Minimal global magnetic millihertz fluctuation level determined from mid-latitude underground observations

Investigation of the ultra-low frequency (ULF) magnetic pulsations, detected at mid-latitude by the underground magnetometer system [SQUID]2, is carried out in order to establish a first baseline for the global minimal magnetic level in the site. This study analyzes 24 quiet days, identified by the Deutsches GeoForschungs Zentrum (GFZ) as magnetically quiet. The results show that the mean magnetic spectrum in the millihertz range (0.1–15 mHz), which corresponds to the global magnetic Earth-ionosphere system, is characterized by several pulsations of noticeable amplitude above the flicker noise. The noise trend clearly shows the difference below and above 3 mHz. We attempt to discern the origin of these pulsations present in the minimum magnetic background baseline, taking into account terrestrial and ionospheric sources. This study provides a complementary analysis of the Earth-ionosphere coupling in the magnetic domain.