A method to predict electric field spectra from empirically modeled geomagnetic ULF activity

A model for prediction of the Earth background electromagnetic field spectra in the ULF range (1 mHz to 10 Hz) is developed. The possibility to model the hourly integrated magnetic wave power spectra with two different mathematical models, a power law or a fourth‒order polynomial, is investigated. Spectral properties of 3 months of ground magnetic data show that the temporal evolution of the power law parameters can be modeled based on the hourly planetary magnetic activity, the K p index. Furthermore, the parameters of the polynomial model are related to the magnetic wave power in two spectral bands within the Pc3 and Pc1 pulsation bands. Empirical models of the magnetic wave power in these bands are developed based on the diurnal variation and the correlation with solar wind velocity of geomagnetic pulsations. Comparison with observations shows that the power law model represents the spectra well for low frequencies. However, the polynomial model based on solar wind velocity provides a better representation for the bulk of the ULF domain with mean errors between 2 and 7 dB, increasing with decreasing frequency. The modeled magnetic wave spectrum and knowledge of the underlying electrical conductivity profile can be used to predict the induced ground electric field spectra. An apparent electric conductivity profile was found via model‒based inversion, and the predicted electric wave power is compared to observations from an electrode systems offshore of the test site. The mean error between the observed and predicted electric field amplitudes is 2–7 dB and is consistently lower than the 95% central range of the data set.