Many clear nights occurred over Europe during the period of April 15 to 30, 2020. As a result of the Corona pandemic, air traffic was halted and emissions on the ground had also been significantly reduced. This fact in combination with many high-pressure fields above Scandinavia resulted in many super clear nights for the BeNeLux. It was not only good to observe in the BeNeLux, it was also stable clear weather in other parts of Europe. In addition to data from Europe, there were also some small datasets from America, Asia and Australia. All in all, the Lyrid data could be well calculated. 55 observers reported 1472 Lyrids via the IMO site. In this article the results of the visual analysis of the Lyrids 2020 are presented and discussed.
1 Collecting data
As usual, the IMO site was first checked for available observations. The author also received observations from one observer who does not report to IMO. When the data was collected, a distinction was immediately made between the observations. The observations had to meet the following requirements:
- Only observations made with a limiting magnitude of 5.9 or higher were used.
- The coverage factor F may not exceed 1.10.
In total, 56 observers conducted 166 observation sessions, resulting in 1472 Lyrids. See also Table 1.
Table 1 – All observers who observed the Lyrids of 2020.
2 Population index r
To determine the population index r, all supplied magnitude distributions were checked on the following rule. The average magnitude of the observed meteors should not differ by more than 4 magnitudes from the observed limiting magnitude. After this check, of the 1019 Lyrids, 838 remained. These 838 Lyrids were used to determine the r value (Steyaert, 1981).
The results are shown in Table 2 and Figures 1 and 2. Most results, as expected, were obtained with r[1; 5]. The disadvantage is that you exclude the Lyrids of 0, –1 and –2, which usually appear around the maximum and this gives a distorted picture for that period. In that regard, r[0; 5] gives a better result and you can clearly see that more bright meteors are observed around the Lyrid maximum. Logically, the uncertainty in the nights around the maximum is also much lower. For 20–21, 21–22 and 22–23 April 87, 506 and 180 Lyrids respectively were used to determine the population index r.
Table 2 – Population index r for the Lyrids in April 2020.
Figure 1 – Population index r[0; 5] of the Lyrids in the period April 14–28, 2020.
The r values from r[0; 5] were ultimately used for the final ZHR calculations. For the maximum the r value could also be determined over periods of one hour, in steps of half an hour. These results are shown in Table 2 and Figure 2.
According to Rendtel (2019) the Lyrid maximum should occur on April 22, 2020 around 06h40m UT (λʘ = 32.32°), but the maximum time will vary from year to year between λʘ = 32.00° and 32.45°. This is between April 21, 2020 22h38m UT and April 22, 2020 09h42m UT. It is known that more bright meteors appear quite soon after the maximum, the population index r then decreases quickly. If we look at Figure 2, we see a steady decrease in the population index r after λʘ = 32.09°. This could indicate that the maximum occurred just before this period. We will discuss this further in the next Section.3.
Table 3 – Population index r for the Lyrids during the night 21–22 April 2020.
Figure 2 – Population index r for the Lyrids during the night 21–22 April 2020.
3 Zenithal Hourly Rates
After all Lyrid data was entered in the ZHR spreadsheet, the data was selected again on the following criteria.
- Radiant heights, minimum radiant height must be 25 degrees or higher.
- In case of too short observation periods, if possible, several short consecutive counting periods were added into longer periods.
- Extreme ZHR outliers were removed.
Among the 1472 Lyrids reported to the IMO and the author, 1083 remained after the first selection process from Section 1. After the second selection process described above, 1046 Lyrids were ultimately left. Table 4 and Figure 3 are the result of the calculations.
Table 4 – Lyrids 2020 ZHR based on 1046 Lyrids.
Figure 3 – The Lyrids ZHR curve based on Table 3. The solar longitude shown represents the period April 13–30, 2020.
From Figure 3 it is clearly visible that the ZHR is between 2 and 4 between λʘ = 24° and 30°. After that, the activity increases to a maximum ZHR of 15. Only after λʘ = 34° to 35° the ZHR drops below 5 again.
Figure 4 – The radio ZHR curve of the Lyrids 2020 by H. Sugimoto.
When did the maximum occur? We now zoom in on the maximum. A nice tool to get an idea when the (possible) maximum has fallen is the graph by Hirofumi Sugimoto. See Figure 4 and also online. Figure 4 shows that according to the radio ZHR method (Sugimoto, 2017), the maximum occurred exactly at λʘ = 32.3°. As the author already wrote in this article about the population index r, it seems that based on the population index r calculations the maximum has taken place around λʘ = 32.09°.
The night 21–22 April 2020 was then examined in detail. The ZHR values in Table 5 and Figure 5 were calculated on the basis of 25- to 60-minute counts. These were then averaged according to the “weighted average” method.
Table 5 – ZHR of the Lyrids between April 21, 2020 21h00m UT and April 22, 2020 12h00m UT.
Figure 5 – ZHR and population index r of the Lyrids during the night April 21–22 together in one graph. Only European data.
From Figure 5 it seems that, based on visual observations, the maximum of the Lyrids has taken place over Europe, around λʘ = 32.074°. This is April 22, 2020 around 00h27m UT. Indeed, we also see the population index r decrease after this maximum. With a ZHR of only 14.6 ± 1.2, this is a weak Lyrid year. In the Meteorshower Calendar 2020 (Rendtel, 2019) it is stated that when the peak is ideal, i.e. at λʘ = 32.32°, the ZHR is usually around 23. The further away from the 32.32° maximum the peak is, the lower the maximum ZHR is, with a minimum ZHR of 14. We now found a peak at λʘ = 32.074° with a ZHR of 14.6 ± 1.2. That is roughly 6 hours earlier than the ideal time and thus seems to support the statement from the Meteor Shower Calendar 2020 of IMO. But there is still a ‘problem’.
It is very unfortunate that the “ideal” maximum was expected on April 22, 2020 at 06h40m UT. The Sun is already above the horizon in Europe, while the radiant from America is still relatively low (except in the north east). Only 4 observers were active around this time. Unfortunately, their data could not be used due to too low limiting magnitudes or too high cloud percentages. A quick calculation of all these data with limiting magnitudes of 4.0, 5.0 and 5.1, and with one observer who had low radiant heights and cloud factors with F = 1.04, 1.85 and 1.66 resulted in ZHR values between 10 and 200! It should be clear why the author prefers not to use observing data with too low limiting magnitudes.
If we then look at American data that meet the requirements described earlier in this article, the observations of two observers remain: Bob Lunsford and Wesley Stone. These were added to the graph in Figure 5. The result of this we find in Figure 6. We must keep in mind that on the one hand it is data from just two observers of which two counting periods coincides, and on the other hand, the overlapping period is very close in terms of ZHR and both are experienced observers who have been active for many years. This indicates that these observations are reliable.
Figure 6 – Detailed graph of the Lyrids at night 21–22 April 2020 between 21h00m and 12h00m UT.
Figure 6 therefore shows the problem of the Lyrids anno 2020: at first sight a maximum above Europe around λʘ = 32.074°. The ZHR is correct, the population index r trend is also what you would expect. However, the good US data immediately indicates higher ZHR values than found in Europe, albeit with larger uncertainties and by only two observers, with immediately a decrease in the ZHR values. This is also what you would expect at a maximum around 32.32° (April 22, 2020 6h40m UT). But unfortunately, there is no good observational data available from around that period.
According to the radio method, a maximum (ZHR 22) is found at the “correct” λʘ = 32.32°. A sub-peak is visible when the European “maximum” was found. The way Sugimoto converts the radio observations to a ZHR curve is described in .
All in all, based on the visual data, it seems difficult to determine where the “real” maximum took place. However, if we look at all observations in comparison with the radio ZHR data, you could cautiously state that the Lyrid maximum probably took place at λʘ = 32.3°.
It is very unfortunate that there are no more observers active in Asia and America, people who observe meteors on a regular basis. Please, try to observe some nights outside the annual meteor shower maxima. Data from the end of July and August is particularly welcome, so the author can make reliable Cp calculations for the observers.
A very big thank you goes to all observers who have observed the Lyrids. Without their efforts this analysis was not possible! Their names are all listed in Table 1. In addition, a word of thanks to Carl Johannink and Michel Vandeputte for reading this article critically and giving advice for this article. And last nut not least a thank you to Paul Roggemans for checking my English!
Rendtel J. (2019). Meteor Shower Calendar 2020, IMO.
Steyeart C. (1981). “Populatie indexbepaling : methode en nauwkeurigheid”. Technische Nota nr. 5 VVS Werkgroep Meteoren.
Sugimoto H. (2017). “The new method of estimating the ZHR using radio meteor observations”. eMetN;2, 109–110.