Sources of Ions in the Near-Mars Environment

A multitude of different populations of ions coexist in the near-Mars environment. Ions are produced from the Martian upper atmosphere and its extended exosphere by photoionization, charge exchange, and electron impact ionization. The solar wind provides another source of ions, primarily hydrogen and doubly ionized helium, but also lower fluxes of various minor ions with higher charge states. Charge exchange reactions with Martian constituents can convert these solar wind ions to different charge states (or to neutral form, in the case of hydrogen). Finally, ionization of interstellar neutral particles results in a population of interstellar pickup ions, which varies as Mars orbits through different heliospheric longitudes. These various populations of ions convey important information about the Martian atmosphere and exosphere, the solar wind, the interplanetary medium, and the interactions between these domains. Each of these ion populations react to the electromagnetic fields in the solar wind and the Martian environment, forming distinctive signatures in both the spatial domain and in phase space. By analyzing ion velocity distribution functions and organizing observations by mass per charge, location, and season, we can in principle separate the various ion populations and extract the information.