Electronic Telegram No. 5259 Central Bureau for Astronomical Telegrams Mailing address: Hoffman Lab 209; Harvard University; 20 Oxford St.; Cambridge, MA 02138; U.S.A. e-mail: cbatiau@eps.harvard.edu (alternate cbat@iau.org) URL http://www.cbat.eps.harvard.edu/index.html Prepared using the Tamkin Foundation Computer Network ETA AQUARIID METEOR SHOWER 2023 A. Egal, Planetarium de Montreal and University of Western Ontario; and P. G. Brown, P. A. Wiegert, M. Campbell-Brown, and D. Vida, University of Western Ontario, report the detection of an outburst of the eta Aquariid meteor shower (IAU shower 31) by the multi-frequency Canadian Meteor Orbit Radar (CMOR) at Tavistock, Ontario, on 2023 May 4-7. Since the shower onset of activity on Apr. 24, the eta Aquariids have been monitored by CMOR's radar systems running at frequencies of 29.85 MHz and 38.15 MHz (cf. website URL https://aquarid.physics.uwo.ca/research/radar), which are sensitive to meteors down to magnitude +8 (i.e., a limiting meteoroid mass of 1.5 x 10**-8 kg). Starting on May 1, CMOR measured a steady increase in the meteoroid flux, reaching a maximum of 41.8 and 55.6 meteoroids/(1000 km**2)/hour around solar longitude (lambda) 45.4 +/- 0.5 degrees at 29.85 MHz and 38.15 MHz, respectively. Using a mass-distribution index, s, of 1.9 (Egal et al. 2020, A.Ap. 640, A58), these fluxes correspond to zenithal hourly rates (ZHR) of 118 and 152 meteors per hour for each frequency. The CMOR-observed geocentric radiant at the time of peak was R.A. = 337.52 degrees, Decl. = -0.39 degrees (equinox J2000.0), with a median dispersion of 1.15 degrees and a geocentric velocity of 64.0 +/- 3.1 km/s. The radar also detected a possible second peak of activity around 47 +/- 0.5 degrees in solar longitude on May 8, reaching a flux of 27.2 meteoroids/(1000 km**2)/hr on the 29-MHz orbital system. Shower rates decreased thereafter until returning close to normal levels on May 9. Global Meteor Network (GMN) measurements (cf. website URL https://globalmeteornetwork.org) in the same interval show no significant increase compared to previous years, with a maximum ZHR of 68 +/- 6 at lambda = 45.3 +/- 0.3 degrees, assuming s = 1.8. The GMN radiants are concentrated at R.A. = 337.70 degrees, Decl = -1.04 degrees, with a median dispersion of 0.51 degrees and a geocentric velocity of 65.64 +/- 1.59 km/s at the time of peak activity. During the peak, the limiting magnitude of the GMN cameras was around +3.5, preventing the network from observing the faint meteors recorded by CMOR during the outburst. The radar outburst confirms the calculations performed at the University of Western Ontario (Egal et al. 2020, A.Ap. 642, A120), which predicted a strong eta Aquariid shower return confined mainly to smaller meteoroids (producing meteors of magnitudes fainter than +4) in 2023 due to meteoroids released by comet 1P/Halley in 390 BCE. Noticeable activity was predicted to occur between lambda = 36.5 and 55 degrees, with above-average rates between lambda = 42.0 and 49.0 degrees. The model predicted a maximum ZHR of 129 +/- 39 meteors per hour around May 5 (lambda = 44.1 +/- 1.1 degrees). The main peak was expected to last until lambda = 46.3 +/- 1.1 degrees on May 7, and to be followed by a moderate second maximum around lambda = 47.5 +/- 1.1 degrees. Despite an offset of 1.3 degrees in lambda (+/- 1.1 degrees) between the CMOR/GMN observed and predicted main-peak time, the model successfully anticipated the dates, duration, and intensity of the shower in 2023 as measured by CMOR. In the simulations, the shower is found to be rich in small particles (s = 1.96 to 2.15), which together with the nearly full moon likely explains why no unusual enhancement of the eta Aquariid rates were reported in visual/video data (cf. website URL http://www.imo.net). The authors thus hereby suggest that the 2023 eta Aquariid outburst was mostly caused by meteoroids of masses between 10**(-8) and 10**(-5) kg, producing meteors that were too faint to be recorded by optical devices. The model also predicts a somewhat-stronger outburst for the shower in 2024, which will be more evenly distributed across small and large masses and also less affected by lunar interference. NOTE: These 'Central Bureau Electronic Telegrams' are sometimes superseded by text appearing later in the printed IAU Circulars. (C) Copyright 2023 CBAT 2023 May 16 (CBET 5259) Daniel W. E. Green