Escape of CO_2~+ and Other Heavy Minor Ions From the Ionosphere of Mars

摘要

Ionospheric escape from Mars is an important process for the erosion of the Martian atmosphere. The heavy ion outflow is dominated by O_2~+ and O~+, however, the Martian ionosphere consists of many more ion species. In the dayside ionosphere below roughly 220 km altitude, CO_2~+ is the second most dominant ion, and higher up and in the nightside ionosphere other ions species with a mass near but below that of O_2~+ (28–30 Atomic Mass Unit [AMU], e.g., CO~+, N_2~+, HCO~+, NO~+, etc.) gain in importance. However, their contribution to the ionospheric outflow is often obscured by the dominant ions O_2~+ and O~+. Therefore, we perform a peak fitting to the mass spectrum of SupraThermal and Thermal Ion Composition onboard Mars Atmosphere and Volatile EvolutioN. This method is validated against the mass spectrum given by Neutral Gas and Ion Mass Spectrometer, and results in the ionosphere agree qualitatively very well. A statistical study is then performed to study the low-energy (<100 eV) heavy ion outflow. We find that CO_2~+ can be seen throughout the Martian magnetosphere, but it escapes only in small amounts at a mean rate of less than 1% of the O_2~+ rate. On the other hand, we find that the ions with a mass from 28 to 30 AMU, which were previously unknown in the Martian ion escape, are an important part of the ionospheric outflow, with a mean escape rate that reaches 32% of the O_2~+ rate.