Hiroshi Ogawa and Hirofumi Sugimoto
Abstract: The Orionids are one of the major meteor showers in October. It is known that the annual maximum occurs around λʘ = 208°. In 2022, some possible unusual activity was detected around λʘ = 205.2° (October 18, 22h30m UT) by the International Project for Radio Meteor Observations (IPRMO). The calculated activity level was 0.7 ± 0.2 and the estimated ZHRr was 39 ± 4. Besides there were more long echoes in 2022 than past years.
The Orionids are one of the major meteor showers during the month of October. The peak occurs at λʘ = 208° with a ZHR = 20 for visual observations (Rendtel, 2021). Although high activity has been observed in 2006–2008 (Arlt et al., 2008) and strong activity was also detected in radio meteor observation (Steyaert, 2014), a very weak activity has been detected in recent years.
Radio meteor observations make it possible to observe even with bad weather and during daytime. The International Project for Radio Meteor Observations (IPRMO) was organized in 2001 (Ogawa et al., 2001). It has as purpose to monitor and analyze meteor shower activity continuously. Besides, Radio Meteor Observations Bulletin (RMOB) is providing worldwide radio meteor data on its website.
Although there was no unusual activity predicted for 2022, this paper reports a possible unusual activity.
This research adopted two methods for estimating meteor shower activity. One is the Activity Level Index which is used by IPRMO (Ogawa et al., 2001). Another method is the estimated ZHRr (Sugimoto, 2017). This index is estimated by using the Activity Level index and a factor named Sbas which translates this to the ZHRr. This method is very useful to compare radio meteor observations to visual observations.
3.1 Actitivity Level Index
Figure 1 shows the result based on the calculation of the Activity Level using 46 sets of observing data from 14 countries. The solid line shows the average value for the period of 2004–2021. The peak occurred at λʘ = 205.21° (October 18, 22h30m UT).
Figure 2 displays the estimated components by using the Lorentz profile (Jenniskens et al., 2000). One is the usual annual activity which had a peak around λʘ = 208.77°. The other component shows a possible unusual activity. The maximum activity level was 0.4 and the Full Width of Half Maximum (FWHM) had –3.0hours/+4.0hours.
3.2 Estimated ZHRr
Figure 3 shows the result of the ZHRr using 40 datasets of observations. The maximum ZHRr reached 39 ± 4 at λʘ = 205.21°. A solid line represents the average value for the period of 2012–2021. The beginning of the unusual activity was observed at λʘ = 205.05° (October 18, 18h30m UT). After the maximum, the unusual activity was over around λʘ = 205.46° (October 19, 04h30m UT). Table 1 shows the numeric details for the profiles of the estimated ZHRr and the Activity Level Index.
Table 1 – The estimated ZHRr and Activity Level Index (AL).
|Time (UT)||λʘ||ZHRr||Activity Level|
|Oct. 18 15h30m||204°.922||8||12±2||13||0.1±0.2|
|Oct. 18 16h30m||204°.963||9||15±3||12||0.1±0.2|
|Oct. 18 17h30m||205°.005||9||18±4||12||0.4±0.1|
|Oct. 18 18h30m||205°.046||9||29±6||11||-0.1±0.1|
|Oct. 18 19h30m||205°.087||10||37±4||12||0.4±0.2|
|Oct. 18 20h30m||205°.129||10||37±6||14||0.5±0.3|
|Oct. 18 21h30m||205°.170||8||37±3||12||0.3±0.3|
|Oct. 18 22h30m||205°.212||8||39±4||20||0.7±0.2|
|Oct. 18 23h30m||205°.253||19||36±5||22||0.5±0.1|
|Oct. 19 0h30m||205°.294||18||30±5||20||0.6±0.2|
|Oct. 19 1h30m||205°.336||16||19±3||19||0.2±0.1|
|Oct. 19 2h30m||205°.377||15||22±2||19||0.4±0.1|
|Oct. 19 3h30m||205°.418||15||21±3||19||0.1±0.1|
|Oct. 19 4h30m||205°.460||17||15±2||18||0.2±0.1|
|Oct. 19 5h30m||205°.501||12||16±3||20||0.2±0.1|
3.3 Long Echoes
A strong overdense meteor echo, also called “Long Echo” has been often observed. An echo during more than 20 seconds is defined as a Long Echo. Figure 4 compares the number of long echoes per day in 2022 with past years in Japan. The number of long echoes was twice the average number during past years on October 19.
Although radio meteor observations detected a possible unusual activity of the Orionids in 2022, there were no visual or video reports confirming this in the world. The best location in the world was Europe. The radiant elevation was around 20° at the time of the maximum. Therefore, it would have been possible to observe some activity. The reason why this is considered is that other meteor shower activity was detected. But it is too difficult to identify the kind of meteor shower in the case of forward scattering. It requires further research to discuss this result.
The activity level was also higher than past average values around λʘ = 207° in Figure 3. It remains uncertain whether the annual peak was shifted or if this was caused by another reason, such as an influence of Southern Taurids.
The worldwide data were provided by the Radio Meteor Observation Bulletin (RMOB) and Radio Meteor Observation in Japan (RMOJ). We wish to thank all radio meteor observers. Besides, we wish to thank Pierre Terrier for developing and hosting rmob.org.
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