1 Introduction

The Orionid meteor shower is a relatively strong and stable regular annual shower with a peak visual hourly rate of 15–30 meteors and broad maximum generally occurring on October 20–23. The parent is comet 1P/Halley, now in an orbit passing a far +0.151 AU from Earth (Jenniskens 2006). The last return of parent comet 1P/Halley caused a renewed interest in the study of both streams originating from this comet, i.e. Orionids and η-Aquariids. Most theoretical works and modeling attempts (Hajduk 1970; McIntosh and Hajduk 1983; Hughes 1987; McIntosh and Jones 1988; Wu and Williams 1993) as well as increasing number of meteor observations mostly by visual amateur observers date around this return. However no enhanced Orionid activity was observed and it was concluded that enhanced rates are not connected with the parent comet returns (Porubčan et al. 1991) but are due to isolated particle concentrations not necessarily in the comets vicinity. The latest outburst was observed in 1993 (Rendtel and Betlem 1993). Emel’Yanenko (2001) in his theoretical work explains enhanced and short-term activity of a shower with a libration of a meteoroid stream, and for Orionids he presents three sets of parameters that describe three possible resonant zones for this shower (Table 1). These librating particles create a resonance substream. The 2006 Orionid activity was caused according to Sato and Watanabe (2007) by the dust trails formed by meteoroids ejected from 1P/Halley in years 1265, 1197 and 910 BC and trapped in the 1:6 mean motion resonance with Jupiter. The exceptional activity of 2006 Orionids Rendtel (2007) is also ascribed to the meteoroids from a resonant zone with the most favorable resonance the 1:6 with Jupiter. Rendtel (2007) also mentions the decrease of the population index, r. During the maximum of the activity the value of r was 1.6 but the long-term average is 2.3–2.9. This fact confirms that the meteors observed during the Orionid 2006 maximum deviated significantly from the average Orionid meteors and on average are significantly brighter. A similar effect was observed during the 1993 Orionid outburst (Jenniskens 1995).

Table 1 Parameters of principal resonance zone near orbit of the Orionid meteor shower (Emel’Yanenko 2001)
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2 Instrumentation, Observations and Data Processing

The multi-station photographic observation of fireballs in fireball networks represents a very efficient and precise method how to record the atmospheric interactions of larger meteoroids. During the short moment of meteoroids ablation we can determine their atmospheric trajectories, orbits, light curves and basic physical properties. One of the most advanced operational fireball network is the Czech part of the European Fireball Network, where each station is equipped with the newest generation camera, modern and sophisticated completely Autonomous Fireball Observatory(AFO) (Spurný et al. 2007). The AFO imaging system consists of a Zeiss Distagon fish-eye objective (f/3.5, f = 30 mm) and a large-format sheet film. All AFOs are equipped with a rotating shutter close to the focal plane to determine fireball velocity. At present we operate 10 stations almost uniformly deployed across the territory of the Czech Republic. The typical precision of measurement of any individual point on the luminous atmospheric trajectory for fireballs up to approximately 200 km distance from the stations is about 10–15 m. This precision is proportionally decreasing with the distance of fireball from stations. In some ideal cases we can determine reliably fireballs at a distance of about 500 km from our territory. It enables us to observe fireballs over large parts of Central Europe. Except direct fireball imaging each AFO includes also an all-sky brightness sensor (radiometer) with sampling rate of 500 measurements per second. Therefore, along with the accurate time of fireball passage and its duration, we also obtain a very detailed light curve. These sensors reliably work even under a cloudy sky, so we have basic information about fireball luminosity and its very approximate location even without photographic records.

This system was in full operation during the 2006 Orionid enhanced activity. During four nights from October 20 we recorded a total of 48 bright Orionids although during this time our observation was strongly affected by unstable weather conditions. The entire data set includes multi-station photographic fireballs and those recorded photographically only from one station, some that were too short (only the brightest flare) to make it possible to compute the trajectory or to determine the velocity, and others that were recorded only by brightness sensors due to a cloudy sky.

Only 10 recorded fireballs were long and bright enough to be recorded photographically from more than one station so that we can precisely determine all important parameters describing their atmospheric trajectories, heliocentric orbits and basic physical properties. In this paper we deal only these multi-station fireballs. All presented Orionid fireballs were measured and processed using our standard procedures (Borovička et al. 1995; Ceplecha 1987).

3 Atmospheric Trajectories, Light Curves and Physical Properties

Atmospheric trajectories for the Orionid fireballs presented here were determined from all available images (Table 2). Because Orionid meteors are very fast, their atmospheric trajectories are often very short (last column in Table 2). This can decrease the precision in determination of other critical parameters. However, in all cases the fireballs listed in Table 2 were recorded from more than two stations, which highly increases the reliability of the data here presented. The only exception being MET06 that was recorded from only two stations. Following the format in the tables, fireballs MET01, MET08 and MET09 were recorded from three stations, MET02, MET07 and MET10 from four stations, MET04 from five stations, MET05 from six stations, and finally MET03 even from eight stations of the Czech Fireball Network.

Table 2 Atmospheric trajectories of Orionids 2006 fireballs
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The results on atmospheric trajectories are collected in Tables 2–4. The time of meteor beginning, geographical position, beginning and end heights and length of observed atmospheric trajectory are presented in Table 2. The beginning heights range from 100 to 114 km and the terminal heights from 78 to 90 km corresponding to a range of the observed trajectory lengths from 15 to 40 km. Physical data of these fireballs are presented in Tables 3 and 4. Zenith distances for the end point, initial velocities (mean measured velocity without deceleration), maximum absolute photographic magnitudes, initial photometric masses, PE coefficients that describe the empirical end heigh criterion and fireball types according to the classification of (Ceplecha and McCrosky 1976) are shown in Table 3.

Table 3 Physical data on Orionids 2006 fireballs
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Table 4 Heights and durations of the flares and dynamic pressures
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The rotating shutter hides one half of the meteor trail so in most of cases the short-term flares are not visible in the photographs at all. These flares are obvious in the light curves from the AFO brightness sensors. This is well documented in Fig. 1 showing the photographic images and radiometric light curves for all 10 Orionid fireballs listed in tables. From known durations of the fireballs and their approximate light curve profile, both derived from the photographic records and from durations of the AFOs light curves, we were able to derive instantaneous heights of flares for each fireball in the atmosphere. Values of these quantities (if visible in the light curve) are presented in Table 4. Since the overlap of the light curves is approximate, the heights listed are rounded-off to kilometers. The duration of the flares varies from several tens of milliseconds (near the mid part of the trajectory) to only several milliseconds in terminal flares. A typical Orionid light curve has a broader maximum and one much shorter very pronounced terminal flare. From such type of light curve we can infer that the material of the Orionid meteoroids easily disintegrates first into bigger particles which gradually ablate and create a longer middle peak and near the end of its trajectory the remaining part of the initial meteoroid completely disintegrates into a large amount of very small particles which ablate and evaporate very quickly. From the values listed in Tables 3 and 4 it is evident that all ten presented Orionid meteoroids consist of very weak and fragile material that is usually assumed to be of cometary origin which corresponds with known parent body—comet 1P/Halley.

Fig. 1
figure 1figure 1

Light curves from AFO’s brightness sensors and images of the Orionid fireballs from all-sky cameras. MF means the position of the maximum brightness, TF the position of the terminal flare (see Table 4). The images from fixed cameras display star trails and interruptions of the meteors caused by rotating shutter (15 breaks/second). The guided images were taken by guided all-sky camera at Ondřejov Observatory and show the entire fireball trails. All fireballs flew from left to right in the images

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4 Radiants and Orbital Elements

Geocentric radiant positions and orbital elements for all 10 Orionid fireballs are tabulated in Table 5. All values are given in the J2000.0 equinox. The fireballs are arranged according to date and time of occurrence, which is given by increasing values of the ascending node. It is evident that the first seven fireballs which were recorded in within a 2-h interval on October 21st all have very similar values. Therefore we conclude that these meteoroids belonged to one very compact filament which slightly differs from a regular background Orionids (Lindblad and Porubčan 1999; de Lignie and Betlem 1999).

Table 5 Radiants and orbital elements (J2000.0) of 2006 Orionids fireballs
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The compactness of geocentric radiants of fireballs belonging to this new filament is shown in Fig. 2 along with the mean geocentric radiant value, the three Orionids recorded during two following nights (MET08–MET10), and the mean radiant positions determined by Lindblad and Porubčan (1999) from IAU MDC photographic data and by de Lignie and Betlem (1999) from DMS video data. Mean orbital elements for filament and both published data sets are also listed in Table 5. Although the differences are not too significant in the statistical sense we can still find some distinctions. The radiant position of the 2006 filament is systematically shifted by about 0.1° or 0.6° (depending on source of data) to higher right ascensions and 0.2° to lower declinations. Also some orbital elements are slightly different: the filament meteoroids have about 0.01–0.03 AU larger perihelion distances and about 0.3°–0.6° smaller inclinations. As shown in Table 5 and Fig. 2 some characteristics of the three Orionids recorded during two following nights differ from the filament. However, this difference is not so obvious to completely exclude the possibility that these meteoroids could belong to the filament (it would need statistically larger set of data).

Fig. 2
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Geocentric radiants of 2006 Orionid fireballs (J2000.0). The radiants are normalised to the node 27.4°, with a radiant drift of 0.70 dRA/Dsol and 0.11 dDec/Dsol. Mean radiant of filament is computed from meteors MET01 to MET07. R1 is the radiant position according to (de Lignie and Betlem 1999) and R2 according to (Lindblad and Porubčan 1999), both normalised to the node 27.4°

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As shown in Table 5 our values significantly differ from those published by Trigo-Rodríguez et al. (2007). They reported Orionid fireballs recorded in the same time interval in the night of October 20/21, 2006, which means that the orbital characteristics of the meteors in this narrow time slot should be very similar. We found that none of the three Orionid meteors detected by the Spanish Meteor Network (SPMN) match the parameters of our filament (Table 5) and that also the radiants and orbital elements of these three Orionids significantly differ among each other. The SPMN results are plainly contradicting the consistent results from our observations of Orionid fireballs within the same short period of time. The discrepancy is unexpected in the face of the very high precision that was reported by Trigo-Rodríguez et al. (2007) that was supported by very low standard deviations of each individual entry. We suggest that the discrepant SPMN results might be either due to an overestimation of the precision in the measurements or a systematic error in computations.

The mean heliocentric orbit of the Orionid 2006 filament has semimajor axis a = 14.8 AU, eccentricity e = 0.959, inclination i = 163.71°, perihelion distance q = 0.603 AU and argument of perihelion ω = 78.7°. According to Emel’Yanenko (2001) (Table 1) it is probable that particles from this filament were in the 1:5 resonance with Jupiter. From detailed analysis of visual observations of the 2006 outburst as well as another Orionid outbursts observed in twentieth century, Rendtel (2007) suggested that these outbursts could be caused by particles from the 1:6 resonance. Similarly Sato and Watanabe (2007) ascribe the 2006 Orionid activity to particles from the 1:6 mean motion resonance with Jupiter. However we have no arguments from our study presented here to decide which value is unambiguously correct. Although all our presented values are determined with high precision and reliability we know that the least-precise value is the semimajor axis (i.e. also period), which is strongly affected by the uncertainty of entry velocity that is objectively difficult to determine with sufficient precision. It is caused by the fact that Orionids are very fast meteors and their atmospheric trajectories are relatively short. Therefore it will certainly need further study to decide this discrepancy.

5 Conclusions

We present results on atmospheric trajectories, orbits, light curves and physical properties of 10 Orionid fireballs recorded by cameras of the Czech Fireball Network during high Orionid activity in three nights of October 2006. The main conclusions are as follows.

  1. (a)

    We determined the precise mean radiant position and orbital elements of the very distinct filament that produced the observed outburst of Orionid activity in morning hours of 21st October 2006. We found that this filament only slightly differs from mean shower characteristics determined from IAU MDC photographic data by Lindblad and Porubčan (1999) or from DMS video data (de Lignie and Betlem 1999). Our values significantly differ from the values for the 2006 Orionid outburst published by Trigo-Rodríguez et al. (2007).

  2. (b)

    From single station photographic data and radiometric records we observed unusual high activity of bright Orionids over relatively long period during four consecutive nights from October 20–24. The lack of multi-station photographic data from second, third and fourth observing nights was partly caused by bad weather conditions over the Czech Republic and partly also by a decreasing number of brighter meteors. In second and third night we recorded only three fireballs from more than one station and their orbital characteristics slightly differ from the filament (Table 5). They rather better correspond to the background values (Lindblad and Porubčan 1999).

  3. (c)

    From very consistent mean values of orbital elements of fireballs belonging to the conspicuous filament of the 2006 Orionid outburst we found that this high activity could be caused by meteoroids trapped in 1:5 resonance.

  4. (d)

    According to analysis of light curves and atmospheric penetration ability defined by PE coefficient we found that all recorded Orionid meteors do not significantly differ among each other and belong to the weak and fragile component of interplanetary matter, as expected since the Orionids are associated with comet 1P/Halley.