The ionosphere of Mars from solar minimum to solar maximum: Dayside electron densities from MAVEN and Mars Global Surveyor radio occultations

Abstract Conditions in the ionosphere of Mars vary from solar minimum to solar maximum due to changes in the ionizing solar irradiance. Although changes in the main ionospheric layers, namely the M2 layer and the lower M1 layer, over the solar cycle have been well-characterized by previous work, corresponding changes in the upper regions of the photochemical ionosphere ( ∼ 140–200 km) have not. Here we investigate those changes using ionospheric electron density profiles acquired at solar zenith angles of 70°–90° under solar minimum conditions by the MAVEN Radio Occultation Science Experiment (ROSE) and under solar maximum conditions by a similar radio occultation experiment on Mars Global Surveyor. Due to the increase in ionizing irradiance from solar minimum to solar maximum, we expect that densities at all altitudes will increase from solar minimum to solar maximum. Due to the eccentricity of Mars’s orbit, observed electron density profiles must be adjusted to a common Mars-Sun distance to highlight solar cycle effects. This process involves increasing observed electron density values by a factor proportional to the Mars-Sun distance, which mitigates for variations in incident irradiance, and expressing altitude relative to the main ionospheric peak, which mitigates for variations in heating and expansion of the neutral thermosphere. We find that the scale height of the upper regions of the photochemical ionosphere is remarkably constant between solar minimum and solar maximum, whereas the corresponding neutral temperature increases by 50%. Yet these two quantities are thought to be proportional. Electron density values in the upper regions of the photochemical ionosphere and peak electron density values increase similarly from solar minimum to solar maximum.