1 Introduction

The Orionid meteor shower is one of the two meteor showers associated with comet 1P/Halley. Its typical maximum ZHR is of the order of 20–25. The broad maximum lasts usually from October 20 to 24. The activity profile during this period is not smooth but shows several submaxima. The years surrounding the comet’s latest perihelion passage yielded no rate enhancement (Porubčan et al. 1991) because of the large minimum distance between the orbits of the parent comet and the Earth. Other outbursts, such as in 1993 (Rendtel and Betlem 1993) were due to isolated particle concentrations far distant from the comet that may occur when the comet is far from its perihelion position (Jenniskens 2006). Most modelling attempts were made 1985/1986 during 1P/Halley’s latest perihelion (e.g. McIntosh and Jones 1988) and later by Ryabova (2003) with a summary of modelling attempts. Speculations about particles in resonant orbits date back to Hajduk (1970).

2 Orionid Data

Since the 80s visual data of meteor showers are available on a global scale as a result of standardized observing techniques and reporting procedures. They are stored the Visual Meteor Data Base (VMDB) of the International Meteor Organization (IMO). The continuous data collection started in 1988, but for some showers older data was added. The Orionids with a well defined radiant and no other active meteor showers occurring at the same time and same area in the sky, fulfill the criteria to be included in the database.

Of course, the information about an individual visual meteor is limited: the shower association of a single meteor is based on its direction, angular velocity and apparent trail length. In the case of reasonable meteor rates (i.e. more than about 15 shower meteors per hour), the statistics is not sensitive against mis-aligned meteors. To keep the data set consistent and avoid effects from low activity periods, we concentrate our analysis to the near-maximum period between \(\lambda_{\odot}=206^{\circ}\) and 212° (all values of the Solar longitude in the text refer to equinox 2000.0).

Data were added from the sources listed in Table 1. The Skalnaté Pleso data (Štohl and Porubčan 1981) include detailed lists for each individual observer that can be directly transformed into the VMDB format. Data lists published by Prentice prior to 1939 (Prentice 1936, 1939) contain only meteor totals for entire nights and are therefore not suitable for rate calculations. Lovell (1954) showed the processed rates from Prentice and Alcock on p. 289. Additional data may be reconstructed from the American Meteor Society publications by Olivier (1935) found in the process of writing this paper, but they are not yet included in the VMDB.

Table 1 Orionid data sources added to the VMDB
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While we were able to calibrate the data back to 1944 according to the present VMDB standards, this was not possible for all older data. In most cases we have to restrict conclusions about the activity of the Orionids at earlier returns to relative values. The number of non-shower meteors reported in the individual count intervals is used to check the qualitative value of the Orionid rates.

3 Population Index Profiles

The Orionid showers typically produce faint meteors while the fraction of fireballs is small as compared with other meteor showers (Rendtel et al. 1995). The population index r was derived from the magnitude data using an adaptive interval length. Data is sampled until 100 meteors are in a bin for an individual value of r. Details are described by Arlt and Barentsen (2006). The interval length was chosen as short as possible in order to detect short term variations which may indicate fine structures in the stream. The population index can be transformed into the mass index by

$$ s=2.5 \log r + 1 $$
(1)

if the shape of the light curve does not change over the magnitude range being considered.

Minimum values of the population index are given in Table 2. If possible, data were analysed per return to avoid averaging over structures or annual peculiarities. Values for the population index commonly listed in the literature are 2.9 (Rendtel et al. 1995) or around 2.5 (Dubietis 2003). Our analysis showed that despite all variations from one return to the next a minimum of the population index r can be found repeatedly at 207.9° ± 0.15° in the annual profiles. As an example, we show the profile of the Orionids 1995 in Fig. 1. Whether the smaller r-values in 1993 and 2006 (Table 2) can be regarded as an indication for a 12-year periodicity as was suggested by Hajduk (1970) must remain unanswered in the absence of sufficient data.

Table 2 Lowest value of the population index r of the Orionids near the rate maximum
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Fig. 1
figure 1

Profile of the population index r for the near maximum interval between \(\lambda_{\odot}=206^{\circ}\) and 212° of the 1995 Orionid return

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The 2006 Orionid return yielded a very distinct data set of 12,012 visual Orionids recorded by 58 observers within 389 h. This allowed us to analyse the shower characteristics with high temporal resolution of 0.1° over most of the period of maximum activity. Numerous bright fireballs were recorded by different techniques (see Spurný and Shrbený 2007, or Trigo-Rodriguez et al. 2007, for example) and both the minimum and average r-values were extremely low (see Table 2 and Rendtel 2007). This resembled very much the parameters of the Leonids 1998 (Tóth et al. 1998; Asher et al. 1999) and the June-Bootids 1998 (Rendtel et al. 1998; Arlt et al. 1999; Asher and Emel’yanenko 2002) and gave rise for the assumption that the Orionids 2006 were caused by resonant meteoroids (Rendtel 2007).

4 Activity Profiles

For the calculation of the Zenithal Hourly Rate (ZHR) we applied the population index profiles of the same year, or the nearest return that yielded sufficient data for the calculation of a profile. The ZHR is calculated

$$ {\rm ZHR}=N_{\rm ORI} \frac{r^{6.5-{\rm lm}}\,F}{T_{\rm eff}\,\sin^\gamma h_R} $$
(2)

with N ORI being the number of Orionid meteors observed in the selected interval, the population index (r) at the time of the observation, lm the limiting magnitude, F a geometric correction for obstructions, T eff the effective observing time and h R the radiant elevation. Values of γ > 1 were proposed to account for different entry angles of meteors (Zvolánková 1983). For this study we selected only intervals with lm better than 5.8 and a radiant elevation of at least 20°. A detailed investigation was made to ensure that there are no spurious artifacts in the selected periods when the observing region changed, for example, from western European to North American locations with different radiant elevations (Rendtel 2007). For this analysis we used a zenith coefficient γ = 1; values of γ > 1 changed the smooth profile and introduced overcorrected ZHRs for radiant elevations in the range between 20° and 45°. Arlt and Barentsen (2006) described in detail the procedure of averaging the individual ZHR values to obtain the profile.

The long-term average maximum ZHR of the Orionids reached values of 20–25 in the period between \({\lambda_\odot}=207^{\circ}\) and 211°. Since we had no magnitude data available for the Orionids prior to 1979, we applied an average r = 2.4 to the 1944–1950 data. The maximum ZHR for each return, or group of returns in case of too few annual data, is shown in Fig. 2. The horizontal error bars indicate that data of several years are combined to calculate an average ZHR.

Fig. 2
figure 2

Maximum ZHRs of the Orionid returns in the period 1944–2006. A horizontal error bar indicates that the value represents an average over several Orionid returns

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In 2006 we found not only a distinct particle size distribution as shown by the population index profile in Fig. 3 but also strongly enhanced ZHR values with peaks of ZHR≈60, i.e. about 2.5 times the usual peak ZHR (Fig. 4). The highest ZHR coincided with the times of the lowest population index, i.e. the densest regions of the stream were characterized by large meteoroids. The only exception was the very late ZHR peak at \({\lambda_\odot}=211.8^{\circ}\) which coincided with a high r-value and thus represented a different particle population.

Fig. 3
figure 3

Profile of the population index r for the near-maximum interval \({\lambda_\odot}=206^{\circ}\)–212° of the 2006 Orionids

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Fig. 4
figure 4

ZHR-profile of the Orionids 2006 showing the same interval as in Fig. 3. For comparison the average ZHR of the 1993 and 1995 returns is shown as a line

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5 Discussion

The unusual 2006 Orionid return with its particle size distributions and duration of enhanced activity similar to the Leonids 1998 and the June-Bootids 1998 led to the assumption that it was caused by meteoroids in resonant zones. Since the parent comet 1P/Halley is not resonant with Jupiter, it may deliver meteoroids into all resonances over a long time. As the minimum distance between the comet and the Earth orbits is quite large, the question is, which resonance could be the most probable one? Model calculations by Emel’yanenko (2001) indicate the 1:6 resonance because it has the greatest width of all discussed resonance zones. Consequently, we could find Orionid rate enhancements that occurred six Jupiter revolutions earlier, i.e. about 72 years earlier than 2006. As described in the Sect. 2, the analysis of old data is not straightforward as in the case of those covering the last six decades. The strongest hint on Orionid rates exceeding the neighbouring years by a factor of about three is found in observations made by Loreta in Bologna in 1936 (Millman 1936). Unfortunately, no data of the same observer is available for other returns. Hence a calibration is difficult. Still, the numbers of non-Orionid meteors reported indicates that the Orionid rates indeed were significantly enhanced. Similar hints can be found in Lovell (1954), although no such calibration is possible and the original papers by Prentice (1936, 1939) do not allow further comparison. So we have to restrict our analyses to relative rates rather than ZHRs as shown in Fig. 5. The fact that the rich 1936 return is by 2 years off from the exact 1:6 ratio may indicate that it extends over a longer portion along the orbit and we observed perhaps different ends of the meteoroids being trapped in the resonant zone. If this is the case, there is a chance of enhanced rates also in 2007 as calculated by Sato and Watanabe (2007). According to their model, the meteoroids near the resonant zone have orbital periods between 70 and 72 years. Just before the manuscript was finished in October 2007, enhanced Orionid rates as expected from the width of the resonance zone and the modelling have been recorded during the 2007 return.

Fig. 5
figure 5

Orionid rates of the 1928, 1936, and 1938 returns derived from visual observations. Data of Prentice and Alcock are summarized by Lovell (1954); data of Loreta are published by Millman (1936). Although these are no modern ZHRs, we find a distinct increase of the activity during the 1936 Orionids

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6 Conclusions

Over most of the period from 1944 to 2006 which has been analysed from a comprehensive data set, the population index r and maximum ZHR of the Orionid meteors show rather small variations with average values of r ≈ 2.4 and ZHR ≈ 20–25. The Orionid 2006 data described the passage of the Earth through a different particle population. The duration of the significantly enhanced activity lasted from \({\lambda_\odot}=207.6^{\circ}\)–210.7° and the magnitude data can be compared with encounters of resonant meteoroids. The unusual meteoroid size distribution was confirmed by photographic data (cf. Spurný and Shrbený 2007; Trigo-Rodriguez et al. 2007) and video data. Recent model calculations by Sato and Watanabe (2007) strongly suggest that the 2006 Orionids are due to meteoroids ejected from 1P/Halley more than 2,900 years ago. The rate increase found in the 1936 data (Fig. 5) emphasizes the 1:6 resonance with Jupiter as the responsible region. The long time lapse between the meteoroid ejection and the observation allowed the meteoroids to move close enough to the Earth’s orbit.