Geomagnetic Observatory at Lampedusa Island: Characterization of local magnetic activity and comparison with the other Italian observatories

At present, the geomagnetic observatory at Lampedusa (south of Sicily — Italy, geographic coordinates 35°31′N; 12°32′E, altitude 33 m a.s.l. - provisional IAGA code: LMP) is the southernmost point of observation in European territory, and since 2007 it contributes at filling the spatial observational gap in the whole south Mediterranean and North African areas. A signature of very low electromagnetic noise is expected at LMP, since it is located in the inner part of a wild park with limited access, far away from the urbanized areas of the island. LMP lies in the middle of the Mediterranean Sea, while the other two Italian observatories (Castello Tesino – CTS and Duronia - DUR, in North and central Italy, respectively) are located in the continental territory.

Comparisons among the three observatories, in both time and frequency domains, allow to magnetically characterize the Italian territory. Both 1-minute and 1-second data for the years 2017-2020 are analyzed under a statistical approach and also single event analysis is performed. Superposed Epoch Analysis (SEA) of geomagnetic data from the three observatories returns individual responses to external triggers during geomagnetic storms as well as SSC and SI events, indicating that in correspondence to impulsive inputs a peculiar feature arises at LMP, probably as contribution of electric currents in the surrounding sea salt water. Magnetic responses in the Ultra-Low-Frequency (ULF, 1 mHz–5 Hz) range from spectral, local Signal-to-Noise Ratio (SNR) analyses under different local time are computed, showing that the signal emerges mainly during morning hours, as expected for upstream waves related ULF source waves: in particular, the distinct narrow band characteristic of SNR at LMP indicates that the ULF signals are here mainly uncontaminated by local Field Line Resonance (FLR) as at DUR and CTS, while lower noise levels estimated at LMP suggest a smaller anthropogenic contamination in this frequency range. Moreover, for the first time at such low latitudes in the Mediterranean region, we find evidence of FLR events on Duronia–Lampedusa intermediate field line with the application of the gradient method, a consolidated technique that provides estimates of the ULF standing wave frequencies.. Results from data retrieved by geomagnetic observatories, whose long time series of data are of primary importance, demonstrate a unique contribution in characterizing the magnetospheric response to external events.