Hiroshi Ogawa and Hirofumi Sugimoto
Abstract: The International Project for Radio Meteor Observations (IPRMO) has detected a meteor outburst of the τ-Herculids 2022 on May 31. Several meteor outbursts occurred at different times. The strongest peak was detected at λʘ = 69.428° (May 31, 04h30mUT). The activity level was estimated to be 1.8 (corresponding to a ZHRr = 34). A sub peak was observed at λʘ = 68.909° (May 30, 15h30m UT) with an activity level = 0.5 and ZHRr = 19.
The τ-Herculid meteor shower is caused by the dust produced by comet 73P/Schwassmann-Wachmann 3. In 1995, 73P/Schwassmann-Wachmann 3 broke up. The possible encounter in 2022 of the Earth with the 1995 trail formed by meteoroids released during this event has been predicted (Rao, 2021).
For 2022, the encounter with the dust trail was predicted around λʘ = 69.44° and λʘ = 69.459° (May 31, 04h55m UT and 05h17m) by Peter Jenniskens (Jenniskens, 2006). Mikiya Sato also calculated λʘ = 69.451° (May 31, 05h04m UT) as most likely time for the passage.
Radio meteor observations make it possible to observe meteor activity continuously even if bad weather interferes or during daytime. Besides, the problem with the radiant elevation is solved by organizing radio observing as a worldwide project. One of the worldwide projects is the International Project for Radio Meteor Observations (IPRMO). IPRMO uses the Activity Level index for analyzing the meteor shower activity (Ogawa et al., 2001).
For Europe and Japan, the 1995-dust trail encounter would appear in twilight time and daytime. The best observing method was radio meteor observations. IPRMO monitored the τ-Herculids activity during this year.
2.1 Activity Level Index and estimated ZHRr
This research adopted two methods for estimating τ-Herculid meteor shower activity. One is the Activity Level Index which used by IPRMO (Ogawa et al., 2001). Another is the estimated ZHRr (Sugimoto, 2017). This index is estimated by using the Activity Level index and a factor named Sbas which translates to ZHRr. This method is very useful in the case of comparing to visual observations.
2.2 Considering the zenith attraction
Since the geocentric velocity of τ-Herculids is very low with 16 km/s (Rendtel, 2021), it is necessary to consider the zenith attraction (Richardson, 1999). This research has considered to take this factor into account.
3.1 Main peak
Figure 1 shows the result of the τ-Herculids 2022 based on the calculation of the Activity Level Index using 37 observing data from 11 countries.
The main peak started at λʘ = 69.228° (May 30, 23h30m UT). The number of meteor echoes increased more and more. The maximum Activity Level reached a value around 1.8. Although a peak was recorded at λʘ = 69.508° (May 31, 6h30m UT), a strong activity remained during a period of a few hours (λʘ = 69.388–69.508° (May 31, 3h30m-6h30m UT)). After the main peak, the activity level became weaker and weaker. At λʘ = 69.668° (May 31, 10h30m UT), the Activity Level felt back at the usual level.
Figure 2 shows the result of τ-Herculids in 2022 based on the calculation of the ZHRr using 42 worldwide data. The estimated ZHRr of main peak reached 34 ± 7 at λʘ = 69.388° (May 31, 3h30m UT). The distinct activity started at λʘ = 69.268° (May 31, 0h30m UT). The end of the activity was situated at λʘ = 69.668° (May 31, 10h30m UT).
Table 1 – Activity Level Index (AL) and estimated ZHRr of the τ-Herculids 2022.
|Time (UT)||λʘ||Activity Level||ZHRr|
|May 30 10h30m||68.709°||11||0.1±0.1||13||7±2|
|May 30 11h30m||68.749°||11||0.0±0.1||10||5±2|
|May 30 12h30m||68.789°||20||0.3±0.2||10||7±1|
|May 30 13h30m||68.829°||21||0.6±0.3||10||11±2|
|May 30 14h30m||68.869°||22||0.3±0.2||17||17±2|
|May 30 15h30m||68.909°||24||0.4±0.2||26||19±2|
|May 30 16h30m||68.949°||21||0.4±0.2||19||17±1|
|May 30 17h30m||68.989°||24||0.4±0.2||28||17±1|
|May 30 18h30m||69.029°||24||0.4±0.1||20||13±1|
|May 30 19h30m||69.069°||20||0.6±0.3||15||11±1|
|May 30 20h30m||69.109°||13||0.3±0.1||18||7±1|
|May 30 21h30m||69.149°||14||0.1±0.1||15||13±1|
|May 30 22h30m||69.189°||13||0.3±0.1||23||12±2|
|May 30 23h30m||69.228°||14||0.6±0.3||23||16±2|
|May 31 0h30m||69.268°||15||0.9±0.3||23||25±2|
|May 31 1h30m||69.308°||15||1.0±0.2||25||30±3|
|May 31 2h30m||69.348°||15||1.3±0.2||22||32±3|
|May 31 3h30m||69.388°||15||1.7±0.4||10||34±7|
|May 31 4h30m||69.428°||25||1.7±0.3||8||34±6|
|May 31 5h30m||69.468°||20||1.4±0.4||7||–|
|May 31 6h30m||69.508°||14||1.8±0.3||16||29±3|
|May 31 7h30m||69.548°||13||0.9±0.1||16||26±3|
|May 31 8h30m||69.588°||13||0.5±0.1||17||17±3|
|May 31 9h30m||69.628°||13||0.3±0.1||17||12±2|
|May 31 10h30m||69.668°||13||0.0±0.1||12||4±2|
|May 31 11h30m||69.708°||11||0.2±0.1||10||4±1|
|May 31 12h30m||69.748°||21||0.3±0.2||12||2±1|
3.2 Sub peak
Half a day before the main peak, a small sub peak has been observed. The sub peak was recorded around λʘ = 68.829°–69.069° (May 30, 13h30m –19h30m UT). The Activity Level was around 0.5. The estimated ZHRr was 19 ± 2 at λʘ = 68.909° (May 30, 15h30m UT).
4.1 Meteor shower components
Figure 3 and 4 shows the activity components of the τ-Herculids 2021 estimated by using the Lorentz profile (Jenniskens et al., 2000).
One component (TAH22C01) had a maximum Activity Level = 1.8 at λʘ = 69.428° (May 31, 4h30m UT) with Full width half maximum (FWHM)= –3.0/+3.0 hours. The ZHRr was estimated to be 35. The other (TAH22C02) had an Activity Level = 0.5 at λʘ = 68.909° (May 30, 15h30m UT) with FWHM = –3.5/ +3.0 hours. The ZHRr was 15 (see Table 2).
It is possible that TAH22C1 relates to the meteoroids of the 1995 dust-trail. This research indicates that the peak caused by the 1995 dust trail occurred earlier than predicted, but no more than one hour. The TAH22C2 component on the other hand, might be caused by the 1892 or 1897 dust trail. These were predicted to occur between May 30, 16h and May 31, 10h (Wiegert et al., 2005).
Table 2 – Estimated components of the τ-Herculids 2022 activity.
|Activity Level||Estimated Zenithal Hourly Rate (ZHRr)|
|TAH22C1||May 31, 4h30m||69.428°||1.8||–3.0 / +3.0||May 31, 4h30m||69.428°||35||–5.0 / +3.5|
|TAH22C2||May 30, 15h30m||68.909°||0.5||–3.5 / +3.0||May 30, 15h30m||68.909°||15||–2.0 / +2.0|
4.2 Another sub-peak?
Before the higher described sub-peak, a very small sub-peak was detected around λʘ = 68.549° (May 30, 6h30m UT) with AL = 0.4 ± 0.2 and ZHRr = 9 ± 1 (Figure.5). It was uncertain activity because the meteor activity level was very weak. It has a possibility of something observed error.
4.3 Poor long echoes
Major meteor showers such as Quadrantids and Perseids show a lot of long echoes (strong overdense meteor echoes). During the period of the τ-Herculid outburst, however, there were few long echoes. It is possible that there were few bright meteors. Also, it could be due to the influence of the very slow geocentric velocity.
The observers who provided data were as following:
Chris Steyaert (Belgium), Felix Verbelen (Belgium), Johan Coussens (Belgium), HFN-R1 (Czech Republic), OBSUPICE-R6 (Czech Republic), VALMEZ-R1 (Czech Republic), DanielD SAT01 DD (France), Jean Marie F5CMQ (France), Balogh Laszlo (Hungary), Istvan Tepliczky (Hungary), AAV Planetario di Venezia (Italy), Mario Bombardini (Italy), Hirofumi Sugimoto (Japan), Hironobu Shida (Japan), Hiroshi Ogawa (Japan), Kenji Fujito (Japan), Masaki Kano (Japan), Masaki Tsuboi (Japan), Minoru Harada (Japan), Nobuo Katsura (Japan), Norihiro Nakamura (Japan), Juan Zapata (Mexico), Rainer Ehlert (Mexico), Salvador Aguirre (Mexico), Karlovsky Hlohovec Observatory (Slovakia), Jochen Richert (Switzerland), Jochen Richert_1 (Switzerland), Ian Evans (UK), Philip Norton (UK), Philip NortonVert (UK), Philip Rourke (UK), Eric Smestad_KC0RDD (USA), Mike Otte (USA), Richard Schreiber (USA), Stan Nelson (USA).
We wish to thank Pierre Terrier for developing and hosting rmob.org.
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