Magnetically controlled ionosphere of Mars: A model analysis with the vertical plasma drift effects

We use our 1-D chemical diffusive model to quantify the physical processes necessary to interpret the dayside ionospheric electron density profiles measured with the Mars Radio Science (MaRS) experiment onboard the Mars Express (MEX) and Radio Occultation Science Experiment (ROSE) onboard the Mars Atmosphere and Volatile Evolution (MAVEN) spacecraft. The electron density profiles selected for this study represent the southern high-latitude region of Mars, where the crustal magnetic field is strong and near-vertical in orientation. These electron density measurements have shown the topside plasma distribution with unusually large electron density (N_e) scale heights presumably in response to downward accelerating solar wind electrons along magnetic field lines. We find that the photochemical control of the Martian ionosphere ceases at a height well above the ionospheric peak. To interpret the measured ionospheric structure at altitudes where plasma transport dominates, we find it is necessary to impose field-aligned vertical plasma drifts most likely caused by the motion of neutral winds. The most interesting finding of this study is that both upward (between 30 ms⁻¹ and 60 ms⁻¹) and downward (between −12 ms⁻¹ and -90 ms⁻¹) drifts are required to maintain the topside N_e distribution comparable with the measured distribution. We also find that a fixed velocity boundary condition at the upper boundary with a sizeable upward ion velocity is needed to encounter any unexpected ion accumulation in the topside ionosphere to limit the Martian ionospheric outflow. Given the complex nature of neutral dynamics and its relationship to plasma transport processes over magnetic anomalies, we consider that a simple model, such as we have developed, is still capable of yielding valuable insights relating to the neutral wind system at Mars.