Attenuation of radio signals by the ionosphere of Mars: Theoretical development and application to MARSIS observations

We investigate the ionospheric conditions required to explain Mars Express Mars Advanced Radar for Subsurface and Ionosphere Sounding topside radar sounder observations of ionospheric attenuation in excess of 13 dB at 5 MHz during solar energetic particle events. We develop theoretical expressions for the attenuation caused by a layer of ionospheric plasma in cases of high, intermediate, and low radio frequency relative to the electron-neutral collision frequency at the ionospheric layer. We apply these relationships to four layers: the M2 layer produced at 120 km by solar extreme ultraviolet photons, the M1 layer produced at 100 km by solar X-ray photons and associated electron impact ionization, the meteoric layer produced at 85 km by meteoroid ablation, and a putative plasma layer produced at 35 km by cosmic rays. Attenuation is weaker in the M2 layer than in the M1 layer. Attenuation in the M1 and meteoric layers are comparable, although their properties are quite variable. The greatest attenuation for radio frequencies above 50 MHz occurs in the predicted plasma layer at 35 km, but its effects are relatively small at lower frequencies. If optimally located with a peak altitude of 50 km, a layer with a peak plasma density of 10⁹ m^−-3 is sufficient to explain the observed 13 dB attenuation. Although the electron densities produced by solar energetic particle events at Mars have not been directly simulated, the required electron densities are plausible. However, the altitude at which solar energetic particles produce plasma is uncertain.