Abstract: The Global Meteor Network successfully covered the predicted enhanced Ursid activity meanly by its RMS cameras in the USA while other parts of GMN had partial or complete overcast sky. As many as 253 Ursid orbits were recorded between solar longitude 269° and 272°. The raw uncalibrated activity profile based on numbers of orbits is in good agreement with radio meteor data. The Ursid orbits recorded during the 2020 maximum consist mainly of a very compact structure with very similar orbits and a compact radiant, apart from few outliers.
In a recent case study on the Ursid meteor shower (Roggemans, 2021) the predictions made by Lyytinen and Jenniskens (Jenniskens, 2006) were recalled for another possible outburst or enhanced activity caused by some old dust trails. In previous years bad weather interfered at many northern hemisphere locations preventing most observers from getting a glimpse of possible Ursid activity. The year 2020 was no exception and most visual observers as well as many cameras faced a completely overcast sky.
Meanwhile we know from radio and radar observations that a distinct enhanced activity of the Ursids took place at the predicted time. The CAMS networks in the US and the United Arab Emirates recorded a nice number with 191 Ursid orbits during the interval 269.80° < λʘ <°270.82° (Figure 1). Unfortunately, the CAMS data is kept under embargo which is indeed not very helpful for anyone to know more about the 2020 Ursid return. Also, the CMOR radar map marks a distinct hot spot at the position of the Ursid radiant but no detailed data is available (Figure 2). Luckily the Global Meteor Network had clear sky during the crucial time span with possible enhanced activity.
2 Global Meteor Network Ursid data
The GMN currently consists of a number of regional networks, several of which are still in full expansion, spread over Canada, Europe, Israel and the US. Most parts of the GMN were badly affected by unfavorable weather most of the time. We limit the scope of this analysis to the time interval of 269. 0° < λʘ < 272.0° or 72 hours. During this time span GMN collected as many as 1492 orbits. 85% of these orbits were recorded by the RMS cameras installed in the US while the other 15% were obtained under less favorable circumstances by the RMS cameras installed in Belgium, Canada, Croatia, France, Germany, Ireland, Israel, Netherlands, Spain and the United Kingdom. It was very unfortunate that the large Russian RMS camera network remained overcast during this time interval.
3 The Ursid shower identifications
The online GMN data lists 265 multi-station meteors identified as Ursids during the 72 hours around the expected maximum. Applying the same method as used in the Ursid case study, the author identified 259 Ursid orbits based on the orbit similarity criteria explained in Roggemans et al. (2019). 12 of the 265 orbits identified as Ursids by the GMN algorithm were rejected for Ursid identification by the similarity criteria, mainly because of a deviant eccentricity, 8 being too low in eccentricity, 4 being hyperbolic. 6 of the 259 Ursid orbits identified by the similarity criteria were listed as sporadics by the GMN algorithm, all six with mainly a slightly deviant radiant position that was probably beyond the limits defined in GMN. All 18 possible Ursids are somehow outliers and therefore these are ignored for the further analysis which finally has 253 Ursid orbits collected by GMN.
4 The mean Ursid orbit based on GMN data
With a total of 253 Ursid orbits collected during 72 hours including the peak activity in a single year, the GMN made a major contribution to the global collection of known Ursid orbits (Roggemans, 2021). Determining a mean orbit (Jopek et al., 2006) for these GMN Ursid orbits using a iterative procedure based on the similarity criteria of Southworth and Hawkins (1963), Drummond (1981) and Jopek (1993) combined, allows to consider different classes of dispersion among the Ursid orbits. These should help to visualize the degree of dispersion and compactness within the meteoroid stream. The different classes of similarity are defined as follows:
- Low: DSH < 0.25 & DD < 0.105 & DH < 0.25;
- Medium low: DSH < 0.2 & DD < 0.08 & DH < 0.2;
- Medium high: DSH < 0.15 & DD < 0.06 & DH < 0.15;
- High: DSH < 0.1 & DD < 0.04 & DH < 0.1;
- Very high: DSH < 0.05 & DD < 0.02 & DH < 0.05.
For each similarity class the mean orbit has been calculated and listed in Table 1. The mean orbits for each of the similarity classes are almost identical for the 2020 GMN data. Most orbits were registered during the peak of the shower activity which consists of very similar orbits. The general Ursids case study (see Roggemans, 2021) was based on combined CAMS, EDMOND and SonotaCo data for the period 2006–2019 covering the entire time span during which candidate Ursid orbits could be detected (256° < λʘ < 283°). Such a long activity period of 27 days with data accumulated from 14 years includes more dispersed particles separated from the main Ursid stream.
In Table 2 the high threshold similarity orbits obtained by GMN in 2020 are compared with the same similarity class mean orbits obtained for CAMS, EDMOND and SonotaCo during previous years. These results agree very well.
Table 1 – The mean orbits calculated for each similarity class according to the threshold of the D-criteria for the Ursids based on the shower identification described in Roggemans et al. (2019).
|Low||Medium Low||Medium High||High||Very high|
Table 2 – The mean orbits calculated for each camera network separately for the Ursids that fulfill the high threshold criteria based on the shower identification by the author.
5 The 2020 Ursid radiant
The compact nature of the 2020 Ursid return also appears very well in the radiant plot. Figure 3 shows the radiant distribution in Sun-centered geocentric ecliptic coordinates to eliminate the radiant drift caused by the Earth moving on its own orbit around the Sun. Apart from few outliers, the very similar orbits form a very compact radiant area of about 3° in diameter. Figure 4 shows the same map but with the velocity color coded. The compact radiant is formed by Ursids with almost identical geocentric velocity, while the few outliers at left have a lower velocity. A gradual increase in velocity in the direction of the Apex can be seen.
6 The 2020 Ursid activity profile
Counting the number or Ursid orbits recorded in time bins of 0.15° in solar longitude (3.6 hours) shifted with steps of 0.05° in solar longitude (1.2 hours) results in Figure 5. The highest numbers of Ursid orbits appeared at solar longitude 270.80°, 270.55° and 270.45°.
Some of the time bins had too few orbits and were removed. With 85% of all the Ursid orbits collected at one region, there may be local observing circumstances as well as statistical fluctuations. The sky conditions will affect the sporadic background in the same way as it does for the number of Ursid orbits. Expressing the number of Ursid orbits as a percentage relative to the sporadic background results in Figure 6. The first peak appears earlier at λʘ = 270.35°, but this may be spurious due to statistical fluctuations with “only” 32 sporadic orbits recorded and 41 Ursids within this time bin. Around λʘ = 270.80°, 125 sporadic orbits were recorded and 76 Ursid orbits. The highest value seen in Figure 5 is reduced to a shoulder on the profile in Figure 6. The more networks and the larger the number of cameras, the better statistical variations may be averaged out. In this case caution is required with activity profiles.
Global radio data (Ogawa and Sugimoto, 2021) saw the enhanced activity beginning at λʘ = 270.29° and ending at λʘ = 270.97°, which is exactly the time span with the best numbers of Ursid orbits in Figures 5 and 6. The radio observers marked two peaks at λʘ = 270.45° and λʘ = 270.55°, peaks which were confirmed by Japanese radio observers. No peak in the radio data at λʘ = 270.80°, but the activity profile for the ZHR equivalent for radio observations also shows a shoulder at this time. The radio data and the orbit data profiles indicate the same sub maxima at λʘ = 270.45° and λʘ = 270.55° as well as a shoulder in the activity profile at λʘ = 270.80°.
7 Ursid orbital elements
Some graphics can help to give insight in the structure of the meteoroid stream. The distribution of the inclination i against the length of perihelion Π (Figure 7) shows how a large majority of the Ursid orbits form a compact concentration with only a small number of outliers. The higher the inclination, the higher the geocentric velocity. The histogram for the length of perihelion Π (Figure 8) seems to suggest that there are some groups of orbits with a slightly different length of perihelion Π. Figure 9 shows the distribution of the inclination i against the perihelion distance q. The increase in velocity is very well visible.
From all these graphs it is obvious that the enhanced Ursid activity in 2020 was caused mainly by a compact component with very similar orbits combined with a more dispersed annual component.
Thanks to the efforts of the Global Meteor Network collaborators, a valuable dataset of Ursid orbits could be recorded although general bad weather conditions prevented most video camera networks on the northern hemisphere to record any glimpse of this shower. The multiple maxima indicate the presence of different dust trails combined with the annual activity.
We used the data of the Global Meteor Network which is released under the CC BY 4.0 license. The author thanks the camera operators of the global Meteor network and Denis Vida in particular for providing the scripts to plot the velocity distribution with a color gradient and to compute the average orbit according to the method of Jopek et al. (2006).
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