Ground-Based Observational Techniques for Meteoroid Lunar Impact Generated Electromagnetic Pulses and Lunar Sub-Surface Structure Detection

Abstract

The lunar surface is pockmarked with large and small craters mostly formed due to meteoroid impacts on the Moon. Most of the craters formed are not erased with time due to lack of "weathering" processes such as no atmosphere and little erosion. The main focus of this research is to develop ground-based observational techniques to search for ongoing hypervelocity meteoroid impacts on the lunar surface. Additionally, to design radar observational techniques to detect and map sub-surface structures that have been buried by the lunar regolith. It is hypothesized that the developing, optically-dense hot ejecta cloud associated with the hypervelocity meteoroid impacts produce an associated complex plasma component that rapidly evolves resulting in a highly-transient Electromagnetic pulse (EMP) in the VHF/UHF spectral region. An observational EMP search was conducted in May 2014 for about 5 hours using an overlapping-band (425-445 MHz) at the Arecibo (AO; Puerto Rico) and Haystack-(HO, Massachusetts, USA) observatories simultaneously to track the common visible lunar surface from two different locations on the Earth. Observations from two locations is helpful in eliminating the false impacts. Interleaved radar observations were used to calibrate the timing and synchronize both the AO and HO systems. As the AO/HO UHF EMP search was interference dominated, an alternative search mechanism using the Arecibo L-band ALFA Array that consists of seven beams arranged in the hexagonal manner was conducted in February 2016. During these observations, at any given time few of the receive-beams were on-Moon and few off-Moon thus allowing discrimination against local interference that might resemble the expected EMP signals. While still encountering local out-of-band radar interference, this observational paradigm did yield a few likely lunar impact EMPs. Additionally, to detect the sub-surface lunar structures, high power large aperture - Jicamarca Radio Observatory (JRO) 50 MHz radar located near Lima, Peru was used to map the lunar surface and subsurface features. This was accomplished by developing or refining various calibration and imaging procedures. This radar provides the ability to map the lunar sub-surface because the 6-meter wavelength radar signal penetrates the low-loss regolith and scatters from larger sub-surface structures allowing study of these structures. This analysis further depends on the (de)polarization of the return signals. Interpretation of lunar radar signal polarization is greatly complicated by the double traverse of the ionosphere at or near wavevector near to perpendicular to the geomagnetic field geometry as described. Preliminary radar observations were conducted in October 2015 by transmitting a circular polarized coded pulse during the lunar transit over JRO. The detected lunar echoes of the duration of 13 minutes were then processed to generate the lunar Range-Doppler maps and identify the (sub)surface features. Preliminary science results from the observations are given. Each of the three observational set-up's along with the signal processing paradigms such as Inverse Synthetic Aperture Radar (ISAR) mapping to form the lunar maps and the time-frequency technique to process the collected observational data are explained. Implications of the observed transient EMP events, processed lunar surface maps, characterization of the observed satellite radar echoes and the difficult radio-frequency interference environment (terrestrial-origin, Moon-bounce signals) surrounding these observations are discussed.