Unexpected outburst of the Perseid meteor shower 2021, 300% more meteors than usual

An unexpected Perseid meteor flare was detected by low-light video observations on August 14, 2021. The outburst peaked at a solar longitude of 141.474 ± 0.005 degrees (J2000.0 equinox) and the activity profile had a full-width-half-maximum of 0.08 degrees solar longitude and a peak ZHR = 130 ± 20 per hour, which is ~45 per hour above the normal annual Perseid activity.

The Perseids had a steeper distribution index in magnitude than the normal annual flux rate. The activity profile is similar to that obtained from visual and direct meteor scattering observations. This activity may be related to earlier smaller enhancements observed in 2018 and 2019.

In 2018, visual observers recorded a narrow peak of Perseid flux activity near solar longitude 140.95°, about ~30 hours after the traditional Perseid maximum, with a peak near ZHR = 25 per hour, which is higher than the normal Perseid activity at this time of ZHR ~ 45 per hour (Miskotte 2019). In 2019, a similar peak was recorded in forward meteor scattering observations collected by the International Radio Meteor Observation Project. That year, the flare peak occurred at a solar longitude of 141.02° with a peak ZHR of ~30 per hour above normal activity (Miskotte 2020a; 2020b).

In this case, we report the detection of a more significant flare on August 14, 2021 (Jenniskens, 2021). This flare was not expected by the known 109P flares and the Swift-Tuttle dust trace.

Perseids are best observed from the northern hemisphere. The 2021 flare occurred between 6 a.m. and 12 p.m. GMT on August 14, 2021, at a time best suited for the CAMS video observation networks of meteoroid orbits in the United States. The networks triangulated meteors with low-light video cameras and determined the meteor’s radiant and velocity during a continuous nighttime observation. The weather was mostly clear for the networks in Texas (coordinator W. Cooney, including D. Selle, F. Sirway, and J. Brewer) and California (P. Jenniskens, D. Samuels, J. Albers, E. Egland, B. Grigsby, and J. Wray). CAMS Mid-Atlantic (coordinator P. Gural), CAMS Florida (A. Howell), CAMS Arkansas (L. Juneau), and LO-CAMS Arizona (N. Moskowitz) also observed some meteors under partly clear skies (see CAMS-website for date August 14).

First results from the new CAMS network in Texas under mostly clear skies and the CAMS network in California under clear skies show an activity profile with a peak zenith hourly rate of ZHR = 130 ± 20 per hour on top of the normal ZHR = 40-45 per hour annual Perseid activity (Figure 1). The full width-half-maximum of the Lorentz profile is 0.08 ± 0.01 degrees solar longitude. The peak was at 141.474 ± 0.005 degrees solar longitude (equinox J2000.0), which corresponds to 8.2 hours GMT on August 14. The combined magnitude distribution index was 3.59 ± 0.36, compared to 2.94 ± 0.04 for the annual component in other years at this solar longitude.

Comparison with other observations

Pierre Martin, a visual observer from Ottawa, Canada, reported “many Perseids per minute with many bursts, sometimes 3-4 per second,” starting at 6 o’clock Greenwich Mean Time. He observed until 9 o’clock GMT, with clear skies and a limiting magnitude of 6.7. From his readings at 5-minute intervals, we have calculated a peak ZHR = 210 ± 20 per hour at a solar longitude of 141.474 ± 0.005 deg. Visually observed meteors well repeat the profile of video data (Fig. 2).

This outbreak was also confirmed by direct radio meteor scattering observations by H. Ogawa of the International Radio Meteor Observation Project. A compilation of data from 49 observers from 14 countries showed that the number of detections increased above normal after 6.4 hours GMT (141.40°S), peaking at about 8.8 hours GMT (141.49°) at three times the peak Perseids level, before dropping back to normal at 12.5 hours GMT (141.65°S). The combined Zenith hourly reading peaked near ZHR = 210 per hour.

The flare cannot yet be identified with the known 109P/Swift-Tuttle dust trace crossing. On the other hand, the width of the flare is similar to that of the returning Perseids (Jenniskens, 2006). The filament is thought to be a cluster of dust in the resonance of mean motions from many past returns. This probably means that this year this dust was directed toward the Earth’s path. These observations may help to better understand the origin and evolution of this dust component.

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