Jürgen Rendtel
Published in WGN, the Journal of IMO 25:4, p. 153 (1997)
Abstract: From data of the 1997 return of the eta-Aquarids, profiles of the population index r and the ZHR have been calculated. The ZHR maximum is located at sol=44.5°±0.2° (May 4, 1997, 22h UT). This is by 1° earlier than the averave derived for the period 1988 to 1995. The minimum value of the population index of r=1.95 occurs about 1° after the ZHR maximum. Also this value is considerably lower than the standard figure of r=2.7. This difference is also the reason for the lower maximum ZHR of 54±3 as compared to the average of 69±4. Analyses of the number densities for different particle populations indicate a mass segregation in the meteoroid stream, with the larger particles appearing later than the smaller meteoroids.
The eta-Aquarids are caused by the passage of the Earth through the meteoroid stream of comet 1P/Halley, as is the appearance of the Orionids in October. Since the minimum distance to the comet's orbit and the stream's core region is smaller in early May, the rates of the eta-Aquarids are higher than those of the Orionids.
While the IMO's VMDB contains a quite large number of eta-Aquarid data over the years, the 1997 return was certainly the best observed for a long time. The ZHR curve shown in the Visual Handbook [1] is a composite profile derived from data obtained between 1988 and 1995. Seen the number of reports available for the 1997 return, it seemed worthwile to do an analysis of this independent data set.
Sanaa Abdo (Jordan), Khalid Al-Tal (Jordan), John Assmus (USA), Mohammad Awadallah (Jordan), Eva Bojurova (Bulgaria), Tim Cooper (South Africa), Hani Dalee (Jordan), Tomasz Fajfer (Poland), Pete Gural (USA), Cathy Hall (Canada), Ibrahim Jamil (Jordan), Khalil Konsul (Jordan), Marco Langbroek (the Netherlands), Robert Lunsford (USA), Adam Marsh (Australia), Norman McLeod (USA), Iris Milja\v{c}ki (Yugoslavia), Nikola Milutinovic (Yugoslavia), Sirko Molau (Germany), Sasa Nedjkovic (Yugoslavia), Mirko Nitschke (Germany), Mohammad Odeh (Jordan), Dragana Okolic (Yugoslavia), Lyna Rashkova (Bulgaria), Luís A.\ Reck de Araujo (Brazil), Jürgen Rendtel (Germany), Petra Rendtel (Germany), Elena Sarbinska (Bulgaria), Branislav Savic (Yugoslavia), Richard Taibi (USA), Valentin Velkov (Bulgaria), Koos Van Zyl (South Africa), Marija Vucelja (Yugoslavia), Graham Wolf (New Zealand), George Zay (USA), Irena Zivkovic (Yugoslavia).
The total 1997 sample comprises 135 hours of net observing time. More than 150 count intervals have been reported during which more than 1000 eta-Aquarid meteors were seen. In contrast, the already quoted composite ZHR curve [1] included 523 only intervals collected over a period of eight years.
The minimum in the population index r found from the 1997 visual data occurred significantly later than the ZHR maximum, indicating a mass sorting in the stream as discussed later.
As to be expected, the minimum figure of r is also smaller than the lowest r reported from the 1990 data of the Orionids [5], indicating that the particle set-up is slightly different along the trajectories of the Earth through the 1P/Halley meteoroid stream with a larger portion of larger meteoroids present closer to the core region of the stream. Nevertheless, both passages are distant from the comet's orbit (0.065 AU for the eta-Aquarids and 0.18 AU for the Orionids [3]).
The short observing window of 1 to 3 hours duration towards the morning twilight - depending on the geographic latitude - and the distribution of the observers around the globe yields a data set which differs from that obtained for other showers in a very important point. There is no temporal overlapping between observations of different regions. The observers cover just three regions: Australia/New Zealand, South Africa/Middle East/Europe, and South/North America. This separation means that there is no period which can be used for calibration of the shower rates. So, we have to trust the reliability of the standard method as found from several other analyses of global data.
Figure 1: Profile of the population index r obtained from the magnitude data of the 1997 return. The sampling interval was chosen as 1° in length, shifted by 0.5° for the period sol=43° - 46°. Further off the maximum, the number of shower meteors remained too low for a determination of a r-profile.
Figure 2: ZHR profile of the 1997 eta-Aquarids, based on 1050 shower meteors reported in 165 count intervals and the r-profile shown in Figure 1.
Another problem to be kept in mind is the low position of the radiant, particularly in the first interval for each data set. Hence, the correction factors for calculating the ZHR are large, and, combined with the small sample of shower meteors which can effectively be seen, the error margins for the individual ZHRs are large. Despite all these limitations, there are no obvious "jumps" or steps in the resulting ZHR curve (Figure 2).
The maximum ZHR of 54±3 is somewhat lower than that of the combined data (69±4) shown in [1]. This is not too much surprising and should be mainly an effect of the different value of r. For a rough estimate, we may assume an average limiting magnitude of 6.0 for all observations. With r=2.0 the ZHR would be just 0.9 of the ZHR determined with the standard r=2.7. The maximum ZHR occurred at sol=44.5°±0.2° (May 4, 1997, 22h UT). This is one day earlier than derived from the combined data set (sol=45.5°). Although no reduction of the various forward scatter data summarized in [6] has been made, the raw data support the maximum at this position rather than a day later.
The ZHR profile is relatively wide with a full width at half maximum (FWHM) of about 5°. Like in the reference profile, there seems to be a slight skewness of the 1997 ZHR profile with a steeper ascending branch as compared to the descending branch. Analyses of radar data [4] hint on a double maximum, with the first one close to the position found here, and another maximum near a solar longitude of 48°. However, the radar data include a somewhat different particle population, and the structure varied considerably from one return to the next. Also, the visual rate profile quoted by Hughes [3] shows some variations with higher rates around sol=49°. The scatter of the averages determined from the current data set, particularly after the maximum is considerable and hence it is difficult to make conclusions from the profile shown in Figure 2.
As already noted, the maximum in ZHR occurs about 1° (about 1 day) before the minimum of the population index r, i.e., the observers recorded a larger portion of brighter meteors in the night after the ZHR maximum (May 5-6) than at the actual ZHR maximum. This implies a kind of mass sorting within the meteoroid stream as it is known from other streams as well. Details can be seen from the figures listed in Table 2.
Number density maxima positions and strengths of the 1997 eta-Aquarids for different meteoroid masses. The values of the number density S are given in particles per 109 km3.
| Magnitude range | Mass range | Number density | Position |
|---|---|---|---|
| <6.5 | >0.3 mg | 74 | 44.0±0.5° |
| <3.0 | >1 mg | 4.7 | 44.5±0.3° |
| <0.7 | >10 mg | 0.74 | 44.7±0.3° |
The value of the population index is r=2.2 at the rate maximum, and reaches a minimum or r=1.95 near sol=45.5°. During the entire period between sol=43.8° and sol=46.5°, the population index is considerably lower than the standard figure of r=2.7. This difference is also the reason for the lower maximum ZHR of 54±3 as compared to the average of 69±4 shown in [1].
Indications for a mass sorting in the meteoroid stream are found from the positions of the maxima in number density for different masses, with the larger portion of bright meteors occurring about 0.7° later than the faint-meteor peak.
[1] J. Rendtel, R. Arlt, R. Koschack, A. McBeath, P. Roggemans, J. Wood: Meteor Shower Descriptions. In: Handbook for Visual Meteor Observers, J. Rendtel, R. Arlt, A. McBeath (eds.), IMO, Potsdam, 1995, pp. 126-279
[2] J. Rendtel: Observing the eta-Aquarids from the Desert. WGN 25:3, June 1997, pp. 121-122
[3] D.W. Hughes: P/Halley Dust Characteristics: a Comparison Between Orionid and Eta Aquarid Meteor Observations and those from the Flyby Spacecraft. Astron. Astrophys. 187, 1987, pp. 879-888
[4] R.P. Chebotarev, S.O. Isamutdinov, A. Hajduk: Radar Observations of Eta Aquarids in 1981-1986 at Dushanbe and Ondrejov. Bull. Astron. Inst. Czechosl. 39, 1988, pp. 82-85
[5] R. Koschack, P. Roggemans: The 1990 Orionids. WGN 19:4, August 1991, pp. 115-130
[6] C. Steyaert: Radio Meteor Observation Bulletin No. 46, June 1997