 40.99-0.13±0.63
Baca Branislav181.00-0.06±0.40Gramer Lew 61.46+0.46±0.44
Bakmann Lars61.45+0.50±0.16Griscik Jurai 81.36+0.35±0.41
Balint Ladislav61.40+0.46±0.13Growe Matthias 40.60-0.71±0.21
Bares Michal280.55-0.94±0.66Habuda Pavol181.26+0.29±0.32
Becvar Petr120.61-1.12±1.25Hadidi Muammar 81.29+0.35±0.10
Belcak Pavol40.49-0.99±0.30Halkova Jaroslava100.72-0.85±1.22
Benyo Igor40.73-0.53±0.61Hall Cathy261.11+0.02±0.61
Beres Rastislav41.13+0.16±0.06Hally Wayne T. 41.21+0.26±0.16
Berg Ray151.23+0.18±0.54Hancar Jozef100.90-0.17±0.32
Berlecky Viktor160.58-1.05±1.00Handjiiski Hristo T. 51.35+0.31±0.53
Bidlen David140.77-0.59±0.93Hansen Torsten 60.82-0.48±0.87
Biliskov Nikola40.61-0.68±0.05Hanusova Katerina240.86-0.25±0.35
Blaho Miroslav111.10+0.11±0.25Harmady Peter171.64+0.64±0.16
Bödefeld Ragnar160.95-0.10±0.31Hashimoto Takema181.16+0.18±0.40
Bojurova Eva200.61-0.78±0.64Haver Roberto120.88-0.31±0.63
Bone Neil321.25+0.20±0.53Heinrich Bernd 61.36+0.39±0.36
Bové Frederick90.75-0.44±0.43Held Branislav 41.49+0.54±0.13
Brezina Emil41.28+0.35±0.07Hemsy Ala'a 60.84-0.28±0.29
Broncek Michal170.64-0.64±0.41Henning Udo190.88-0.21±0.32
Brown Bob40.59-0.89±0.16Hevesi M\'onika 60.61-0.71±0.33
Broz Miroslav41.43+0.47±0.27Hevesi Zoltán 60.62-0.66±0.11
Cabala Milos40.82-0.39±0.66Hiriak Mario 81.03-0.01±0.38
Cernak Milan41.37+0.43±0.09Holodnak Rudolf 90.97-0.12±0.54
Cerveny Lukas81.00-0.11±0.58Hontel\'e Nathalie 40.78-0.32±0.10
Chakarov Decho161.20+0.16±0.45Hribar Stanka 51.38+0.40±0.37
Chladny Pavol101.56+0.54±0.40Hrusovský Vladimír101.08+0.04±0.47
Cillik Vratislav131.41+0.45±0.20Hubner Dusan 41.15+0.12±0.48
Cioroianu Ovidiu151.35+0.35±0.37Hughes Robert 81.02-0.12±0.73
Collier Matthew90.94-0.12±0.19Husnaj Milan 41.00-0.02±0.19
Dalee Hani41.30+0.35±0.03Huziak Richard100.92-0.17±0.44
Deconink Goedele170.68-0.57±0.42Imamura Osamu 40.78-0.39±0.13
Diallova Monika140.90-0.23±0.49Ishikawa Chiaki 41.02-0.12±0.79
Dielen Didier80.57-0.95±0.88Ishiwata Noriko 41.09+0.12±0.20
Diko Lukas41.58+0.63±0.18Ito Daiyu 60.99-0.03±0.09
Dlhopolcekova Dagmar110.68-0.53±0.30Ivanov Peter 40.94-0.09±0.08
Dobrovoda Lubomir81.01-0.05±0.43Izuhara Sinitirou 70.89-0.17±0.06
Dolinsky Peter41.19+0.16±0.53Izuhara Yumi 81.11+0.07±0.56
Dotzinsky Doytchin40.90-0.25±0.60Izumi Kiyoshi100.72-0.50±0.16
Drengubiak Peter60.66-0.62±0.41Jedlicka Miroslav 40.99-0.03±0.11
Dreveny Radek60.66-0.64±0.46Johannink Carl391.59+0.60±0.23
Drga Jozef61.01-0.13±0.67Jonderko Wojciech210.55-0.83±0.25
Dujava Milan101.28+0.32±0.26Kalas Vaclav280.66-0.64±0.53
Dygos Jaroslaw440.57-0.82±0.48Kaminski Krzysztof 41.14+0.17±0.25
Dzafic Oliver141.63+0.62±0.23Kania Maciej100.91-0.50±0.98
Enzlein Frank101.58+0.61±0.21Kanuk Pavol 81.39+0.45±0.11
Erben Frantisek81.48+0.53±0.22Karabas Jan221.51+0.52±0.30
Everaert Bert131.12+0.09±0.46Kasparová Jana270.88-0.23±0.43
Fajfer Tomasz401.42+0.43±0.33Kerekesova Katarina 61.24+0.29±0.15
Fedor Juraj201.05-0.05±0.54Keresztessy Michal120.96-0.16±0.58
Fialova Karolina60.81-0.50±0.85Kereszturi Ákos 60.90-0.18±0.25
Fodor Tamás60.81-0.44±0.74Knöfel André 40.84-0.24±0.02
Foyt Charles60.58-0.94±0.37Kohout Jan 40.74-0.46±0.31
Friebel Andrea61.10+0.09±0.36Koide Hideki 40.82-0.30±0.15
Fukuda Nobuyuki40.86-0.26±0.37Komarek Zdenek181.49+0.50±0.39
Fukui Keiiti50.65-0.72±0.68Konopka Marcin170.87-0.27±0.47
Gaarder Kai41.09+0.11±0.17Konsul Khalil 40.59-0.85±0.68
Gajdos Vladimir100.87-0.31±0.64Korec Matej121.66+0.62±0.41
Gajos Marcin41.37+0.43±0.07Koromhaz Ratislav120.74-0.52±0.56
Gehlhaar Robert120.47-1.10±0.57Koschack Ralf 70.94-0.09±0.13

Observer#ObscPDelta lmObserver#ObscPDelta lm
Koschny Detlef 81.71+0.70±0.24Odwan Ibrahim 41.08+0.08±0.31
Kosiyama Nobuyuki 40.90-0.17±0.19Oka Masayuki121.21+0.29±0.17
Kovalova Alzbeta110.60-0.97±1.04Olech Arkadiusz330.95-0.09±0.19
Kovarik Jaroslav230.80-0.38±0.56Olesen Jens O. 80.85-0.30±0.51
Kraj\v c\'{\i}rová Anna 70.92-0.15±0.35Ondrus Jan 60.83-0.29±0.33
Kral Lukas 40.73-0.44±0.26Onufrak Peter130.67-0.60±0.37
Krawietz Andreas200.56-0.83±0.38Osada Kazuhiro 81.60+0.61±0.13
Krcmarova Dita120.92-0.13±0.26Osaki Kazuhiko 61.05+0.01±0.38
Krestianko Imrich 81.19+0.23±0.10Pagacova Katarina 41.06+0.04±0.48
Krumov Vladimir 80.77-0.38±0.32Panos Moya Andres Rafael140.62-0.86±0.79
Kucharski Tom 60.68-0.61±0.62Papista Adrian 61.45+0.50±0.24
Kucman Roman 41.46+0.50±0.17Pekarik Ladislav180.80-0.43±0.62
Kudor Gy\"ongyv\'er 60.80-0.34±0.25Pelckmans Simon 70.59-0.73±0.37
Kupco Alexander261.36+0.39±0.31Piekarzova Katerina200.65-0.65±0.51
Kuschnik Ralf211.28+0.30±0.30Pisara Peter 31.08-0.11±0.87
Kusnirák Peter 41.13+0.15±0.17Plazar Janja 41.66+0.67±0.25
Kwinta Maciej340.83-0.28±0.30Polakova Ivana 41.03+0.01±0.33
Langbroek Marco271.38+0.39±0.35Popanastasov Kostadin120.96-0.21±0.73
Lenza Libor141.08+0.08±0.25Porozhanova Lilia 61.13+0.07±0.57
Liska Robert 60.74-0.42±0.13Pospechova Zuzana120.72-0.45±0.17
Litavsky Milan 41.19+0.24±0.08Pospieszny \L{}ukasz100.45-1.15±0.37
L\"o{}wenherz Richard111.48+0.49±0.34Potucek Peter 61.74+0.76±0.09
Lunsford Robert251.23+0.15±0.59Rankin Mel 40.82-0.38±0.52
Maciejewski Gracian231.03-0.02±0.42Rapavy Pavol131.18+0.18±0.28
Majnik Szabolcs 60.61-0.68±0.17Rashkova Lina Hristova 81.32+0.32±0.29
Mala Miroslava230.72-0.48±0.38Rendtel Andreas 80.66-0.80±0.63
Malek Tomas 40.96-0.06±0.12Rendtel Ina 61.10+0.12±0.24
Mameta Katuhiko 40.96-0.10±0.40Rendtel J\"urgen181.32+0.39±0.30
Manak Roman 60.53-0.97±0.56Rendtel Petra101.30+0.41±0.27
Marecek Robert 80.93-0.18±0.49Richter Janko140.48-1.03±0.52
Marek Ales141.44+0.40±0.48Ridzyova Jana141.30+0.24±0.51
Martin Pierre341.35+0.35±0.36Rodiger Vanja111.34+0.26±0.74
Martinak Boris 81.46+0.50±0.07Rombauts Dirk 40.31-1.62±0.26
Maturkanic Michal jr. 40.38-1.32±0.18Rosina Milan 41.23+0.27±0.12
Mazak Miroslav 41.08+0.07±0.32Ruiz Ruiz Victor 60.60-0.85±0.66
McBeath Alastair 61.85+0.75±0.51Ruzicka Stefan251.29+0.28±0.38
McLeod Sherri 41.24+0.31±0.42Ruzickova Blanka121.36+0.41±0.20
Mecir Lukas160.51-1.08±0.76Sajdl Jaroslav140.46-1.83±1.63
Medl\'{\i}n Rostislav 60.48-1.59±1.75Sampson Russ 61.45+0.59±0.26
Metakhov Dimiter161.35+0.33±0.52Sandel Jeffery 40.95-0.14±0.46
Mi\v{c}ek Ivo 61.14+0.14±0.35Sanocki \L{}ukasz 60.74-0.43±0.31
Micikova Jana 41.30+0.35±0.12Sárneczky Krisztián 60.65-0.60±0.21
Micu Vasile 40.66-0.62±0.46Sato Koetu 90.60-0.71±0.34
Mikulka Pavel 61.35+0.40±0.23Schreyer Thomas 40.73-0.41±0.16
Mikusinec Roman101.10+0.09±0.35Scurbecq Ren\'e171.28+0.29±0.39
Milja\v{c}ki Iris 40.98-0.02±0.07Sedlak Peter181.90+0.81±0.28
Miseje Ivan 60.59-0.95±0.94Sergey Ivan M.111.40+0.38±0.39
Miskotte Koen441.24+0.27±0.22Shulist Brian191.00-0.05±0.30
Mizoguchi Hidekatu 41.24+0.13±0.68Sill Godfrey 41.07+0.08±0.10
Mocek Jan 40.62-0.67±0.04Sioi Hiroyuki 60.76-0.46±0.44
Mokrisova Hava120.59-0.79±0.41Skoczewski Andrzej100.99-0.03±0.23
Molau Sirko271.18+0.21±0.13Skreka Marcel 60.72-0.46±0.11
Momcheva Ivelina 40.60-0.71±0.24Slavkovi\'c Vesna150.99-0.02±0.15
Morgan Thom100.95-0.16±0.48Sliz Julius 80.94-0.21±0.70
Mori Sigehiro 40.92-0.13±0.16Sli\v{z}ová Jana 81.15-0.04±0.98
Mraz Michal 61.04+0.04±0.20Smahel Lukas 91.56+0.60±0.13
Muraki Minoru120.79-0.51±0.74Smith James N.180.80-0.40±0.53
Nakayama Sin 40.33-1.73±0.09Sobczak Tadeusz281.04+0.02±0.32
Nasku Tomas 60.49-0.98±0.09Socha Krzysztof100.97-0.07±0.33
Nedeljkovi\'c Sasa 41.14+0.17±0.25Sochan Milos171.49+0.47±0.39
Nitschke Mirko 41.27+0.32±0.06Solomon Jan120.68-0.77±0.87
Nonay Terry 60.79-0.42±0.34Srba Ji\v{r}i 61.58+0.63±0.06
Novak Matus201.20+0.21±0.29Stancel Jan 61.14+0.06±0.61
Ocenas Daniel 81.12+0.09±0.48Stefanikova Katarina 61.11+0.08±0.47
Odeh Mohammad 71.33+0.39±0.12\v{S}tefe\v{c}ek Ján 60.98-0.21±0.80
Observer#ObscPDelta lmObserver#ObscPDelta lm
\v{S}tefe\v{c}ek Svetozár 61.73+0.74±0.14Vandenbruaene Hendrik 71.20+0.24±0.23
Stehlik Jaroslav 40.72-0.47±0.27Vandenbruaene Jan 40.66-0.59±0.15
Stijn Calders 80.74-0.41±0.33Vanko Martin101.56+0.58±0.25
Suskova Eva191.38+0.40±0.37Vargovic Peter 61.24+0.23±0.49
Sustr Matej 40.52-0.91±0.22Varju Jozef 60.94-0.10±0.23
Suzuki Masafumi131.31+0.41±0.09Velkov Valentin 91.13+0.12±0.27
Svozil Pavel 40.87-0.19±0.10Verbeeck Cis101.31+0.34±0.32
Svrcina Rudolf 41.67+0.69±0.08Verbert Jan141.43+0.44±0.34
Szaruga Konrad201.31+0.31±0.36Veren Suzana 40.97-0.12±0.55
Szczerba Robert410.93-0.13±0.30Vince Gabriel 41.51+0.53±0.36
Takanasi Masaaki 81.08+0.12±0.06Vingerhoets Myriam 60.66-0.67±0.65
Takanasi Mika 61.13+0.18±0.10Vucelja Marija 60.92-0.16±0.41
Tanaka Syoiti 81.08+0.11±0.18Wáchter Sabine 60.62-0.65±0.20
Tell Khaled 71.26+0.30±0.14Wagner Bruno 41.44+0.49±0.13
Toda Masayuki 40.73-0.48±0.01Weiland Thomas220.88-0.22±0.36
Tomcik Jiri201.11+0.12±0.21Witzler Gudrun 41.39+0.45±0.17
Tornyos Tomas 40.67-0.58±0.30Wtorek Krzysztof100.74-0.55±0.61
Toth Daniel 41.52+0.54±0.33Wünsche Nikolai 60.97-0.11±0.51
Tóth Tamás 60.67-0.59±0.31Wusk Oliver 91.30+0.29±0.47
Trenn Manuela141.45+0.47±0.32Yabu Yasuo110.75-0.42±0.14
Triglav Mihaela 80.99-0.04±0.33Yanagi Sinitirou 40.82-0.31±0.22
Trybus Pawel161.20+0.24±0.21Yonekura Yasuyuki 40.97-0.04±0.09
Ulbricht Heiko 40.35-1.55±0.70Yosizaki Katuhiro 61.06-0.14±0.77
Urban Juraj121.12+0.09±0.42Zapletalova Eva 60.67-0.62±0.48
Vajdova Bohdana 81.36+0.40±0.27Zaunick Hans-Georg180.31-1.66±0.60
Van de Weyer Geert100.73-0.53±0.60Zay George391.28+0.27±0.44
van Loo Peter 81.24+0.24±0.36Zibar Martin 60.61-0.71±0.43
Van Olmen Christophe 40.46-1.37±1.15Znásik Miroslav171.03-0.01±0.35
Van Olmen Glenn170.76-0.39±0.26Zsolnai Imro 61.16+0.19±0.23
van Weerden Anne 61.11+0.03±0.61

Figure 5 shows the details of the first Perseid peak, derived from observing periods no longer than 40 minutes. Although the averaging period is only about 30 minutes, the profile is still very smooth and has a maximum at sol=139.71±0.01°; the peak ZHR is 137±5. It should be noted that this number represents the activity if peak rates would have lasted for 1 hour. In fact, the maximum lasted only about half an hour, whence the term "equivalent ZHR" (EZHR) would be more appropriate here [11].

Figure 5 - Magnification of the ZHR profile at the first, sharp activity peak. Shorter averaging periods than in Figures 3 and 4 were used, whence the higher peak rates.

The second strongest activity component is the broad traditional maximum at sol=140.03±0.03, with a maximum ZHR of 94±2. The actual level of activity may be altered by the true population index which is unknown as can be seen in Figure 3. The r-value is interpolated between the neighboring points at sol=139.8° and sol=140.2°.

Finally, we observed a third slight increase of activity after the traditional maximum of the Perseids at sol=140.35±0.03°, reaching a ZHR of 68±5. It should be noted that the peak was more prominent before the correction for individual observers' perception was applied.

Figure 6 shows the meteoroid flux per hour and square kilometer derived from the activity and population index profiles. This quantity refers to particles which produce meteors brighter than magnitude 6.5. The flux rho is very sensitive to the population index and varies roughly proportionately to r2. High population indices cause higher fluxes than small r-values. The maximum flux of the first peak occurs about at sol=139.58±0.01°, which is 0.13° (3 hours) before the ZHR peak. The ZHR gives the number of meteors an observer sees at lm=6.5; for the flux, we need the true number of meteors up to magnitude 6.5. This is one of the reasons why the population index is again involved in the computation of fluxes, and the high $r$ before the actual visible peak implies maximum meteoroid fluxes. The low r at the peak means that the observers did not "miss" too many faint meteors, because they were just less numerous.

Figure 6 - Profile of the meteoroid flux of Perseid particles causing meteors brighter than magnitude 6.5.

The post-maximum increase, however, is well-pronounced in the flux profile, because of the relatively high population index of r=2.24±0.06 at sol=140.31°. The peak is in fact of about the same level as the first peak. The flux strength at the traditional maximum is less significant, since we have no population index for that period.

Discussion

It is very astonishing how well the first peak of the Perseids was predicted, the difference being only 0.01° (15 minutes) in solar longitude [1]. Fortunately, the peak ZHR was significantly underestimated, leaving us with the hope for future activity peaks from that stream component.

Another feature of the 1997 ZHR profile can be used to argue against this, however. The third, additional maximum at sol=140.35° has not been observed in previous years. However, the 1989 profile shows a distinct shoulder of activity after the traditional maximum lasting until sol=140.5°. In the 1988 ZHR-profile, the traditional maximum was very broad and lasted until about sol=140.35°. These facts suggest that such a third activity increase accompanies both the first years of appearance of the new-filament peak as well as the last years when it is assumed to vanish. Consequently, we should be prepared to see nothing of the first peak anymore in one of the next years, probably in 1999.

A particle simulation of meteoroids ejected at the 1862 perihelion passage of Comet 109P/Swift-Tuttle by Wu and Williams [12] used three sets of orbital parameters of the comet to integrate the particle motion. The model with closest results to the observed rates between 1988 and 1997 indicates enhanced activity until the end of this century. The computation did not extend beyond 2000 AD.

The strong prominence of the post-maximum increase in 1997 may be due to a number of high-perception observers who were active in Europe on August 12-13. Although their influence was diminished by the perception correction, it might not have been compensated completely. Another explanation involves the population index which will also be affected by high-perception observers and which is not corrected by a similar limiting-magnitude shift Deltalm. The easier detection of faint meteors moved the r-value up and affected the ZHR by too high lm-corrections (if lm<6.5), despite the Deltalm being correctly applied in the activity calculation. We computed another population index profile after a Deltalm-correction in the magnitude distributions. In fact, some features change: the minimum in r becomes even more distinct with r=1.73±0.06, but the r-dip at sol approx. 135°-136° is much less prominent; the rest of the profile remains the same.

This way of population index correction is, however, based on ZHRs with the uncorrected r-profile. We should in turn recalculate the ZHR profile and derive new perception values. It becomes clear that we deal with an iterative process; such an analysis needs much more time and will not be given here. It may just be noted that the post-maximum increase remains distinct, but turns into a plateau between sol=140.3° and sol=140.4°.

The lowest population index value, r=1.80±0.04, occurs at sol=139.70±0.04, coinciding with the first activity peak. This dip implies that the actual particle density in the Perseid stream (causing m<=6.5mag) is much lower than the visual impression. Peak fluxes are found before the ZHR peak, very close to the passage time of the comet's orbital node. The high population index after the traditional maximum implies high fluxes at the third peak as well. Enhanced fluxes of about the same level before and after the traditional maximum are also present in the 1989 profile of spatial number densities (spatial number density is flux multiplied by geocentric velocity, a constant). We conclude the following:

The flux profiles for 1990 to 1996 should be revisited for traces of a similar twin structure besides the strong visual activity peak.

References

[1] R. Arlt, S. Molau, M. de Lignie: Observers' Notes for the 1997 Perseids. WGN 25:3, June 1997, pp. 123-124

[2] R. Koschack, J. Rendtel: Determination of Spatial Number Density and Mass Index from Visual Meteor Observations (II). WGN 18, August 1990, pp. 119-140

[3] E. Öpik: Analysis of 1436 Meteor Velocities. Tartu Obs. Publ. 30:5, 1940, pp. 3-86

[4] L.R. Bellot Rubio: Effects of a Dependence of Meteor Brightness on the Entry Angle. Astron. Astrophys. 301, 1995, pp. 602-608

[5] R. Koschack: Single Body Theory and Zenith Correction Factor. In: Proceedings IMC, Belogradchik, 1994, pp. 31-43

[6] R. Koschack, P. Roggemans: The 1989 Perseid Meteor Stream. WGN 19:3, June 1991, pp. 87-98

[7] R. Koschack, R. Arlt, J. Rendtel: Global Analysis of the 1991 and 1992 Perseids. WGN 21:4, August 1993, pp. 152-167

[8] R. Arlt, J. Rendtel: First Analysis of the 1997 Perseids WGN 25:5, 1995, pp. 207-209

[9] J. Rendtel, R. Arlt: Perseids 1995 and 1996 - An Analysis of Global Data. WGN 24:5, 1996, pp. 141-147

[10] P. Brown, J. Rendtel: The Perseid Meteoroid Stream: Characterization of Recent Activity from Visual Observations. Icarus 124, 1996, pp. 414-428

[11] J. Rendtel: A First Global Analysis of the 1994 Perseids. WGN 22:6, 1994, pp. 205-209

[12] Z. Wu, I.P. Williams: The Perseid Meteor Shower at the Current Time. Mon. Not. R. Astron. Soc. 264, 1993, pp. 980-990

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