UPPER LIMITS TO THE NIGHTSIDE IONOSPHERE OF MARS

The nightside ionosphere of Mars could be produced by electron precipi tation or by plasma transport from the dayside, by analogy to the Venu s, but few measurements are available. We report here model calculatio ns of upper limits to the nightside ion densities on Mars that would b e produced by both mechanisms. For the auroral model, we have adopted the downward traveling portions of the electron spectra measured by th e HARP instrument on the Soviet Phobos spacecraft in the Martian plasm a sheet and in the magnetotail lobes. For the plasma transport case, w e have imposed on a model of the nightside thermosphere, downward flux es of O+, C+, N+, NO+ and O2+ that are near the maximum upward fluxes that can be sustained by the dayside ionosphere. The computed electron density peaks are in the range (1.3 - 1.9) x 10(4) cm-3 at altitudes of 159 to 179 km. The major ion for all the models is O2+, but signifi cant differences in the composition of the minor ions are found for th e ionospheres produced by auroral precipitation and by plasma transpor t. The calculations reported here provide a guide to the data that sho uld be acquired during a future aeronomy mission to Mars, in order to determine the sources of the nightside ionosphere.