Arecibo ALFA Array Observations in Search of Lunar Meteoroid-Strike EMPs

We present the preliminary results of a search for transient Electromagnetic Pulses (EMP) associated with the impact of meteoroids on the lunar surface as observed with the Arecibo Observatory ALFA (Arecibo L-band Feed Array) system. The ALFA system is a cluster of seven, dual-linear polarization feeds/beams arranged in a hexagonal manner and operated in the protected L-band region centered at 1.41 GHz. We analyzed 8 TB of data totaling nearly 5.5 h of on- and off-moon observations made in February 2016. We demonstrate the observing strategy and time–frequency methods for the detection and removal of the local-radar transient interference signals while identifying potential EMPs. Local out of band radar interference signals are observed as intermodulation artifacts in the protected L-band. Seven transient wideband EMP events with time scales of less than 10 μs have been detected following the extensive vetting process we describe. Assuming that these EMP-like events originate from gram-sized meteoroid strikes and using very approximate hypervelocity impact, plasma production theory, and EMP generation theory, we estimate the progenitor impact meteoroid kinetic energy to be approximately 1.8 × 10⁷ J. Assuming that the observed EMPs are the result of 10 g meteoroid impacts, the resultant meteoroid flux is 3 × 10⁻⁷ km⁻² h⁻¹ based solely on lunar surface area observed and net observing period. Implications of the observed transient EMP events, measured lunar noise temperature and the comparison with energy estimates derived from the existing lunar impact optical observations are also discussed.