Hiroshi Ogawa and Hirofumi Sugimoto
Abstract: Worldwide radio meteor observations recorded a strong activity of the Ursids in 2021. The outburst peak time occurred at λʘ = 270.40° (December 22, 08h UT) with an estimated Activity Level Index = 0.8. The estimated ZHRr was 41 ± 4. The enhanced activity remained for about four hours (December 22, 06h – 09h UT).
The Ursid meteor shower is one of the major showers at the end of the year. Although it shows only a weak annual activity level in most years, sometimes outbursts of activity have been recorded such as in 2008, 2009, 2014 and 2016 (Ogawa and Steyaert, 2017). Also in 2020, worldwide radio meteor observers have recorded an activity level twice the usual annual level (Ogawa and Sugimoto, 2020).
For 2021, a dust trail encountering was predicted by Peter Jenniskens (Jenniskens 2006) expected to occur at around λʘ = 270.33° (December 22, 06h47m UT).
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).
This research adopted two methods to estimate the Ursid meteor shower activity. One is the Activity Level Index which is used by IPRMO (Ogawa et al., 2001). The second is the estimated ZHRr (Sugimoto, 2017). This index is estimated by using the Activity Level index and a factor named Sbas which translates the activity to the ZHRr. This method is very useful to compare radio observations with visual observations.
3.1 Activity Level Index
Figure 1 shows the result for the Ursids 2021 based on the calculation of the Activity Level Index using 42 observing datasets from 14 countries. The gray line indicates the average for the period 2004–2019. An unusual activity has been detected around December 22, 06h – 09h UT (λʘ = 270.32°–270.45°). The activity during this period remained at the same level (see Table 1).
Table 1 – Activity Level Index (AL) and estimated ZHRr for the Ursids in 2021.
|Time (UT)||λʘ||Activity Level||ZHRr|
|Dec 22 03h||270˚.19||25||0.2±0.1||13||19±3|
|Dec 22 04h||270˚.23||26||0.3±0.1||18||16±2|
|Dec 22 05h||270˚.28||25||0.4±0.1||16||20±3|
|Dec 22 06h||270˚.32||27||0.7±0.2||19||26±3|
|Dec 22 07h||270˚.36||28||0.7±0.2||19||37±3|
|Dec 22 08h||270˚.40||21||0.6±0.2||13||41±4|
|Dec 22 09h||270˚.45||17||0.8±0.2||14||38±5|
|Dec 22 10h||270˚.49||17||0.3±0.2||10||24±3|
|Dec 22 11h||270˚.53||14||0.4±0.1||11||22±3|
|Dec 22 12h||270˚.57||18||0.5±0.2||13||23±3|
|Dec 22 13h||270˚.62||20||0.1±0.1||12||15±2|
|Dec 22 14h||270˚.66||24||0.1±0.1||10||6±1|
Figure 2 shows the activity components of the Ursids 2021 estimated by using the Lorentz profile (Jenniskens et al., 2000). One component (URS21C01) had a maximum Activity Level = 0.3 at λʘ = 270.36° (December 22, 07h UT) with Full width half maximum (FWHM) = –7.5 / +5.5. The other component (URS21C02) had an Activity Level = 0.5 at λʘ = 270.40° (December 22, 08h UT) with FWHM = –2.0 / +2.5 (see Table 2). Although the peak time occurred earlier than during the usual annual activity, it seems that URS21C01 is corresponding to the traditional activity profile. It is possible that URS21C02 represents the outburst activity produced by the dust trail.
Table 2 – Estimated components of the Ursids activity in 2021.
|URS21C01||Dec 22nd 07h UT||270˚.36||0.3||-7.5/+5.5|
|URS21C02||Dec 22nd 08h UT||270˚.40||0.5||-2.0/+2.5|
3.2 Estimated ZHRr
Figure 3 shows the result for the Ursids in 2021 based on the calculation of the ZHRr using 42 datasets worldwide. The estimated ZHRr reached 41 ± 4 at λʘ = 270.40° (December 22, 08h UT). The enhanced activity started at λʘ = 270.02° (December 21, 23h UT). The unusual increase started at λʘ = 270.32° (December 22, 06h UT). The activity ended at λʘ = 270.66° (December 22, 14h UT) (see Table 1). The activity level was the same as for the 2020 Ursids when a ZHRr = 39 ± 3 at λʘ = 270.54° was recorded.
The worldwide data were provided by the Radio Meteor Observation Bulletin (RMOB). We thank the following observers for their contribution:
Johan Coussens (Belgium), Chris Steyaert (Belgium), Felix Verbelen (Belgium), HFN-R1 (Czech Republic), ZVPP-R7 (Czech Republic), Jean-Marie F5CMQ (France), DanielD SAT01_DD (France), WHS Essen (Germany), Balogh Laszlo (Hungary), Istvan Tepliczky (Hungary), Mario Bombardini (Italy), Oss Monte San Lorenzo DLF (Italy), GAML Osservatorio Astronomico Gorga (Italy), AAV Planetario di Venezia (Italy), Kenji Fujito (Japan), Masaki Tsuboi (Japan), Hirofumi Sugimoto (Japan), Hironobu Shida (Japan), Tomohiro Nakamura (Japan), Masaki Kano (Japan), Hiroshi Ogawa (Japan), Nobuo Katsura (Japan), Ikuhiro Yokoyama (Japan), Rainer Ehlert (Mexico), Juan Zapata (Mexico), Kees Meteor (Netherlands), Rafael Martinez (Puerto Rico), Karlovsky Hlohovec Observatory (Slovakia), Jochen Richert (Switzerland), Ian Evans (United Kingdom), Philip Norton (United Kingdom), Philip Rourke (United Kingdom), Stan Nelson (United States of America), Richard Schreiber (United States of America), Eric Smestad_KC0RDD (United States of America).
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