Comparative study of Mars and Venus ionospheres using ionospheric photoelectron measurements by Mars and Venus Express

The Analyser of Space Plasmas and Energetic Atoms (ASPERA-4) instrument package on Venus Express (VEX) has a nearly identical counterpart on Mars Express, called ASPERA-3, which provides a unique opportunity to study the Venus and Mars ionospheres with minimal instrumental interpretation side effects. Previously we used the Electron Spectrometer (ELS) on ASPERA-4 to study the structure and variability of the Venus ionosphere, which is of interest for both plasma escape and the dynamics of the neutral upper atmosphere. We discovered a dawn-dusk asymmetry in the Venus ionosphere, uncorrelated with Interplanetary Magnetic Field strength or direction. Here, we have adapted an automatic algorithm (filter) from this previous work to detect the presence of photoelectrons near Mars in ASPERA-3 ELS data. With information about ionospheric photoelectrons at both Venus (> 6 years of data) and Mars (~9 years of data), we are able to directly compare the ionospheres of the two planets. The filter has identified approximately 200,000 and 3 million measurements at Venus and Mars, respectively, that contain significant photoelectron peaks in the range of 20-30 eV. We find that the Martian ionosphere is more extended vertically in comparison to the Venus ionosphere, relative to the planet's radius. Relative to Venus, the Mars ionosphere is denser in sunlight, less abundant in eclipse, and has a larger tailward extent. The Mars ionosphere also responds to external drivers, such as solar EUV intensity, differently than the Venus ionosphere. We present some plausible ideas to explain these differences based on photoionization and plasma transport mechanisms, and differences in external conditions at the two planets.