Ion escape dynamics in the Martian upper ionosphere: a self-similar plasma expansion approach

ABSTRACT This study investigates the dynamics of positive ion escape in the Martian upper ionosphere at altitudes of 850–1200 km using a self-similar hydrodynamic plasma expansion model. The model incorporates kappa-distributed suprathermal electrons and two oxygen (${\rm O}$) positive ion species, ${\rm O}^{+}$ and ${\rm O}_{2}^{+}$, with plasma parameters based on Mars Atmosphere and Volatile Evolution mission observations. Numerical solutions of the coupled nonlinear fluid equations reveal that the ${\rm O}_{2}^{+}{-}{\rm e}$ temperature ratio and the suprathermal electron $\kappa$-index strongly govern the escape profiles – density, velocity, and electrostatic potential, whereas variations in ${\rm O}^{+}$ temperature and density ratios exert only minor influence. The estimated total escape rates are approximately $7.4 \times 10^{23}$ ions s$^{-1}$ for ${\rm O}_{2}^{+}$ and $5.7 \times 10^{22}$ ions s$^{-1}$ for ${\rm O}^{+}$, corresponding to mass loss rates of 0.039 and 0.0015 kg s$^{-1}$, respectively. These values provide quantitative constraints on Martian atmospheric escape processes and align well with the observational estimates. Therefore, our model offers valuable insights into the mechanisms governing ionospheric plasma expansion, thereby contributing to a deeper understanding of the long-term evolution of planetary atmospheres and the potential habitability of Mars and similar terrestrial planets.