A Model Analysis of the Northern Ionospheric Structure Observed with the MAVEN/ROSE at Mars

The northern hemispheric electron density (N_e) data acquired by the Radio Occultation Science Experiments (ROSE) onboard the Mars Atmosphere and Volatile Evolution (MAVEN) have indicated more complicated ionospheric structure of Mars than previously thought.  Large variations in the topside N_e scale heights are observed presumably in response to the outward flow of the ionospheric plasma or changes in plasma temperatures.   We use our 1-D chemical diffusive model coupled with the Mars - Global Ionosphere Thermosphere Model (M-GITM) to interpret the northern upper ionospheric structure at Mars.  The primary source of ionization in the model is due to solar EUV radiation. Our model is a coupled finite difference primitive equation model which solves for plasma densities and vertical ion fluxes.  The photochemical equilibrium for each ion is assumed at the lower boundary of the model, while the flux boundary condition is assumed at the upper boundary to simulate plasma loss from the Martian ionosphere.  The crustal magnetic field at the measured N_e locations is weak and mainly horizontal and does not allow plasma to move vertically.   Thus, the primary plasma loss from the topside ionosphere at these locations is most likely caused by diverging horizontal fluxes of ions, indicating that the plasma flow in the upper ionosphere of Mars is controlled by the solar wind dynamic pressure.  We find that the variation in the topside N_e scale heights is sensitive to magnitudes of upward ion fluxes derived from ion velocities that we impose at the upper boundary to explain the topside ionospheric structure.  The model requires upward velocities ranging from 50 ms^-1 to 90 ms^-1 for all ions to ensure an agreement with the measured N_e profiles. The corresponding outward fluxes in the range 1.1 x 10⁶ – 5.8 x 10⁶ cm^-2 s^-1 are calculated for O₂⁺ compared to those for O⁺ in the range 3.8 x 10⁵ – 6.7 x 10⁵ cm^-2 s^-1.  The model results for the northern N_e profiles will be presented in comparison with the measured N_e profiles.