MAVEN Observations of Ionopause-like Density Gradients in the Martian Ionosphere

The top of an ionosphere is often marked by a sharp change in electron density and other plasma properties, called an ionopause. Here we present a statistical study of dayside ionopause-like density gradients observed in ion and electron density profiles from the MAVEN spacecraft at Mars. Prior studies of the Martian ionopause have lacked simultaneous comprehensive measurements of plasma and magnetic field properties. Therefore, we use MAVEN observations of the electron density, magnetic field, and ion and electron energy spectra to study the factors that influence properties of the ionopause. We present statistics on how the solar wind conditions and crustal field strength affect the presence or absence of an ionopause and its altitude, and consider how plasma and field properties change across the boundary. We find that, on average profiles with an ionopause are accompanied by a higher energy flux of protons at high altitudes and stronger magnetic field at low altitude than profiles without. At altitudes above ~300 km, the O+/O2+ ratio is significantly larger for profiles with an ionopause than those without an ionopause. These findings enhance our understanding of this important plasma boundary at Mars.