The variability of the topside ionospheres of Venus and Mars as seen by recent radio science observations

Abstract

This study combines the observations of the Venus Express Venus Radio science experiment (VeRa), Mars Express Mars Radio Science experiment (MaRS), and MAVEN Radio Occultation Science Experiment (ROSE) at Venus and Mars with proxies for environmental parameters, insolation and solar wind characteristics to compare the effects of individual drivers on the variability of the topside ionospheres of both planets. The average extent and variability of the ionospheric topside of both planets decrease with increasing solar wind dynamic pressure. On Venus, the average altitude and variability of the ionospheric topside increase with rising solar EUV irradiation. On Mars, the extent of the ionospheric topside on the northern hemisphere under very low solar wind conditions is similarly affected by solar EUV, while the southern hemisphere is influenced by the presence of strong crustal magnetic fields and heliocentric distance. The relationship between the ionosphere on the southern hemisphere and heliocentric distance indicates that increased solar irradiation and atmospheric dust affect the extent of the ionospheric topside during southern summer. A wide variety of ionospheric topside features, including traditional ionopause gradients, are observed in low noise level and near-vertical electron density profiles on the ionospheric dayside of both planets. All topside structures observed in the Venus ionosphere during the VEX mission are also seen at Mars. This suggests that despite the influence of unique drivers on each planet's ionosphere, the overall interaction of Mars with the solar wind shows many similarities to that of Venus during a weak solar cycle.