Morphological study of the electromagnetic environment at a ULF-ELF observatory at Asahi, Chiba, and methods for suppressing narrowband and pulsed interferences

This paper presents a morphological study of the electromagnetic environment in the Ultra-Low and Extremely-Low Frequency ranges (ULF and ELF) at our observation site near Asahi (geographic coordinates: 35.770774N, 140.695007E), Chiba prefecture in Japan.Initially, we determined an "electromagnetically quiet" nighttime period, approximately from 16:30 to 19:20 UT (01:30 -04:20 JST), when man-made noises drop down and natural intrinsic phenomena in these frequency ranges can be observed clearly.The main source of the noise was identified as interference caused by trains' traffic on two nearby railway lines, located between 2 and 3 km from the observation site.We also analyzed various types of quasi-monochromatic and pulsed noises with unknown origins.We developed methods to minimize the effects of these interferences and improve the signal-to-noise ratio when measuring natural ULF and ELF signals such as Pi2, Pc4, Schumann resonances, and others.These methods will be used in our future research.