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Large meteoroids in the Lyrid stream

ABSTRA C T The outburst of the Lyrid meteor shower in 1803 was remarkable for being rich in bright fireballs and the generation of electrophonic sounds. The implications implicit to the detection of electrophonic sounds are studied in this paper. We present estimates for the minimum-sized Lyrid meteoroid capable of generating electrophonic sounds, and compare these lower limits with the largest meteoroid that might reasonably be ejected from a cometary nucleus through coupling with the sublimation gas outflow. A difference of a factor of order 30 is found between the two limiting sizes. A minimum diameter of order 1 m is required for a Lyrid meteoroid to satisfy the conditions necessary for generating electrophonic sounds. The mechanisms responsible for the placement of large, metre-sized meteoroids into the Lyrid stream are not well defined, but they possibly relate to surface ageing effects of the parent comet, Comet C/1861 G1 Thatcher, and to a history of nuclear fragmentation.