GNGTS 2021 - Atti del 39° Convegno Nazionale

201 GNGTS 2021 S essione 2.1 The field strength E in mV/m on the ground, from a transmitter at a distance d in km, can be calculated by (Wait, 1992) E = 300 [P λ /R h 2 sin(d/R)] ½ exp[-i(k d + π /4)] Σ n Λ n exp(-ikS n d) , (1) where P is the irradiate power in kW, R is the Earth’s radius in km, λ is the free space wavelength, k = 2π/λ, Λ n is the excitation factor for the mode n , kS n is a propagation constant, and h is the height of the ionosphere equal to 70/90 km in day/night. The excitation factor Λ n gives the relative amplitude and phase of each mode of order n excited in the Earth-ionosphere wave guide by the source. The real part of the propagation constant kS n contains the phase information for each mode while the imaginary part determines the attenuation rate. The Λ n and S n factors depend on wavelength, ionosphere eight, the ground electrical propertires, and the spherical reflection coefficients of the ionosphere. These last depend on the vertical distribution of electron density and collision frequency, the direction and magnitude of the Earth’s magnetic field, the frequency and angle of incidence. The electron density distribution is a function of latitude, season, solar cycle, time of day, and whether or not ionospheric disturbances are presents or not. The influences of these various parameters have been summarized in past works (Al’pert et al., 1967; Morfitt, 1977; Davis and Berry, 1977), and can be used to discover the physical link or guide the survey of geophysical phenomena related to VLF and LF monitoring. Several ionospheric perturbations have long been known to be linked to tropospheric activity like Hurricane (Bauer, 1958), and tectonic activity like strong earthquakes (Davies and Baker, 1965). VLF signals are currently used as an efficient probing tool for monitoring lower ionospheric changes (Inan et al., 2010; NaitAmor et al., 2018; Pal et al., 2020; Das et al., 2021). Specifically, measurements of the electromagnetic field concerned disturbances in VLF radio signals related to seismic activity (Molchanov and Hayakawa, 1998). A statistical results by the superimposed epoch analysis in Japan was studied (Maekawa et al., 2006), it shown that the ionosphere was definitively disturbed in terms of both amplitude and dispersion when strong earthquakes occurred. Another statistical correlation between earthquakes and VLF signals by the Japanese VLF network over 10 years, revealed perturbations 3 - 6 days prior (Hayakawa et al., 2010). Ionospheric perturbations with seismic activity have been studied including wave paths of VLF and LF transmitters in Italy (Biagi et al., 2001) by ICV and NSC transmitters described in Tab. 1 and shown Fig. 3. Finally, the geohazard risk reduction can be obtained thanks to a completely statistical evaluations of strong event forecasting, be it of tectonic or atmospheric origin ( TA ), based on the relation between covariance and correlation applied to the surface events and ionospheric digital events ( IO ), so to obtain the conditional probability (Fidani, 2018) P(TA|IO) = P(TA) + corr(TA,IO) Sqrt{P(TA)[1−P(TA)][1−P(IO)]/P(IO)} , (2) where TA is considered as “1” if the earthquake or weater magnitudes are greater than certain thresholds, and IO is considered as “1” if the VLF intensity or phase measurements overcame their respective thresholds.