Hiroshi Ogawa and Hirofumi Sugimoto

Abstract: The Arids outburst released by the 2014 dust trail from comet 15P/Finlay has been observed on October 7 using worldwide radio meteor observations. The observing conditions were difficult for many radio meteor observers, being located at the Northern hemisphere. This time, the outburst was mainly caught at Mexican observing stations. The outburst occurred at October 7, 0h – 1h UT (λʘ = 193.67–193.71°) with an Activity Level Index = 1.7 ± 0.1 and an estimated ZHRr = 80 ± 10.


1 Introduction

An encounter with meteoroids of comet 15P/Finlay, named Arids, had been predicted for 2021 by several researchers (Rendtel, 2020; Vaubaillon, 2020). The first outburst was already detected on September 27–30 (Jenniskens, 2021). After that, another encounter caused by the 2014 dust trail was expected on October 7. It was calculated for the period of October 7, 00h30m–01h30m (UT) (Ye et al., 2021).

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). The first outburst of the Arids was not detected by the worldwide radio meteor observations because the radiant elevation was too low to be observed at almost all observing stations.


2 Method

2.1 Activity Level Index and Estimated ZHRr

This research adopted two methods to estimate the Arids 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.

2.2 Considering the zenith attraction

Since the geocentric velocity of the Arids is very slow with 11 km/s, the zenith attraction needs to be taken into consideration (Richardson, 1999). This analysis has taken this factor into account.


3 Results

3.1 Activity Level Index

Figure 1 shows the result for the Arids 2021 based on the calculation of the Activity Level Index using 30 observing stations in 11 countries. Almost all of the observing stations had difficulties to observe this shower because of the low radiant elevation even with the zenith attraction taken into account). Some unusual activity has been recorded around October 7, 0h–1h (λʘ = 193.67°–193.71°). The maximum occurred at October 7, 0h UT (λʘ = 193.67°) with an Activity Level Index = 1.7 ± 0.1. This outburst was mainly observed by Mexican observing stations.

Figure 1 - The Activity Level Index for the Arids 2021.

Figure 1 – Activity Level Index of Arids 2021.

3.2 Estimated ZHRr

Figure 2 shows the result for the Arids in 2021 based on the calculation of the ZHRr. A strong activity was recorded at October 7, 1h UT (λʘ = 193.71°) with ZHRr = 80 ± 10.

Although the time of the maximum is different between the Activity Level Index and the estimated ZHRr, it seems that this depends on the rather few observations (only a couple of observing stations in Mexico).

Figure 2 – The estimated ZHRr of the Arids 2021.

Figure 2 – The estimated ZHRr of the Arids 2021.


Table 1 – Activity Level Index (AL) and estimated ZHRr of Arids 2021.

Time (UT) λʘ (°) Activity Level ZHRr
Oct 6 20h 193.507 1 2 14±3
Oct 6 21h 193.548 2 0.2±0.0 0
Oct 6 22h 193.589 3 0.4±0.2 3 4±3
Oct 6 23h 193.630 3 0.4±0.2 3 12±4
Oct 7 00h 193.672 2 1.7±0.1 2 59±14
Oct 7 01h 193.713 3 1.4±0.3 2 80±10
Oct 7 02h 193.754 9 0.6±0.1 1 2±4
Oct 7 03h 193.795 4 0.2±0.1 1 (-7±3)
Oct 7 04h 193˚.836 6 -0.1±0.2 1 (-6±2)



The worldwide data were provided by the Radio Meteor Observation Bulletin (RMOB). Although most observing stations suffered bad conditions such as a low radiant elevation, many observes provided data.  We thank the following observers for their contribution:

Chris Steyaert (Belgium), Felix Verbelen (Belgium), Johan Coussens (Belgium), Jean Marie F5CMQ (France), WHS Essen (Germany), Balogh Laszlo (Hungary), Istvan Tepliczky (Hungary), GAML Osservatorio Astronomico Gorga (Italy), Mario Bombardini (Italy), Oss Monte San Lorenzo DLF (Italy), Hirofumi Sugimoto (Japan), Hironobu Shida (Japan), Hiroshi Ogawa (Japan), Kenji Fujito (Japan), Masaki Kano (Japan), Masaki Tsuboi (Japan), Nobuo Katsura (Japan), Tomohiro Nakamura (Japan), Juan Zapata (Mexico), Rainer Ehlert (Mexico), Salvador Aguirre (Mexico), Rafael Martinez (Puerto Rico), Karlovsky Hlohovec Observatory (Slovakia), Jochen Richert (Switzerland), Philip Norton (United Kingdom), Philip Rourke (United Kingdom), Simon Holbeche (United Kingdom), Eric Smestad_KC0RDD (United States of America), Richard Schreiber (United States of America).


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