A one‐dimensional multispecies magnetohydrodynamic model of the dayside ionosphere of Venus

The behavior of the plasma and magnetic field in the dayside ionosphere of Venus was studied for both time‐dependent and steady state conditions, using a one‐dimensional multispecies MHD model. The coupled continuity, momentum, and Maxwell's equations were solved simultaneously for several ion species. The multispecies MHD model includes the momentum coupling terms for all ion‐ion and ion‐neutral interactions. The present model is more complete than the “one‐major‐ion” model of Shinagawa et al. (1987). The results of the MHD model and the “one‐major‐ion” model are compared; they indicate that the one‐major‐ion treatment was a fairly good approximation. The magnetic field in the lower ionosphere is maintained by the magnetic flux transported downward from the magnetic barrier region by plasma motion, and the typical time scale for the decay of the magnetic field is about several hours. Two new cases are presented: (1) steady state conditions for the magnetized ionosphere, and (2) inclusion of ion loss due to horizontal transport in the magnetized region. For steady state conditions, the electric field in the ionosphere should be independent of altitude in a one‐dimensional model. Above 160 km, the convective motion contributes to the electric field, while below 160 km the ohmic resistivity term makes the main contribution to the electric field. The effects of the divergence of the horizontal plasma velocity were approximately included by solving a simplified horizontal ion momentum equation. The resulting calculated profiles of the magnetic field and the electron density agree much better with the observations at high altitudes than those without the ion loss terms. These results indicate the importance of the horizontal transport processes in the ionosphere of Venus at high altitudes.