The upper ionosphere of Mars: A comparison of Mariner 9 radio occultation and MARSIS measurements

Electron density profiles of the Martian ionosphere show that the dayside ionosphere can be divided into two regions, one controlled by diffusion where the electron density decreases exponentially with altitude, and one at lower altitudes in which electron densities follow the basic predictions of Chapman theory. Models and data generally place the transition between the two regions near 200 km, but measurements from the Mars Advanced Radar for Subsurface and Ionosphere Sounding (MARSIS) on Mars Express suggest that the transition occurs closer to 275 km. This altitude also marks the transition between the two MARSIS operational modes: the electron density above 275 km is determined from local plasma frequency measurements, while at lower altitudes it is obtained remotely from radio sounding. In this study we compare Mariner 9 radio occultation electron density profiles to the Němec et al. (2011) empirical model based on MARSIS data to evaluate the accuracy and biases of the MARSIS measurements. We investigate whether the Mariner 9 electron density profiles can be well described by the Němec et al. (2011) empirical model. We also identify the typical Mariner 9 transition region altitude, as measured by a change in scale height, to determine whether the transition at 275 km observed by MARSIS is a consequence of differences in the two MARSIS operational modes or is physically meaningful. The Mariner 9 radio occultation measurements of the Martian ionosphere have recently been digitized and reanalyzed and are ideal for our study because they extend as high as 400 km, spanning the transition region between the two MARSIS data types, while similar measurements from Mars Global Surveyor rarely extend beyond 200 km. Our findings will help resolve discrepancies between the two MARSIS data types and validate the MARSIS electron density measurements.