Schumann resonances as a tool to study the lower ionospheric structure of Mars

The study of the propagation of extremely low frequency (ELF) waves is essential for electromagnetic sounding investigations planned for some of the future Martian missions. Future surface stations will have the possibility of continuously recording low-frequency electromagnetic field fluctuations. Natural electromagnetic waves produced near the surface by electrostatic discharges in dust storms (dust devils) or by geological activity can be trapped in the resonant cavity formed by the surface and lower ionosphere as it occurs on the Earth. Low-frequency electromagnetic waves can also travel along the magnetic field lines of the recently discovered magnetic anomalies from the magnetosphere to the surface and may produce resonant structures in the cavity. The structure of the resonant frequencies, also called Schumann frequencies, is mainly determined by the geometry of the cavity and by the global electrical conductivity of the ionosphere/atmosphere. Measurements of Schumann frequencies by surface stations can be used for remote sensing of the electrical conductivity of the lower ionosphere/atmosphere. We present a numerical model of electromagnetic wave propagation based on the transmission line modeling (TLM) method with the aim of calculating the resonance frequencies on Mars and their dependence on solar activity and various possible ionization sources like meteoroids. The model has been previously validated by application to the terrestrial case. The numerical results obtained for the Earth are very close to the experimental ones, which supports our predictions on Mars. Our model can be used to study the global atmospheric conductivity using future real ELF measurements by surface stations or even balloons on Mars.